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    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Hanging around with Comet Garradd – September 2011 Event

The first three days of September offer an excellent binocular challenge that is rewarding even if you fail – and for that matter, this rewarding part can be done on any clear night in the month!

But September 1, 2, and 3 offer us a really cool opportunity to spot a faint comet as it skims past one of my favorite binocular asterisms, the Coathanger. Just learning to find the Coathanger is a reward in itself because it’s a neat cluster of stars that actually looks like it’s name implies – a coathanger.  So first we’ll give you some guidance on finding the Coathanger – then, with dark-adapted eyes in a dark sky location you can see if you can find the current interloper, a comet known as C/2009 P1 Garradd.

You can start your search for the Coathanger by locating the Summer Triangle with your naked eye. It is high overhead during an early September evening.

Face south in September in the early evening, then look high overhead. Vega is the brightest star of the three that make up the summer triangle. Identify it - then try to identify the four - much dimmer stars that make up Sagitta - the arrow. (Click image for larger version.)

As you can see, Sagitta will fit in a low-power, binocular field. It's four main stars are fourth magnitude and can be seen with the naked eye if your light pollution is not too bad and your eyes have been dark adapted about 15 minutes. Finding the charming asterism of the Coathanger requires moving just one field away in the direction shown by the arrow. Yes, the Coathanger is upside down - unless you happen to be in the Southern Hemisphere. Its brightest star is magnitude 5 and the fainter ones are magnitude six and 7 - but all should show in just about any size binocular. Click image for larger version of this chart.

Finding Comet Garradd

Comet Garradd will be visible in binoculars for several months – but it is much easier to find in the first three days of September because on those days it is passing near the Coathanger. However, it is a very faint and a challenging object for beginners who expect it to look like its pictures. It won’t. It will simply be a faint cloud. It can be seen in standard 50mm binoculars, but is much easier with larger ones, such as the inexpensive 15X70mm ones I use. Here are three charts showing its position on September 1, 2, and 3, 2011.

Click to enlarge.

Click to enlarge.

Click to enlarge.

And if you are clouded out on September 1, 2, and 3?

Don’t despair.

1. Find the Coathanger and enjoy the sight!

2. And if you want to pursue  Comet Garradd, see the chart below for its path over the entire month.

Click image for larger version. (Prepared using Starry Nights Pro screen shots. )

Finally, i strongly recommend that you go play with the controls on an orbit calculating utility at the Jet Propsulsion Lab site. This will show you how the positions of the Earth, other planets, and Comet Garradd changes with time. And if you use the slider on the side, you will see how Comet Garradd’s orbut is nothing like the orbit of the planets – how it comes in at a very steep angle and how its distance from us – and from the Sun is constantly changing.  It could be a brighter object this coming winter, for example, though then it will be in our morning sky.

Here are a couple examples of what you can see. This is what the JPL utility shows you for the Comet’s location on September 1. (This is just a screen shot – a picture – you can’t play with the settings here! For that go to JPL.)

That makes the comet appear to be very close to Earth. But it's really an illusion based on your angle of view. When you play with the slider on the right of the utility this bird's eye view turns into something like this second view.With the second view we see the comet is really coming into our solar system at a steep angle and so is "above" the earth as it appears to pass "near" us in early September, 2011.

How bright the comet appears depends on how close it is to the Sun – the utility tells us this in “AU” – Astronomical Units – which equal 93 million miles – the distance between Earth and Sun – and how close it is to Earth. However, there’s another factor impacting brightness.  A comet is like an onion with several layers to it. Sometimes one layer contains more volatile material then another layer – so sometimes a comet will brighten when it’s still quite far from the Sun, then not get any brighter even though it gets closer. So take all predictions of brightness with a grain of salt.

For more details on the past, present, and future of Comet Garrad, go here.

Look north in September – the king’s on the rise!

Yes, that’s Cepheus, the King – remember that Cassiopeia (the “W” ) is the Queen. Though Cepheus makes a familiar “home plate” asterism, it’s not nearly so memorable as the “W” of Cassiopeia, primarily because its stars are dimmer than those of the “W.” In fact, you might have difficulty picking it out at first, but here’s a tip: Follow the familiar “Pointers” of the Big Dipper to the North Star – then keep going. The first bright star you meet will mark the tip of the Cepheus home plate. (It’s about one fist away from Polaris – the Pointer stars are nearly three times that far in the other direction.)

Also coming up below the “W” is the “Bow” asterism that marks Perseus, who is carrying the head of Medusa, which contains the “Demon Star,” Algol. We’ll take that up next month when they’re higher in the sky and easier for all to see. Here’s a chart.

Click image for a larger version. (Developed from Starry Nights Pro screenshot.)

For a printer-friendly version of this chart, download this.

To review the connecting mythology, which helps me remember the related constellations, here’s the story in brief. Cepheus and Cassiopeia have a daughter Andromeda whose beauty makes the sea nymphs jealous. They enlist Poseidon to send a sea monster to ravage the coastline of Ethiopia, the kingdom of Cepheus and Cassiopeia. To appease the monster, the good king and queen chain Andromeda to a rock along the coast, but Perseus rescues her and together they escape on Pegasus, his flying horse. You meet Andromeda and Pegasus – the flying horse is much easier to identify as the “Great Square” – in the “look east” post this month. Also in that post we detail the “Three Guides,” three stars that mark the zero hour in the equatorial coordinate system used to give a permanent address to all stars. The first of those Three Guides is Beta Cassiopeia, visible in our northeastern sky, and so on the chart with this post.

Moving from mythology to science, Cepheus is probably best known today for a special type of star called a Cepheid variable. This is a star that changes in brightness according to a very precise time table. What’s more, it was discovered that the length of a Cepheid’s cycle – that is the amount of time it takes to grow dim and then brighten again – is directly related to its absolute magnitude. The absolute magnitude of a star is a measure of how bright it really is as opposed to how bright it appears to us. How bright it appears is, of course, related to how far away it is. That makes Cepheid variables a sort of Rosetta Stone of the skies.

It is relatively easy to time the cycle of a variable, even if the star is quite faint from our viewpoint. These cycles usually cover a few days. If you can identify the length of this cycle, you then can know the absolute magnitude of a star. And if you know its absolute magnitude, it’s a simple matter to compare that to how bright it appears to us and thus determine its distance from us.

This is a huge breakthrough. Without Cepheid variables astronomers were at a loss for determining the distance of anything more than a few hundred light years away. The distance to such”close” stars could be determined using a very common method known as parallax – that is, determining how the star appeared to change position slightly from opposite sides of the Earth’s orbit. But that change in position is extremely tiny and difficult to measure even with very close stars. With the Hipparcos satellite and computer analysis, it has been possible to use this system for stars as far as 3,000 light years. But that still is close by astronomy standards. (Keep in mind our galaxy is about 100,000 light years across.) But Cepheid variables can even be found in other galaxies. In fact, they played a huge role in proving that “spiral nebulae” were real other “island universes” – that is, other galaxies. The Hubble Space Telescope has found Cepheids out to a distance of about 100 million light years – a huge leap from the 3,000 light years we can reach with the parallax method.

There are other ways of making an educated guess at an object’s distance, and they frequently are quite complex and indirect. But the Cepheid variable has been one of the most important tools in the astronomer’s tool kit for the past century. It was in 1908 that Henrietta Swan Leavitt, a $10.50 a week “calculator” at Harvard Observatory noticed a pattern while doing tedious work cataloging stars and saw it’s importance. Though she published a paper about it, she never really received the credit she deserved during her lifetime for this breakthrough discovery.

So when you look at this “home plate” in the sky, see if you can find the fourth magnitude star, Delta Cephei – it’s not hard to spot under good conditions. (See the chart above.) When you find it, pay homage to it for the key role it has played in unlocking the secrets of the universe – for once astronomers know the distance of an object they can make all sorts of deductions about its composition, mass, and movement.

Look east In September 2010 – a pair of asterisms and three brilliant ‘guides’

There are three new asterisms this month – well, I’m not sure one of them should really be called an asterism. This third asterism is really just three stars that serve as a special marker for the equatorial coordinate system – so we’ll take that up last. As we travel September skies we’ll also move from the age of mythology to the age of science. To get started, here’s a chart of what you can expect to see in the east about an hour after sunset.

Click image for larger view. (Developed from Starry Nights Pro screen shot.)

For a printer-friendly version of this chart, download this.

First, let’s look at the “Great Square” – or perhaps we should say “Great Diamond,” since that’s what it looks like when rising. Once overhead, it is certainly a square, and it forms the heart of the constellation known as Pegasus – the flying horse. The stars are all second and third magnitude – about the brightness of the stars in the Big Dipper – so wait until about an hour after sunset, then look east and you should be able to pick this out. Its stars mark out a huge chunk of sky that is nearly empty of naked-eye stars, which is why I sometimes call it the “Great Empty Square.”

The second asterism, Andromeda’s Couch, ties to the northern corner of the square. In fact, it shares a star with this corner. “Andromeda’s Couch” is just my memory device – others would simply call this “Andromeda” because that’s the name of the constellation it dominates. I have difficulty seeing the lovely maiden chained to a rock by looking at these stars and their companions, however. Like most constellations, with Andromeda you need a huge imagination to see the figure these stars represented to the ancients. But knowing that in myth Andromeda was a lovely woman who was rescued by Perseus, I like to think of this graceful arc of stars as her couch with her a misty fantasy figure lying there in alluring fashion. That said, notice three things about it:

1. The bright star at the right – southern – end is also a corner of the Great Square, as we mentioned. In fact, it is the brightest star in the Great Square.

2. The three brightest stars in the “couch” – I’m ignoring the second star which is fainter – the three brightest are about as close to being identical in brightness as you can get – magnitude 2.06, 2.06, and 2.09. They also are pretty equally spaced. Hold your fist at arm’s length and it should easily fit in the gap between these stars, which means there are 10-15 degrees between each star. That’s similar to the spacing between the four stars in the “Great Square” as well.

3. The second star, as mentioned, is dimmer by more than a full magnitude (3.25), but it’s what gives this asterism a couch feeling to me – or maybe a lounge chair – marking a sharp, upward bend.

And where’s the hero Perseus? he should be nearby, right? Well he’s on his way, rising in the northeast after Cassiopeia, but we’ll leave him for next month when he’s more easily seen.

Now for the pièce de résistance!

This is a group of stars that are new to me, at least in this role, and I love them! They’re called “The Three Guides,” but I think of it as four guides, and in the fall of 2010 there’s even a fifth – the bright planet Jupiter. They can all be tied together by a long, graceful arc that represents the great circle of zero hour right ascension – thus the significant tie to the equatorial coordinate system.

If you’re not familiar with this system, it is essentially a projection of the Earth’s latitude and longitude system onto the sky to enable us to give a very precise address for any star or other celestial object, as seen from our planet. On Earth we require an arbitrary circle be chosen as the zero longitude line, and this is the circle that passes through Greenwich, England.

In the heavens we also need such a circle, and the one chosen is the one that passes through the point where the Sun crosses the celestial equator at the vernal equinox. But that point is not represented by any bright star, so how do we know where this “zero hour” circle is? We need it to put numbers to the entire system. Enter “The Three Guides.”

They start with the star Beta Cassiopeia. This is the western most star in the familiar “W,” which is rising in the northeast on these September evenings. In the early evening in September this is the “top” star in the “W.” From there draw an arc to Alpha Andromedae. This is the star mentioned before where Andromeda and the Great Square are joined – they both share this star.

The third star of this trio is Gamma Pegasi – the star that appears to be at the bottom of the Great Square when we see it as a diamond when rising. (If this is not clear, one glance at the accompanying chart should make it so.)

When I look at this great arc, however, I always start to trace it right from the North Star, Polaris. All the great circles representing meridians of right ascension pass through the north and south celestial poles.

And in September 2010, this same arc connecting Polaris, Beta Cassiopeia, Alpha Andromeda, and Gamma Pegasi points right to the most brilliant “star” on our eastern horizon, the giant planet Jupiter. This, of course, is true just for 2010, for like all good “wandering stars,” Jupiter will be moving on. But this month it serves as a special guide as well – more about that in the September “Events” post. Here’s a wider view to the east bringing in all these connections.

Taking a wide view of the "Three Guides" to incorporate the North Star and Summer Triangle as well. Here's what we should see about an hour after sunset in September 2010. Click image for larger version. (Derived from Starry Nights Pro screen shot.)

For a printer-friendly version of this chart, download this.

What’s important is to be able to visualize this one circle in the sky and connect it with the other circle crossing it at a right angle – the celestial equator. If you can do that, you will have identified the two zero points on the equatorial coordinate system and moved your knowledge of finding things in the sky from the mythological arena to the scientific one. That’s why these three “guides” excite me so. When you can look up at the night sky and see not only a dome, but a curved grid projected on it, and on this grid be able to attach meaningful numbers, then you have graduated to sky explorer, first class!

. . . and the rest of the guideposts?

If you’ve located the new September asterisms and identified The Three Guides, then it’s time to check for the more familiar stars and asterisms you might already know, assuming you have been studying the sky month by month. (If this is your first month, you can skip this section.) So here are the guidepost stars and asterisms still visible in our September skies.

  • The Summer Triangle is now high overhead, though still favoring the east. Vega, its brightest member, reaches its highest point about an hour after sunset and moves into the western sky. Altair and Deneb are still a bit east, but will cross the meridian within about three hours of sunset.
  • The “Teapot,” marking the area of the Milky Way approaching the center of our galaxy, is due south about an hour after sunset. Well into the southwest you’ll find the red star Antares that marks the heart of the Scorpion.
  • Arcturus (remember, follow the arc of the Big Dipper’s handle to Arcturus) is due west and about 25 degrees above the horizon as twilight ends.
  • The Keystone of Hercules and the circlet that marks the Northern Crown can both be found high in the western sky by tracing a line between Vega and Arcturus.

Events for September 2010 – center stage for the Georgian Star as introduced by Jupiter!

Uranus - the Georgian Star - as seen by Voyager 2. When you view it in binoculars you may notice the bluish tint but it's doubtful. In fact, while the blue is attributed to the methane in its atmosphere, some folks say they see it in telescopes, some don't.

OK, so no one calls Uranus the Georgian Star these days, but it beats heck out of the adolescent giggles you get if you’re not very careful about how you pronounce Uranus – and “Georgian Star” was its given name. But whatever you call it, the seventh planet will be center stage this month, as it does a complex dance with its brilliant partner, Jupiter. We also have Venus and Mars with us in the evening sky in the west and – just barely – Saturn. In the morning sky Mercury puts in a nice appearance, and I really love what’s going on September 22 – we get the Fall Equinox, of course, plus a full Moon – the Harvest Moon – and we have both Jupiter and Uranus joining the Moon at opposition to the Sun. In fact, Jupiter is at its closest approach to Earth – by a hair – in nearly half a century.

But let’s start with the third largest planet – what was once dubbed the “Georgian Star” – and its close encounter with Jupiter. This is the second such encounter this year, but the first was in the early morning hours of June and the third will come right as the new year begins, which usually means in these latitudes, very cold observing. Such a “triple conjunction” of Jupiter and Uranus happens about once every 14 years. So I’m looking forward to watching, and charting, the two planets starting in September. It’s this close encounter with Jupiter, which dominates our eastern sky during September evenings, that makes The Georgian Star so easy to find this fall, though you’ll need binoculars. Here’s a movie made with Starry Nights Pro software showing you how Uranus appears to march past Jupiter between September 1, 2010, and October 15, 2010. The red circle is 5 degrees – the typical field provided by 10X50 binoculars. Later I’ll provide some charts as well.

“The Georgian Star” is also the title of a very readable new book by Michael D. Lemonick that details the astronomical explorations of William and Caroline Herschel and particularly William’s discovery of Uranus. But I’ve had enough of adolescents of all ages giggling when I forget the one way you can pronounce Uranus – YOUR-a-nus – without it sounding like a reference to a body part we usually don’t mention. So if the professional astronomers can demote poor little Pluto, then I’m for restoring Uranus to its original name, the “Georgian Star.” After all, that’s the name its discoverer gave it. Of course for Herschel this was frank flattery of King George III in a blunt attempt – successful, too – to get a life-time assignment as an astronomer with royal patronage.

For the rest of us the name “Georgian Star” is a good way to connect the planet to history and remind us of the approximate discovery date – George III was king during the American Revolution, and the Georgian Star was discovered in the spring of 1781 while that revolution was still going on. Up until then all the planets had been known since ancient times, and it was assumed that was all there were. So for several months Herschel and the rest of the astronomical brain trust of that day all tried to squeeze this odd object into some known category other than planet:

  • It was, of course, a comet!
  • But it had no tail.
  • Yes, but it was too big to be a star.
  • Ah, but it was not changing size the way a comet does – nor was it fuzzy!

Thus went the debate and so it wasn’t until the following fall that they pretty much settled on the idea that it was another planet – and then the name game started. Herschel leaped in with the Georgian Star, but this didn’t win much favor, except with King George – after all, many astronomers were not English and besides, the other planets had names that connected them to ancient gods. A couple of years later, the name Uranus was proposed by a German astronomer, Johann Bode – in German the name didn’t have the giggly implications that it has in English – and after about 50 years it pretty much stuck.

None of which makes the Georgian Star any easier to find. It looks like another star and a very faint one to boot. What gave it away to Herschel was that it appeared larger than a star when viewed in his telescope. Stars are pinpoints of light – this was a tiny, round disc. What’s more, when viewed over the course of several nights, it changed position relative to the “fixed” stars. And though it is barely on the edge of naked eye visibility in the best of conditions and hard to pick out from many other stars of similar brightness, you can find it easily this fall using nothing but ordinary binoculars – and you can watch it move! So step into the shoes of Herschel. How exciting it must have been in that spring of 1781, for this musician-turned-amateur- astronomer to have been the first to identify a new planet and in so doing double the known size of our solar system!

When and where to find Jupiter

You can see Uranus and Jupiter by hand holding just about any ordinary binoculars. However, for a nicer set-up, mount your binoculars on a camera tripod. This is especially useful - and might show you some of Jupiter's moons as well - when the planets are low in the sky. As Jupiter gets high in the sky, this will become less comfortable, however - in short, a pain in the neck.

Jupiter rises in the east near sunset. That means you have to wait a few hours for it to get high enough to easily see it and Uranus with your binoculars. Hint – if you can mount your binoculars on a tripod, that’s even better. A very good chance they will then show you one to four moons of Jupiter, as well as Uranus. Using binoculars on a tripod is awkward for objects that are high in the sky, but easy for objects that will be low, such as Jupiter at the times mentioned. (Go here for a utility that will tell you the position of Jupiter’s moons at any date or time.)

Jupiter cannot be missed, Just look between east and southeast for what will be, by far, the brightest “star.” Here’s when:

September 1, 2010 – three hours after sunset: Jupiter and Uranus will be 23 degrees above the horizon – a bit more than two fists.

September 17, 2010 – two hours after sunset: Jupiter and Uranus will be about 20 degrees above the horizon  – two fists – and for the next night or two about as close together as they get this month.

September 30, 2010 – two hours after sunset: Jupiter and Uranus will be about 25 degrees above the horizon – about two and a half fists.

Most binoculars will attach to a tripod with a simple "L" bracket. Before getting one, however, check to see if your binoculars have the necessary threaded hole on the brace between the lenses. This is usually hidden by a small cover that is easily removed.

When you locate Jupiter, use your binoculars and the appropriate chart from below to figure out which star is the Georgian Star – Uranus. When you have done so, pause and reflect for a moment. Do you understand why the ancients never spotted Uranus? Can you see it without the binoculars? And if you use a small telescope – think about William and Caroline Herschel and colleagues and how puzzled they would be by this object – star like, yet not a star. Like them, you can easily track the path of Uranus against the background stars on a chart of your own making. Just remember, both planets are moving and Jupiter, being closer to the Sun by far, is moving faster – and don’t forget that you are watching this from Earth, which is moving even faster than either of these “wandering stars.”

View through binoculars of Jupiter and Uranus about three hours after sunset on September 1, 2010, as depicted on a Starry Nights Pro chart.

Printer-friendly version of this chart.

View through binoculars of Jupiter and Uranus about two hours after sunset on September 17, 2010, as depicted on a Starry Nights Pro chart. (This is the period of the closest approach of these two to each other this month.)

Printer-friendly version of this chart.

View through binoculars of Jupiter and Uranus about two hours after sunset on September 30, 2010, as depicted on a Starry Nights Pro chart.

Printer-friendly version of this chart.

Finally, if you would like to track the motions of Jupiter and Uranus over the next month or two, use the chart below and download the “printer-friendly (black on white) version.) Doing this will give you some sense of what it was like for the astronomers in 1781 who were trying to figure out what this new object Herschel had discovered really was!
Download this for a printer-friendly version of the above chart.

Jupiter is of special interest to telescope users this season as it showed up in June with a major belt missing. Even with the smallest telescopes, amateur astronomers are used to seeing two dark belts either side of the equator on Jupiter. But the southern one of these belts went missing while Jupiter was out of sight behind the Sun. There are signs now, however, that it is re-emerging, making each time you view Jupiter in a small telescope, something of an event – will the belt be there, or not?. What’s more, the Great Red Spot is located in the general area where the belt is now missing. The result is the Red Spot is easier to see, but Jupiter rotates very quickly on its axis and to see the Red Spot you need to be looking at the right time. Sky and Telescope has an online calculator you can use to find out when the Red Spot will be well placed for observing. Go here to use that calculator.

The equinox – and Harvest Moon, and Belt of Venus

On or near September 22 will be a fun time to watch a sunset – especially if you have a clear horizon to both east and west. I love to do this on a local point of land that juts southward into the ocean and gives me such a view in both directions. What’s so special about September 22?

1. It’s the fall equinox, which means the Sun will be setting due west and day and night will be of nearly equal length.

2. It’s the night of the full Moon – the Harvest Moon – and that means it will be rising directly opposite the Sun as the sun sets.

3. It’s very close to the time when Jupiter – accompanied by Uranus – will be in opposition to the Sun – meaning they rise in the east as the Sun sets in the west.

4. And finally, it’s a good time to look for some natural phenomena that you have probably seen many times but not realized what you were seeing – the Earth’s shadow and the Belt of Venus.

Here’s a wonderful picture taken by Doug Miller and used some years ago as the Astronomy Picture of the Day. Note the dark band at the horizon – that’s the Earth’s shadow – and the rosy band above it – that’s the Belt of Venus. This reddish sky is caused by the atmosphere reflecting light from the setting Sun on the opposite side of the sky. You will see this in the EAST within the first 20 minutes or so after sunset.

At sunset look EAST and you will see something like what Doug Miller captured here. The dark band near the horizon is the shadow of the Earth, and the rosy band above it is the Belt of Venus. Click image for a larger version.

Now, of course, on the 22nd you will also see the Harvest Moon in this area – or perhaps just above it. It rises about three-quarters of an hour before sunset on that evening. (Technically, the Moon isn’t full until nearly sunrise the next morning. )

This shows where the Moon and Jupiter will be at sunset, but you probably won't be bale to pick Jupiter out for another half hour or more. It will tag along with the Moon as they both rise. (Prepared from Starry Nights Pro screen shot.)

What’s more, at sunset, just below the Moon, a brilliant Jupiter will be rising – though you may have to wait a while to see it because of haze near the horizon. As Jupiter gets higher in the sky – an hour or more after sunset – you might be able to pick out Uranus in the same binocular field of view as explained earlier in this post. (I say “might” because a full moon about five degrees away will wash out the sky making it hard to see Uranus, though Jupiter will stand out since it’s so bright.) If the 22nd isn’t clear, you can observe this phenomena at any sunset, but to see Jupiter rising about this time you’ll want to look within several days either side of this date. (On other nights the Moon will be farther from Uranus and so it should be easier to see it than on the 22nd.)

Jupiter is actually at opposition on September 20 when it will be closer to the Earth – and thus appear larger in our telescopes – than at any time since 1963. But don’t get too excited – closer means a percent or two closer than it was last fall or will be next fall. It will also be extremely bright – magnitude -2.9. That means the only things brighter in the sky at this time will be the Moon and Venus. At magnitude -4.6 Venus will be really dazzling – about as bright as it gets. However, it also will be quite low in the southwest – at sunset just about one fist above the horizon. Half an hour later – when it should be dark enough to see it easily if your horizon is clear, it will be little more than five degrees above the horizon – about the same height the Moon was at sunset.

If you point your binoculars at Venus in the southwest about half an hour after sunset, then you should be able to pick out Mars off to the right and up a bit. It will be about 7 degrees away – which means it may not fit in the same binocular field of view as Venus, but you should only move a little bit to find it. At magnitude 1.5 it will be much dimmer, looking like a little red star. Saturn, which will be visible with a careful binocular search earlier in the month, is now blotted out by the Sun. (If you want to get a final look at it for this season, you will find it below Venus and well to the right – close to due west – during the first week of the month. )

Fleeting Mercury

Fleeting Mercury – it’s not just a cliche – it’s a reality. The fleet-footed messenger of the gods is aptly connected with the fastest-moving planet. Its average orbital velocity is about 107,000 miles an hour as compared with 66,000 miles an hour for Earth, or 29,000 miles an hour for Jupiter. So Mercury can really zip around the Sun, and in doing so our view of it changes rapidly. Note this sequence of events:

September 3 – Mercury behind the Sun and thus out of sight to us.

September 13th – Mercury visible a half hour before sunrise about 8 degrees above the eastern horizon and about 5 degrees below Regulus. Two days before. Mercury was fainter than Regulus. But on the 13th it’s brighter and will continue to brighten.

(Prepared from Starry Nights Pro screen shot.)

Printer-friendly version of this chart.

September 19th – Mercury is as far west of the Sun as it gets this time – and thus easily visible in the eastern morning sky and blazing at magnitude -0.3 – much brighter than magnitude 1.3 Regulus.

After that, it plays a little game with us – on the one hand it continues to get brighter, but on the other hand it also drops nearer to the Sun, so it starts to get lost in the glow of dawn. By the end of September, it is just 6 degrees above the horizon about half an hour before dawn – visible, but no longer easy to see unless your morning horizon is very clear.

Click image for larger version. (Prepared from Starry Nights Pro screens hot.)

September 2010 Calendar

  • 1- Last quarter Moon
  • 1-7 Look for Venus, Spica, Mars, and Saturn in the western sky half an hour after sunset. Binoculars needed for all but Venus.)
  • 8 – New Moon
  • 10- Venus, Mars, Spica, and a 3-day old crescent Moon make a nice gathering very low in the southwest after sunset. You should be able to cover them all with a fist held at arm’s length.
  • 13-30 Time to start looking for Mercury in the morning sky, about an hour to 30 minutes before sunrise.
  • 15 – First quarter Moon
  • 17 – Uranus and Jupiter less than a degree apart for several nights.
  • 22 – Sunset special – Venus and Mars in the west, Jupiter and Uranus in the east, plus a Harvest Moon and a good time to look for the Earth’s Shadow and Belt of Venus. However, Uranus will be difficult to see because the Moon will be so near to it.
  • 23 – Full Moon
  • 30 – Last quarter Moon

The incredible variety of stars – and how much we can learn from simply noticing their color!

Even a candle flame has a lot to say when you dissect it's light. Think of what the stars can tell us!

Even a candle flame has a lot to say when you dissect it's light. Think of what the stars can tell us!

Look at a star. Can you tell it’s color? Don’t be surprised if you can’t. At first all stars just seem white to many people.  Yet amateur astronomers joyfully describe them in exciting hues of red, orange, yellow, and blue.  The truth is, it would probably be more accurate to talk of these “colors” as “taints.”  A red star doesn’t look like a red Christmas tree light. But once you’ve taken a good close look – once you’ve made comparisons between key bright stars, such as Spica and Antares in the summer sky, or Rigel and Betelgeuse in the winter sky – well, you definitely should see that Antares and Betelgeuse are tainted red and Spica and Rigel are tainted an icy blue.

This drawing - details and copyright can be found here - does a better job than anything I've seen of depicting star colors as we see them with our naked eye. Keep this in mind as you learn more about the OBAFGKM classification system.

This drawing does a better job than anything I've seen of depicting star colors as we see them with our naked eye. Keep this in mind as you learn more about the OBAFGKM classification system. For more details and copyright information click image.

And from that single piece of information – the color –  you can make reasonable guesses about a star’s size, temperature, intrinsic brightness, life expectancy – even how rare it is and the way it is going to die. How? By knowing the secret of a wonderful little classification system that uses the sequence of letters OBAFGKM – a system built on more than 150 years of breaking star light into its constituent colors, studying the result, and comparing those results with what we can learn in the laboratory about light and its relationships to various elements.  Oh – and if you don’t mind risking a little political incorrectness, you can remember that sequence of letters by this wonderful little  mnemonic device – Oh Be A Fine Girl (Guy) Kiss Me.

And what all of this tells us about stargazing in general is that what we see when we look at the stars on a typical night are not the ordinary stars, but the extraordinary ones.  That is,  the stars we see with our naked eyes are a sampling of the unusually bright, unusually close – and in some cases – unusually large . With our naked eye we cannot see a single example of the most common type of star – not one.

From a BB shot to a mountain

Color and brightness diffeences can be obvious, but the real differences are only revealed through careful study with sophisticated instruments.

Color and brightness diffeences can be obvious, but the real differences are only revealed through careful study with sophisticated instruments.

And while we certainly notice the differences in brightness of the stars, and we may have now tuned ourselves to detect the subtle tints of color that separate red stars from yellow and orange from blue, there’s little we can discern with our naked eye that prepares us for the incredible variety of stars. Take one measure alone – spatial dimensions. All stars appear to us as a point source of light – they show no disk, except as an artifact of our telescopes. In short – they all look the same size – very, very small.

Yet this sameness is a far cry from reality. In reality stars have an incredible range of sizes. Let’s scale things down to the graspable. We’ll reduce our home planet to the size of a small bead, 2mm in diameter – less than one tenth of an inch. We’ll let that small bead represent the size of a white dwarf star. On that same scale, our star – the Sun – is about 9 inches in diameter. And on that scale the largest stars we know would be almost 2,000 feet in diameter.  Think of it! Stars range from something smaller than a BB shot to something larger than the tallest man-made structure. Then consider this – that BB shot represents something the size of the Earth, a typical size for a white dwarf star – but there’s actually a type of star much smaller than a white dwarf.  It’s called a “neutron star,” and while it may be as massive as our Sun, it’s about as big as a large city! I couldn’t figure out how to include that in our little model without using a powerful microscope.

(Magic interlude: to really get a quick handle on the size of planets and stars, go here (link opens in a new window) – then return 😉

This incredible size range alone should tell you that stars are quite different, one from the other. We first began to notice how different about 150 years ago when scientists combined a prism with a telescope and broke down the light from the stars into its various wavelengths, displaying a spectrum. In such spectra we see not only colors but thousands of dark lines at locations where specific wavelengths of light are absent, and these dark lines hold the key. From them, scientists can deduce chemical composition, temperature, and much more.

The Harvard connection

In the late 19th Century astronomers at Harvard University started to classify the spectrum of stars. There were obvious vast differences, and they applied an alphabetical system to rank stars by the strength of the lines that represented hydrogen.  At first it all made sense and the letters were in alphabetical order, but as their knowledge grew, the letters got scrambled until we finally ended up with a system that runs in this order:


So Astronomy students, in an age when we were less tuned in to sexism, learned the simple mnemonic Oh Be A Fine Girl Kiss Me.  More recently they have messed things up a bit with additional letters, L and T, and within each class there are nine different sub-classes, which are represented by numbers.  But for now let’s stick with the basic letters, as they cover the vast majority of stars.  Learn it anyway you like, but learn it. The order of the spectral letters is one of the few things I find worth memorizing, for it turns out to be an almost ideal classification system covering in a single order several major characteristics of the stars.


The mains pectral classes of stars with the class designation on the left, a represenative spectrum for that class of star, and then the temperature for that spectral class on the right in degrees Kelvin.Clcik on image for information about its source, copyright, etc.

First, these letters represent a temperature sequence. The hottest stars are at the top. and classified as “O” stars. “M” stars are the coolest common stars.

And OBAFGKM represents a mass sequence, with the stars at the top being the most massive.  (Don’t get mass and spatial dimensions confused, however.  A star can have the same mass, yet be as tiny as the Earth, or far, far larger than our Sun. )

But these same letters also represent a color sequence going from blue-white to red, with yellow stars in the middle. Our Sun, for example, is a “G” star – basic yellow.

The sequence also indicates something about life span. The hotter a star is, the shorter its life. “O” stars will live a few million years, furiously exhausting their nuclear fuel. “G” stars such as our Sun, are destined to live billions of years – about 10 billion for the Sun. And “M” stars may go on for trillions – much longer than the universe has existed to date.

And the sequence tells you something about how the stars will die – those near the top, the “O” stars, will go out with a bang, those near the bottom, a whimper.  So temperature, mass, color, life span, and the end game are all related.

They also represent a sort of frequency distribution. “O” stars are one in a million – very rare.  And “M” stars are most common. In fact, about 80 percent of the stellar population is believed to consist of “M” stars, yet we don’t see a single normal “M” star with our naked eye.  (Catch the hedging there?  We do see some abnormal “M” stars. More on that later.)

Know the spectral type, know the star

But the OBAFGKM sequence really hits the jackpot when it comes to dropping stars into convenient little boxes.  If you look up at Vega and note that it is blue/white, then you know it’s nearer the upper end of the list – probably a “B” or “A.” This means it is hotter than the Sun, more massive, and will live a much shorter time. That’s a lot to know just from a crude estimate of its color, but it’s pretty close to what astronomers have learned with more sophisticated techniques. Vega is classified as an “A” star which  has a temperature of 9600K compared to the 5777K of our Sun.  It has half again as much mass as the Sun and is expected to live less than a billion years, compared to the 10 billion for our Sun. In short, know the color – or better yet, the letter assigned to a star and you immediately know something about its color, temperature, mass, life expectancy,  and death,  as well as how common it is or isn’t.

But this really intersting thing come when you make a very simple graph that ranks stars by just two qualities – their actual luminosity  and their temperature/color. “Luminosity” means the actual brightness of a star independent of its distance from us. Color and temperature are indicators of the same thing. The result is what’s known as a Hertzprung-Russell diagram. Here’s an example with each dot representing a star.


What you should notice is the bulk of stars fall into three areas of the charts – the lower left quadrant where oyu find the white dqaefs, the upper right region where you find various giants, and a thick, swirling band that cuts across from one corner to the other. This band hold the most stars by far and it is known as the “main sequence.”

One thing I don’t like about the word “sequence,” however, is people assume that to be “on the main sequence” means a star is sort of sliding down it – that the sequence is a developmental or evolutionary sequence.  In fact, that was what scientists believed at first – that stars started out hot (top left), then got cooler as they aged, eventually ending up in th ebottom right.  Sounds logical. But the main sequence is not a developmental sequence.  Stars don’t start out life at one end and end up at the other. Instead they stay on the main sequence until their final stages of life, at which time they may move off the main sequence into another realm entirely.  Such moves signal dramatic changes in the star’s life, size, behavior, appearance, and life expectancy.

But start witht he idea that when a star starts life it does so at some particular point ont he mains equence and it stays at that point for most of its life. Where stars go as they leave the main sequence and exactly what happens to themt hen is a matter of mass. In fact, mass is probably the single most important characteristic of a star. You would think it would be chemical make up, but notice we haven’t talked much about star chemistry. That’s because for most stars its basically the same – 92 percent hydrogen, 8 percent helium, and everything else crammed into a fraction of one percent, if it’s there at all.  In a few cases, however, chemistry is significantly different and those stars get their own special classifications. Again, we’ll leave the exceptions to another time.

To appreciate the importance of mass you need to understand what a star is. Reaching for a definition that will cover the whole range of stars, James B. Kaler calls stars “self-luminous condensates of the fragmented dusty gases that fill interstellar space.” OK. As you get near the low mass end, stars do get a bit freaky and it’s difficult to fit them into the general picture. But let’s stick with the general picture where it’s easiest to think of a star as a huge ball of gas, expanding under the pressure of the nuclear furnace at its core, and held in together by the opposing inward pressure of its own gravity.

And that’s the key – gravity. There’s another word we could explore forever without getting an ultimate answer.  Gravity is the force that causes stuff to be attracted to other stuff resulting in more stuff. Honest. The interesting thing about gravity is it works on “the more the more.” That is, the more stuff you have, the more attractive the force of gravity, and so the more likely you are to have even more stuff.

In the ISM – the interstellar medium where stars are born – there’s a lot of stuff – mostly gas and dust – that is spread quite thinly. But various disturbances pass through the ISM, sort of priming the gravity pump, encouraging stuff to get together in clumps which then become bigger clumps until you finally have so much stuff in one spot that the inward pressure becomes crushing. At this point a star is born, for what happens is the hydrogen atoms start bumping into one another, and when they do this, they fuse together and form helium atoms.  At the same time, a small fraction of the matter in the hydrogen atoms gets consumed by the process – matter is turned directly into energy by that most wonderful formula E=MC square – and, of course, the amount of energy is tremendous as we found out when we built atomic bombs and later atomic power plants. A little matter goes a very long way, for we’re multiplying the converted mass by the speed of light (186,200) squared, a very big number. (Mind you, I am not a physicists. I try to recount these things as I understand them from my study. But having read a great book that traces the history and development of that equation it still leaves me completely bewildered why the energy produced should have anything to do with the speed of light. )

What’s so elegant about a star undergoing nuclear fusion is the wonderful way it counterbalances the force of gravity. Left on its own, gravity would have just kept squeezing the stuff tighter and tighter. But the very force of gravity causes the nuclear ignition and that creates the outward force to work against the inward force of gravity.

When we see a star we’re not really staring into a nuclear furnace.  The nuclear fusion in the core is a just the starting point for an exchange of energy that takes something in the order of a million years to reach the surface of the star and send out radiation in the form of the light that we see.  But oh my, what a mighty engine!  A star such as our sun “burns”  4 million tons of hydrogen a second, and according to Leon Golub and Jay Pasachoff in “Nearest Star,” the result is that in just one-thousandth of a second the Sun emits enough energy “to provide all of the world’s current energy needs for 5,000 years.”

And we look up there and it’s hard to not hear the echoes from our nursery –  “Twinkle, twinkle, little star.” Twinkle indeed!

OK – back to OBAFGKM.  How hot a star gets – and many other characteristics – depends entirely on how much stuff is gathered together in this ball of gas. The usual measure of this is to compare the amount with how much is in our Sun. Count the Sun as “one” and a typical “O” star is made of somewhere between 10 and 100 times as much stuff, while an “M” star may have just one tenth what our Sun has.

Other boxes for the stars

Without going into tremendous detail – it’s enough to try to swallow the main sequence in a single gulp – there are four other ways that stars are classified.  I want to mention them here because you’re bound to hear these terms, and some of them sound downright crazy – like calling our Sun, which is larger and brighter than most stars, a “dwarf.”

Stars fall into two broad evolutionary categories, with a third one that exists only in theory. Stars we see belong to either Population I or Population II.  Our Sun and most other stars we see are Population I. The oldest stars we see – such as the ones that are found in globular clusters – are Population II stars.  Population III stars were the first stars formed after the Big Bang. No one has yet seen a Population III star – they are a theoretical concept, but generally well accepted as such. A Population III star would be almost entirely hydrogen and helium. Population II stars were formed next – after the Population III stars had exploded and added “metals” to the universe – metals being elements other than hydrogen and helium.  So Population II stars have a more complex chemistry. Population I stars tend to be richer in metals.  They are the stars being formed today.

(Confused? Do you get the feeling astronomers count backwards?  Well stay tuned. )

Where dwarfs aren’t dwarfed by anything but giants

Another classification system looks at the size – the spatial dimension, not mass – of stars, and it would be confusing, if it weren’t so laughable. I’m not quite sure how astronomy ended up in this quandary, but I assume it’s another instance where new discoveries played havoc with the established naming process. Sort of like trying to straighten up a room as the kids are playing in it. Thus we have a size classification system that goes from smallest to largest stars and reads like this:

VII – white dwarfs
VI –  subdwarfs
V –  dwarfs – main sequence
IV –  subgiants
III –  giants
II –  bright giants
I –  supergiants, and yes
0 –  hypergiants

Oh boy! Wonder what happened to “normal?” Well it’s there. Normal is “dwarf.” In fact the main sequence holds about 95 percent of the stars, and you can consider “dwarf” and “main sequence” synonymous.  And that, of course, means our Sun is a “dwarf.”

OK – maybe I’m being too hard onthe astronomers. As mentioned earlier, the difference in the size of stars is mind-boggling. Here’s a good graphic thatc aptures only the top half od these differences – the difference between our “dwarf” Sun and the giants.

Star sizes 2

Remember, stars do not evolve along the main sequence, but they can fall  – or jump – off it. That’s where these classifications come in. Essentially, as a star nears its life’s end, it goes careening off the main sequence in what results in graceful curves when you start plotting temperature and mass on a graph.  The path to the end is complex and varies according to the initial mass of the star. In the later years of their lives, stars can swell up to incredible sizes and become red giants. An example is Betelgeuse, the star that marks the right (eastern) shoulder of Orion. Such a star doesn’t grow in mass, but it expands like a balloon and while its surface is relatively cool – thus the red color – it’s huge and so the star radiates a lot of light despite being relatively cool..

Stars actually can make the climb off the main sequence more than once and the second time they do this, they may turn into slowly pulsating giants, varying their output significantly over long periods. (Long, that is, from our viewpoint.) Such a star is Mira, “the wonderful.” It goes through an 11-month cycle where it reaches a peak making it easily visible to the naked eye, then it falls back to a point where it is so dim that’s it difficult to detect with binoculars. Chi Cygni is another such star, and there are many more.

In these end games, stars may go through a nearly explosive stage where they blow off a huge amount of their substance and create an expanding cloud that we see as a beautiful planetary nebula. The Ring Nebula in Lyra and the Dumbbell Nebula in Vulpecula are examples.

Stars can also undergo an incredible collapse where their core shrinks to the size of the Earth. Such stars still retain a significant amount of their mass, and they are known as “white dwarfs.” In this case the word “dwarf” makes sense, but the word “white” is another name that has been overtaken by new discoveries. White dwarfs can be red, or any other color normally associated with stars.

And, of course, stars can go out with a bang if they start life with enough mass. The result is a supernova that momentarily shines with incredible brightness, then leaves behind a ragged cloud such as we see with M1, the Crab Nebula.

This doesn’t mean, however, that the star was destroyed in the massive explosion.  In fact, although everyone seems to write about the life and “death” of stars – and I’ve fallen into that pattern as well – I’m not at all sure it’s appropriate. Stars don’t die. They go through incredible changes that may make them difficult or impossible for us to see – but some of the stuff of the star is still there and in most cases it still continues to radiate light. In the case of the Crab Nebula, the star that exploded left a significant core in the form of an incredibly compact neutron star – a star where all the atoms have been stripped of their electrons and protons and are crunched together so tightly that they are about the size of a large city.  Such a neutron star spins as it collapses, much as a figure skater does when she pulls in her arms. It also beams radio energy in one direction and this beam sweeps the heavens like a powerful lighthouse. When we’re aligned with such a neutron star, our radio telescopes pick up a regular pulse of energy, many times a second, from the rapidly spinning neutron star. We call this a pulsar, and again, many examples have been discovered.

In the case of our Sun the current betting is that it will “burp” a time or two creating a complex planetary nebula, then retreat to the white dwarf stage – and white dwarfs go out with a whimper, not a bang. But while theys top radiating, there is still star stuff there so I’m not sure if this really represents the detah of a star.

Whatever the evntual fate, however, it all depends on the initial mass. A star like our Sun ends up as a white dwarf. A star 2-to-3 times larger than our Sun ends up as a neutron star. And a star that starts out even bigger, ends up as a black hole.

The bigger they are, the farther they fall!


I know we’ve covered a lot of ground. There’s actually much, much more that could be said about stars.  But the basic message is simple. Most stars fit into a single classification system that tells us at a glance several major things about the star. Learning that sequence of letters – Oh Be A Fine Girl Kiss Me – goes a long way in helping you make some sense  out of those distant,  twinkling,  dots of light.

Here’s a summary of this system in table form


Click table for a larger version.

  • Ninety-five percent of all stars are on the main sequence.  Most of the stars that are not on the main sequence are white dwarfs. Roughly one percent of the stars fall into one of the giant categories.
  • Stars near the top of the main sequence are rare, as are giants of any spectral class, yet when we look at the constellations we are seeing mostly A and B main sequence stars, and a variety of giants, since these are the brighter stars.
  • Notice that stars in the last two categories,  L and T, are either barely visible to us as red stars, or not visible to our eyes at all because they shine only in the infrared. We don’t know how many of these stars there are, but they could be as common as the main sequence “M” stars.
  • The lower limit for the mass of a star is 1/80th the mass of our Sun – or about 13 times the mass of Jupiter.
  • Temperatures are for a star’s surface. The interior is much hotter. The Kelvin temperature scale is the same as the Celsius one, except it starts at absolute zero. This adds 273 degrees to the Celsius scale, a minor consideration when you look at the typical temperature of a star.
  • Age – we can date star clusters by seeing what class of star remains. “O” stars die first, then “B,” etc. No dwarf “K” or “M” star has died yet – the universe isn’t old enough.
  • Understand in all of this we’re dealing with a continuum, so the numbers are just guides.

The stars are not a WYSIWYG world.  What you see is NOT what you get. But what you get is far more exciting, interesting, and elegant than what you see. That said, don’t forget that the goal here is to see, really see – to go out in the night and let the light from these unimaginably distance fires ping your brain, and when it does, mix that experiential knowledge with your abstract knowledge in the hopes of a greater awareness. Good luck!

Simply mind-boggling: Universal Measuring Sticks and Observing Logs

Measuring an 11-foot (meters) strip. (Click image for larger version.)

Measuring an 11-foot (3.4 meters) strip. (Click image for larger version.)

While simple, this project is next to impossible to depict well in a photograph because each “measuring stick” is just a few inches wide and more than 10-feet long. But build one and I bet you’ll find it a mind-bending experience!

I call this project the  Universal Measuring Sticks and Observing Logs and together these “measuring sticks” serve a simple function – they put into perspective the distance of each object you observe. And even if you don’t observe, they’ll help you get a handle on the incredible distances to the planets, stars, and other galaxies.

To do this our basic measuring unit will be the speed of light – 186,200 miles a second (300,000  kilometers a second). That, according to Einstein, is the speed limit for the universe. Nothing can go faster. So we simply ask ourselves how far will light go in a minute? An hour? A year?  Just starting with the distance travelled in a second boggles the mind – the distance that light travels in a single second would take it all the way around the Earth more than 7 times.  That is, it’s 24,902 miles around the Earth at the equator and if you divide that into 186,200 you get (rounded) 7.5 ( 40,076 km divided into 300,000 kmps gives you 7.5 as well). So our most basic unit, the light second, is already far larger than anything most of us have experienced. But at least it gives us a starting point to begin to get even more mind-numbing distances into perspective.

Materials needed for this project:
(2) 11-foot (or 3.3meter) lengths of adding machine tape*
pen or pencil(s)  – different colors helpful, but not necessary
calculator (helpful) or scrap paper

*You  can use any 11-foot strip of paper you have or create – but I found adding machine tape the easiest way to do this and it’s commonly available. Some might want to use four lengths and not use each side – or you might want to use a dozen sheets of ordinary paper. The goal is to make four scales each 10 feet (120-inches) long with little extra paper on each end to keep it neat.

We will actually make four measuring sticks, each a bit over 10 feet ( 3 meters) long. We need four because it is impossible to fit everything on a single scale and still have it readable.

Well, not absolutely impossible.  For example, the first and smallest scale used is for the solar system. On that scale, one inch  equals two light minutes. (If you’re using the metric system, then start with a scale of 25mm equals two light minutes – very nearly the same. ) That scale puts the moon just 1/100th  of an inch (a quarter of a millimeter) from Earth at one end with Neptune (and Pluto) near the other end.  But if we were to include the nearest star we observe in our northern hemisphere skies other than the Sun, it would  require a piece of adding machine tape 36 miles (57.9 kilometers) long! Possible – hardly practical. Oh – and were we to include the nearest galaxy we observe, the Andromeda Galaxy, we would need about 10 million miles (16 million kilometers)  of tape. Sort of defeats the purpose of a scale model! So it is for very practical reasons that we have created four measuring scales.

To use each scale start by putting a vertical line across your tape about 6-inches from the left hand edge. This is your starting point, which is, in all cases, Earth. To the left of this, put down the name of your measuring stick and the scale being used for that stick. To the right of this, calculate the distance to each item you observe, mark it and identify it on the scale. If you like, include the date observed.The result should look something like this measuring stick for the solar system – keep in mind this is just the first part – the whole “stick” goes onf or 10 feet.

This is the starting end of a solar system measuring stick.

This is the starting end of a solar system measuring stick.

General notes that apply to each stick

  • Light travels at 186,282 miles (300,000 km) a second. We use the speed of light as our measuring unit.
  • On the second and third scales in particular you may find that several objects are at, or near, the same distance, so to mark and identify them you will need to use the full width of the paper.
  • Distances up to 1,000 light years are pretty well known now and reasonably accurate because of measurements taken by the Hipparcos* satellite. Distances beyond this get increasingly fuzzy with many different indirect methods to determine them. For this reason you should regard all these distances as reasoned approximations. For close objects (within 1,000 light years) use sources written after the Hipparcos* measurements which were published in 1997.
  • Our distances are also an indicator of time – each distance tells us how long ago the light we see left an object. You may find it fun to mark all but the first scale with historical, evolutionary, and geological events on Earth. Such time references add to your perspective.

You can look up distances and calculate them to scale any time, but the real goal here is to reinforce the observing experience, so if you are observing , I suggest you use this more as a log and  mark your scale either when planning an observing session, or when reflecting on that session after observing. That way the abstract experience of learning and calculating distances is in your mind along with the real-life experience of observing the object.

The Scales – (If you prefer to work in metric, just change “1-inch” to 25mm – it will be close enough for these purposes.)

#1 The Solar System

Scale: 1-inch = 2 light minutes (or for those who want more precision, 120 light seconds*

Minimum distance in light hours, minutes, and seconds,  from the Earth to the moon and planets are:

  • Moon: 00:00:01.2  (that is 1.2 light seconds)
  • Venus 00:02:07  ( 2 minutes, 7 seconds)
  • Mars  00:03:02
  • Mercury  00:04:18
  • Sun 00:08:19
  • Jupiter  00:32:43
  • Saturn 01:06:28  ( 1 hour, 6 minutes, 28 seconds)
  • Uranus  02:23:35
  • Pluto 03:58:07**
  • Neptune 03:59:25

To calculate the distance on your Universal Measuring Stick, simply divide the time in minutes by 2, or the total time converted to seconds, by 120.

Example: Jupiter is 32 minutes, 43 seconds away. In seconds that is (32X60) + 43 or 1,920 seconds plus 43 which is 1,963 seconds. 1963/120 = 16, so Jupiter will be 16 inches away.

*Use 2 light minutes for reasonable approximations, or get more precise with seconds.

** yes, Pluto when closest to us is closer than Neptune when closest to us!

If you’ve made the solar system measuring stick, you should have the basic idea how and find the others easy.

#2 Our Stellar Neighborhood – to 2,600 light years

Scale: 1-inch = 21.6 light years

This scale covers most of what you can see with your naked eye –  as well as many things you can not see with the naked eye because they are too faint, but still fairly close to us. Well, close as astronomical objects go, but incredibly far away when it comes to what we’re used to.

(The entire solar system scale would be so small, it would be impractical to represent it on this scale with anything except the thinnest of lines right at the start.)

We’ll use some of our bright guidepost stars just for starters. Here are their distances in light years:

  • Polaris  430
  • Arcturus  37
  • Spica 262
  • Antares 600
  • Vega  25.3
  • Altair 16.8
  • Deneb 1,400
  • Big Dipper  80*

*This is an approximation covering the main stars of the Dipper which are really part of an open cluster.  With most asterisms the stars would be at various distances.

#3 Our home galaxy, the Milky Way – to 100,000 light years

Scale: 1-inch = 833 light years

(The previous scale would take up little more than the first three inches of this scale.)

This measuring stick takes us to some of the more distant open clusters, typical globular clusters, and some nebulae that are easy to observe with the naked eye, binoculars,or small telescopes.


  • Pleiades M45 440
  • Dumbell Nebula M27  1250
  • Orion Nebula M42  1,300
  • Ring Nebula M57 2,300
  • Open Cluster M37 4,400
  • Globular Cluster M13 25,000

#4 Our observable universe – to 100 million light years

Scale: 1-inch = 833,000 light years

(The entire previous scale would take up about the first one eighth of an inch on this one.)

While the Andromeda galaxy can be detected with the naked eye and observed with ordinary binoculars, most of what we include on this measuring stick takes us to the limit of what we usually observe with a backyard observatory that includes at least a  6-inch telescope. We can reach farther into the universe than this, but with anything past the middle point on this scale you see very little – and most of what you see at these distances justifies the term amateur astronomers usually use for these objects – faint fuzzies!


  • Andromeda Galaxy M31  2.5 million
  • Pair of galaxies beahind the Great Bear’s ears –  M81, M82  12 million
  • Whirlpool Galaxy M51 23 million
  • Leo Triplet Galaxies M65, M66, NGC 3628 35 million

Finally, getting the measure of the universe – here’s a brief tribute to the measurers – ancient and modern . . .

“HIPPARCHUS OF NICEA must have been an interesting fellow. He was a second-century B.C. mathematician, philosopher and astronomer. Using the only astronomical instrument available to him — his eyes — Hipparchus took on the daunting task of measuring the positions of the stars and planets as they passed overhead each night. He came up with a catalog of 1,080 stars, each of which he described simply as “bright” or “small.”
“Hipparchus wasn’t the first astronomer to pursue the science of astrometry, as the astronomical discipline of positional measurement is now called. However, his star catalog was the first of many compiled over the centuries by astronomers using ever-better instruments and techniques. From those measurements — all made from the Earth’s surface — astronomers have derived everything from basic stellar properties to estimates for the age of the universe.
“On August 8, 1989, the science of astrometry took a long-awaited leap to the stars. Riding aboard an Ariane rocket was the High Precision Parallax Collecting Satellite, otherwise known as Hipparcos. For the next three and a half years, Hipparchus’s 20th-century namesake measured the parallaxes and brightnesses of more than a million stars — despite a potentially crippling accident that sorely challenged the project’s architects.”

The above is quoted from this Web site: http://tinyurl.com/yurcq2 Go there for more details.

Prime Time observing for September 2009 – a square, a couch, dancing moons, and more!

Please note: All charts with this post are for observers in mid-nothern latitudes centered on 40° N. If you are 10 or more degrees south or north of that – or if you’re not sure of your latitude – please go here to make your own custom star charts.

Our focus as always is the eastern sky, 45 minutes after sunset, where in September 2009 we’ll find a brilliant Jupiter whose moons play a fascinating game of hide and seek. But our main goal will be to locate and remember  this month’s  two new asterisms the Great (empty) Square and Andromeda’s Couch.

Let’s start with Jupiter, though, because no prime time observer can fail to find Jupiter in the eastern sky starting about half an hour after sunset – there’s simply nothing brighter except the Sun and Moon – well nothing brighter in the eastern early evening sky.  Venus gets brighter than Jupiter, but it never appears in the eastern sky after sunset, though it is in the eastern sky these September mornings an hour or so before sunrise. If you’re one who likes to be up then, be sure to take a look – you can’t miss it!

Though not visible to the naked eye, what’s most fascinating about Jupiter is its four brightest moons. Yes, they look a lot like little stars, even in the telescope, but they are in a rough line with the planet’s mid-section and they continuously change positions around the planet from night to night. In fact these changes can be seen over the course of an hour or so, though at the least you need good binoculars that are held very steady in order to see them. Any small telescope, however, should reveal them easily. For an introduction to observing these four moons see the video and text here. This describes moon events for an extraordinary evening – September 2/3, 2009 – but at some time on many evenings you can observe one or two such events, so even if you miss the events of September 2/3, watching the animation and reading this should help you understand similar events that happen quite often whenever Jupiter is visible.

Of course Jupiter is not going to help you learn the rest of the night sky because like all planets it is constantly changing its position relative to the background stars. But our two bright asterisms for September will help and they are as simple as they come – a square with an arc of three bright stars attached to it.

Click chart for a much larger version.

Click chart for a much larger version.

The first is known as the “Great Square.” I call it the “Great (empty)  Square” because the area inside it is almost completely empty of other naked-eye stars.  The other asterism ties to it like the tail of a kite flying sideways.  It streams off one corner and I think of it as “Andromeda’s Couch.” Of course this is just my memory device – others would simply call this “Andromeda” because that’s the name of the constellation it dominates. I have difficulty seeing the lovely maiden, chained to a rock by looking at these stars and their companions, however. Like most constellations, with Andromeda you need a huge imagination to see the figure these stars represented to the ancients. But knowing Andromeda was a lovely woman who was rescued by Perseus, I like to think of this graceful arc of stars as her couch.  That said, notice three things about it:

1. The bright star at the right – southern – end is also a corner of the Great Square. In fact, it is the brightest star of the four that make up the square, but only by a little.

2. The three stars are pretty equally spaced. Hold your fist at arms length and it should easily fit in the gap between the stars which means there are 10-15 degrees between each star. That’s similar to the spacing between stars inthe Square.

3. There’s another dimmer, but fairly bright star, between the first star ( the one at the corner of the Square)  and the middle one.

And where’s the hero Perseus? he should be nearby, right? Well he’s on his way, rising in the northeast after Cassiopeia, but we’ll leave him for next month when he’s more easily seen.

Looking north

Meanwhile, for those in the northern hemisphere, the bright stars circling Polaris and always visible are well represented this month with the Big Dipper starting to move towards the horizon in the northwest and the “W” of Cassiopeia starting to take the dominant role in the northeast opposite it.

Click chart for a larger image. Northern skies as seen from about 40° N latitude in mid-September..

Click chart for a larger image. Northern skies as seen from about 40° N latitude in mid-September..

Our chart shows the northern celestial pole region about 90 minutes after sunset when skies are about as dark as they get. Will you see all these stars? Depends. First, on how much light pollution there is where you observe. Second, on how well your eyes are dark adapted. You must avoid white light for at least 15 minutes – better still, half an hour – if you wish to see the fainter stars. If you want to test how good your skies and night vision are, look at the Little Dipper. In light-polluted suburbs you will probably see just the three brightest stars. In good rural conditions you should see all seven.  And if you can see them, then this is a good opportunity to try to trace out Draco, one of a handful of constellations whose connect-the-dots pattern actually suggests the mythological figure of a dragon.  I love Draco, but quite honestly, I have to look for it – it doesn’t jump out at me the way the Big Dipper and the “W” do.  And as far as learning the sky – well, you learn the “W,” the Big Dipper and Polaris so you can then find stuff like Draco when you want to find it.

The arrows on the chart indicate the general direction in which the sky appears to move. Stay out an hour and this motion should become obvious to you.

. . . and the rest of the guideposts?

If you’ve located the new September asterisms then it’s time to check for the more familiar ones you might already know, assuming you have been studying the sky month by month.  (If this is your first month, you can skip this section. ) So here are the guidepost stars and asterisms still visible in our September skies.

  • The Summer Triangle is now high overhead, though still favoring the east. Vega, its brightest member, reaches its highest point about an hour after sunset and moves into the western sky. Altair and Deneb are still a bit east, but will cross the meridian within about three hours of sunset.
  • The “Teapot,” marking the area of the Milky Way approaching the center of our galaxy, is due south about an hour after sunset. Well into the southwest you’ll find the red star Antares that marks the heart of the Scorpion.
  • Arcturus (remember, follow the arc of the Big Dipper’s handle to Arcturus) is due west and about 25 degrees above the horizon as twilight ends.
  • The Keystone of Hercules and the circlet that marks the Northern Crown can both be found high in the western sky by tracing a line between Vega and Arcturus.

. . . our journey and September’s planets (2009)

In the course of a night you can still get a glimpse at all the planets – technically – but the truth is both Saturn and Mercury are very difficult to see this month, and Pluto is always just a faint speck visible in large amateur telescopes. Jupiter, as we’ve noted, dominates the evening sky in the southeast. Nearby – visible in binoculars or small telescopes – is Neptune. And an hour or so later, if you want to track it down with binoculars, Uranus will make a good test of your star-hopping skills.  In the morning sky both Mars and Venus are prominent, though Venus gets closer to the Sun throughout the month. At the start of the month Venus rises about three hours before the Sun – by the end of the month this is cut to about two hours – but even in twilight it is so bright it’s hard to miss.

Charts to help you find the  planets follow, but first, let’s look at the solar system from the perspective of someone in a spaceship hovering above it. This shows us where we are in our journey around the Sun and also gives us a chance to examine where the other planets are in relation to us. See if you can translate this perspective into what we see in our sky. The chart below was created with the Solar System Live capability found here. I added the arrows in Photoshop Elements simply to indicate the horizon and directions relating to the earth’s rotation on its axis.

Click image for larger view. Arrows indicate the western and eastern horizons at sunset on September 15, 2009. Smaller arrows show the direction these horizons move at the earth turns on its axis in the course of the night.

Click image for larger view. Arrows indicate the western and eastern horizons at sunset on September 15, 2009. Smaller arrows show the direction these horizons move as the earth turns on its axis in the course of the night. (Planets are not drawn to scale.)

Looking at the horizon line going out to the west – left – you can see that at sunset Saturn is nearly on the horizon.  Use the arrow going to the east (right) and you can see Uranus isn’t quite visible in our night sky at sunset, but Jupiter, Neptune, and Pluto are well beyond the eastern horizon.  Draw an imaginary line from Earth through Jupiter and you’ll see it comes near Neptune – which is why Neptune appears relatively close to Jupiter in our night sky this month, though you’ll need binoculars to find it. (Notice also that Neptune, while a giant planet, is more than twice the distance from the Sun as Jupiter – which is why it is so dim and small in our night sky while Jupiter is bright – and in a telescope – quite large. As these horizon lines rotate,  Saturn sets, followed by Mercury and  then several hours later Pluto and eventually Jupiter. Meanwhile, Uranus rises in the east, followed in the morning hours by Mars and Venus.  Notice also that Pluto is just a tad beyond Neptune these days, though the distance between them will slowly increase.  The chart does show, however, that for a while Neptune was our most distant planet. See how Pluto’s orbit was inside that of Neptune? Don’t forget, Pluto takes 248 earth years to get around the Sun once. These events hold generally true no matter where you are in the world, but they need to be fine tuned for your latitude. Folks in the southern hemisphere, for example, get a much better view of Saturn and Mercury early in the month, than those in the north.

Finding Uranus

Uranus can be found with binoculars – or in exceptional conditions the naked eye – but locating it is an advanced project for those already comfortable with finding the naked eye bright stars and asterisms. You need full darkness, your eyes should be dark adapted, and you should be in an area where light pollution isn’t a serious problem.  That said, finding this planet is relatively easy if you have a decent pair of binoculars and patience.  Here’s a chart to use. After reading the directions below, click on the chart to get a larger version.

This Uranus finder chart is meant to be used firstw ith the naked eye, then binoculars. The red circle represents the typical view with wide field 7X or 8X binoculars. See text for instructions. Click on chart for larger view.

This Uranus finder chart is for September, 2009, about two hours after sunset. It is meant to be used first with the naked eye, then binoculars. The red circle represents the typical view with wide field 7X or 8X binoculars. Included on this chart are many faint stars that can be seen only with binoculars. See text for instructions. Click on chart for larger view. (Made from Starry Nights Pro with modifications.)

Start your search by locating Jupiter and the Great Square. You may also see Fomalhaut, a first magnitude guidepost star that will be introduced in October.

Next look below the Great Square for the “Circlet.” This is a well-known asterism in the constellation Pisces – but in typical suburban skies it is a difficult object and you may be able to pick out just three of the brightest stars in it with your naked eye. In rural skies you should be able to see most of these stars with the naked eye, but try to locate them with binoculars. The entire Circlet probably will not fit in a single binocular field of view, but enough of it should so you know what you are seeing.

Now use your binoculars to try to locate the trapezoid of fainter stars below the Circlet. This little unnamed trapezoid will probably fit in your binocular field of view. The faintest star of these four is just a bit brighter than Uranus, so that gives you an idea of what you seek.

Finally, with your binoculars scan up and to the right (west)  of this trapezoid and you should pick up an arc of three stars all about the same brightness. The third – the highest – of these is Uranus. While you won’t see a disc, you may notice that it shines with a steadier light than the other two. This is typical of planets. In a good telescope Uranus will show a tiny disc and perhaps a greenish tinge, but to the casual observe may be easily mistaken for a star.

Finding Neptune

Neptune is both easier and harder to find than Uranus. Again, binoculars and a dark sky are needed. What makes it easier is it’s near Jupiter. What makes it harder, is it’s signifcantly fainter than Uranus – so faint that whether you see it or not will depend on how dark your skies are.  You will need this little finder chart, however, to pick it out of the starry background.

Finding Neptune requires binoculars, or a small telescope, and patience. Fortunately, Jupiter drops us right in the neighborhood! See text for complete directions - and click on chart to get an elarged version. (Made with screen shot from Starry Nights Pro. I added names and arrow.)

Finding Neptune requires binoculars, or a small telescope, and patience. Fortunately, Jupiter drops us right in the neighborhood! See text for complete directions - and click on chart to get an enlarged version. This charts is for mid-September, 2009, about two hours after sunset. Neptune will appear to move slightly towards Jupiter during the course of the month. (Made with screen shot from Starry Nights Pro. I added names and arrow.)

Step 1 – find Jupiter, the brightest “star” in the eastern sky. The red circle represents a widefield binocular view. Your binoculars may show a smaller field. Also see if you can spot the two bright stars in our chart that are to the left – east – of Jupiter. They are bright enough so you should be able to see them even in typical suburban skies. In any case, you certainly should be able to find them with binoculars by first locating Jupiter, then scanning to the left – eastward.

Step 2 – after locating the two bright stars, use binoculars to look for the arc of three dimmer stars above them. These three are about the same brightness as Uranus and just at the edge of naked eye visibility under excellent, dark skies. For most people this means they will be seen only in binoculars. Neptune is to their left – east – as indicated.

And for early risers – Venus and Mars!

A crescent moon and Venus dominate the morning sky in the east, along with Mars and half a dozen bright "winter" guidepost stars. Click chart for larger image. SLightly modified screen shot from Starry Nights Pro.

A crescent moon and Venus dominate the morning sky in the east, along with Mars and half a dozen bright "winter" guidepost stars. Click chart for larger image. SLightly modified screen shot from Starry Nights Pro.

Don’t miss the autumnal equinox!

OK – if you’re in the southern hemisphere, this marks the start of spring. In the northern hemisphere, it’s autumn. In either case, it’s when the Sun crosses the celestial equator and day and night are almost of equal length.

The autumnal equinox this year is on September 22, 2009, at  21:28 Universal Time.

So what? Well, if you’re just starting out in star gazing, this is a great time to get your bearings at your observing site. That is, on or about September 22 – a few days either way won’t matter much – note where the Sun either rises or sets. That marks the due east – or due west – point on your horizon and from that you can easily figure out where north and south are.

It’s also the day on which the reading of your equatorial sundial switches from one plate to the other. That is, in the north you go from the north-facing dial plate to the south-facing (underneath) one.  See our equatorial wrist dial project. if you want to know more about this.

And finally, I find it cool that day and night are nearly of equal length. For one thing, that means the stars get a break. For the next six months here in the north we’ll have longer nights and thus more time to enjoy the night sky.

. . . and Saturn’s rings vanish too!

Yes they do – and they do it this week (September 4, 2009) , and it’s fascinating, too – but not nearly so easy to observe as Jupiter and its moons – unless you’re int he southern hemisohere.

I had been so wrapped up in the vanishing  act of Jupiter’s moons this week I had forgotten that this is also the week that Saturn’s rings vanish as well! Then today Spaceweather.com published a wonderful little video – not a simulation, but  a real picture animation made by an amateur – showing our changing perspective of the ring system over a period of several years. You can see it here. Spaceweather.com introduces it this way:

On Sept. 4, 2009, Saturn will turn its rings edge-on to Earth, and for the first time in 14 years they will seem to disappear. “To mark the occasion I’ve made an animation combining six years of Saturn observations,” says New York amateur astronomer Alan Friedman. “It shows the changing plane of the ring system as viewed from my Buffalo backyard from 2004 to 2009

Now this really isn’t as much of a shocker to me as the Jupiter’s moon act  simply because the rings have been slowly getting more and more difficult to see and were actually edge on for a period in August  – and because while Saturn is still visible, it’s very, very difficult to see – and dangerous – because it is so close to the Sun.  At sunset on September 4 it will be just 4 degrees from the western horizon and will set 25 minutes after the Sun for my location at 42-degrees north.  That means you need a sparkling clear western horizon, rare indeed, to see it.  Yes,  sophisticated telescope users will tell you that you can see planets in daylight and you can. But when something is this close to the  Sun that’s far too close for comfort and safety as far as I’m concerned.  Make a slight mistake and you get an unfiltered glimpse of the Sun and that will damage your eyes for sure, if not  blind you. For what? To see Saturn without it’s rings? Nope. Not for me. Though it is rare enough – it won’t happen again for 16 years – it just doesn’t have the same appeal to me as Jupiter’s moons.

The long term phenomena is interesting – the animation put together of it is terrific and instructive. Enjoy this event that way. Here’s the link once more. But live viewing? Not worth the try this time from my perspective unless you live in the southern hemisphere. From Sydney, Australia, for example, Saturn will be a respectable – though challenging – 10 degrees above the western horizon at sunset. Thirty minutes after sunset Saturn will still be about 4 degrees above the horizon, beneath a slightly brighter Mercury which will be about 17 degrees high.  I love Saturn – and I love it for its rings. But I’ll wait to greet it again until at least October, but more likely November. Then it will be solidly above the horizon an hour or more before dawn – and yes, it’s rings will be visible then, as once more we’re at an angle to the huge planet so we can see them. Meanwhile, I’ll let Jupiter play its role as the King of Planets, well placed high in the early evening sky and I’ll continue to enjoy the dance of its moons.

Jupiter’s moons pull a vanishing act – and other neat dance moves!

(For an observing report on this event, go here.)

Jupiter’s four bright moons always put on a great show – ask Galileo – changing positions slowly in the course of the night. But for viewers in North America the night of September 2-3 offered an unusual opportunity because the four bright moons all played hide-and-seek at once – something that happens  about 20 times a century and I suspect much less for any given geographic area. That said, the following video and text I believe remainr elevant andhelpful for anyone interested in watching Jupiter’s moons. The  video is an animated simulation depicting events of one night. It was created in Starry Nights Pro astronomy software and time has been speeded up so  that what you see in about four minutes is what actually takes place in about seven and a half hours.

(For a complete explanation of what’s going on in this animated simulation – and what you should see September 2-3, 2009 – you can take this shortcut  to  the sequence of events below.)

Here’s why I love watching  Jupiter’s four brightest moons:

  • You can see them with any small telescope – even binoculars if you can hold them real steady.
  • They do something! Most astronomical objects don’t change much over our lifetimes. Jupiter’s moons, as you can see from the video simulation, go through significant changes in a single night.
  • These four bright moons played a major role in changing our view of the universe.
  • They even helped us determine the speed of light a couple hundred years ago, something next to impossible to determine on Earth without modern, sophisticated instruments.
  • For the telescope user they:
  1. duck in and out of Jupiter’s shadow (eclipse)
  2. hide behind the planet and suddenly pop out (occultation)
  3. cross in front of the planet providing a challenge for telescope users to spot them  (transit)
  4. and from time to time they cast their shadows on the giant planet – shadows visible in a backyard telescope as pefect round circles
  • Hubble and modern spacecraft have shown us that Jupiter’s moons are full of surprises.  No two are alike and all four are different than what scientists imagined before the spacecraft got out there and gave us an up close and personal view.
  • All of which is incredibly awesome when you understand that the little lights you see moving with grace, precision, and predictability are complete worlds in themselves the size of our moon or larger. (Ganymede is about 1.5x the diameter of our moon.) Newton is playing the tune, and the moons do the dance – music of the spheres indeed!


You can enjoy Jupiter’s bright moons any night the planet is visible. What’s special about the night of September 2 and 3 of 2009 is they’re going through their complete routine in one night and for a couple hours all four of them are out of sight for all practical purposes, but still providing an interesting challenge for the telescope user. What’s more, Jupiter is very close to the moon on this particular night  and very bright in the southeastern sky,  so it is easy to find. And any small telescope will reveal the moons. Seeing them with binoculars is possible, but it takes sharp eyes and a steady hand. I’ve never been able to see them with binoculars unless I can steady the binoculars on a tripod, or against the corner of a building or some other support.

The moons were discovered by Galileo 400 years ago and he didn’t waste any time writing about his findings in his “Starry Messenger.” What he had to say shook up the religious/philosophical/scientific establishment of the day. Although Copernicus had argued otherwise more than 50 years before, the common belief remained that the Earth was the center of the universe and everything revolved around the Earth. But a few nights of observing Jupiter’s moons and that whole business of us being at the center of everything went out the window. Obviously Jupiter was at the center of its own little system and these moons were revolving around it, not us.

The  13-day-old Moon and Jupiter dominate the eastern sky near the horizon for North American observers on September 2, 2009. This screen shot from Starry Nights software captures the positions, but don't get the idea that Jupiter looks thatbig - the size represents its brightness - and its is fa rbrighter than any of the stars, though to the naked eye it will look like a star.

The 13-day-old Moon and Jupiter dominate the eastern sky near the horizon for North American observers on September 2, 2009. This screen shot from Starry Nights software captures the positions, but don't get the idea that Jupiter looks that big - the size represents its brightness - and its is far brighter than any of the stars, though to the naked eye it will look like a star.

Now, with any small telescope, you can travel in the footsteps of Galileo, observe Jupiter’s moons, and make your own drawings. This month (September, 2009) is a good time to start because Jupiter appears in our eastern sky as the Sun is setting in our western sky. By about half an hour after sunset Jupiter will put in an appearance. The only thing brighter than it in the eastern evening sky this month is the moon. And on the evening of September 2 folks in North America will see a nearly full moon pretty close to Jupiter – exactly how close depends on where you are and when you look. On the East Coast at 8 pm EDT the moon will be less than 3 degrees from Jupiter in the southeast and pretty close to the horizon. About 7 hours later it will be in the southwest and the moon will still be with it, but the separation will have increased to about five degrees. (Typical binoculars have a field of view of about 7 degrees, so you should easily see both the moon and Jupiter in the same binocular field.  The question – which I honestly can’t answer – is how easy will it be to see Jupiter’s moons with our own moon shining so close to it? Will the moon drown them out? I’m pretty  confident this won’t be a problem for telescope users. It may make it  difficult for binoculars users since the moons of Jupiter are roughly as bright as the faintest stars we can see with our naked eye.)

What should you look for an when?

First, if you want to know which of Jupiter’s moons is where on any given night, use this neat little online utility provided by Sky and Telescope magazine.

Now the basics. Jupiter has 63 moons, but only four of them are easily seen in small telescopes. Here are their names – in order moving outward from the planet – and links to more details about each.

Sequence of events September 2-3

Here is the schedule of events in  EDT, with  24-hour format Universal Time in parenthesis. (Data is from a listing in the September Sky and Telescope.)  If clouds, sleep, or work cheat you out of a live view, you still might refer to the following list as you watch the animation.)

September 2

7:19 pm (23:19)Callisto is occulted – goes behind the planet. (This will be in daylight for US observers, but when you start observing Jupiter later,  know that Callisto is already behind it.)

11:43 pm (3:43 Sept. 3) Io is occulted – goes behind the planet. (This is fun to watch – how long does it take Io to vanish? )

11:58 pm  (3:58 Sept. 3)Europa transit begins on the opposite side of the planet from where you saw Io vanish a few minutes before. Seeing the moon against the bright disc of the planet is possible in small telescopes, but varies in difficulty depending on exactly what part of the planet is behind the moon, some parts being darker in hue than others. But keep in mind, these moons are so far away that even in a large, backyard telescope they barely show a disc under ideal conditions. So what you are look for is a point of light not much different than a star.

September 3

12:43 am (4:43)Ganymede follows Europa onto the planet’s disc. Is it easier to see than Europa? It’s significantly larger, so might be a tad easier, but again, it takes a large telescope to show the moons as even a small disc.

12:56 am (4:56)Europa’s shadow follows it onto the face of Jupiter – but it may be easier to see as it gets nearer the center. It also may help you pick up the disc of Ganymede – the shadow of Europa during the first half of its journey is very close to the disc of Ganymede – at least as shown by the Starry Nights Pro simulation.

2:29 am (6:29)Io pops back into view – but look where it is! It’s not close to the planet when it does this because after it was hidden by the disc (an occulation) it went into the planet’s shadow – technically, an eclipse. So what you see is it emerging from the shadow, already some distance out from the planet.

2:42 am (6:42) Ganymede’s shadow enters the disc.  Notice how far behind Ganymede it is? Europa’s shadow was much closer to it. Why? Because Europa is much closer to the planet then Ganymede and here’s proof!

2:49 am (6:49) Europa emerges from the disc – but if you have not been able to follow it when it was on the disc it may be difficult to pickup for few minutes because it will still be close to Jupiter and lost in its glare – at least to the smallest telescopes and binoculars. However, Europa’s shadow will still be visible on the disc for almost another hour.

3:47 am (7:47) Europa’s shadow goes off the disc.

At this point East Coast observers are seeing a Jupiter that is very close to the southwestern horizon and the moons will be difficult to observe. For my location – at 71 degrees longitude (Westport, MA). the giant planet sets about 4:30 am.

4:20 am  (8:20)Ganymede’s transit ends, but it’s shadow is still on the planet.

4:35 am (8:35) – At last! Here comes Callisto! For me it has been out of sight all night. It goes behind the planet before it is dark enough to see it and doesn’t come out until after Jupiter has set. However, folks farther west should certainly see this exit event.

6:21 am (10:21)Ganymede’s shadow exits the planet’s disc. Hey – that’s all folks – for tonight. But there are many other nights when there are interesting events.

Be sure to check Sky and Telescope Javascript utility.  Any night Jupiter is well placed for observing I always check this little utility to see if there are any neat events coming up at a convenient time. Nights like September 2-3, 2009 are rare. But with four moons there are frequent times when one or the other is doing something interesting.

How to time a light beam!

Oh – and about determining the speed of light. Think about it.  I believe Galileo once took a stab at this by stationing observers facing one another from different mountain peaks. They then uncovered a lantern at a predetermined time.  No luck. Light is much too fast for this kind of experiment. Hey – light could go completely around the Earth more than seven times in a second! But here’s how Jupiter’s moon helped determine the speed of light more than 300 years ago! These kinds of discoveries always leave me in awe at how brilliant the discoverer’s were and how precisely they were able to make observations with tools that were not nearly as good as the inexpensive telescopes available to anyone today.  The  account which follows can be read in full here. It is from a posting by Michael Fowler of the University of Virginia Physics Department. One more thing to appreciate as you watch Jupiter’s moons.

The first real measurement of the speed of light came about half a century later, in 1676, by a Danish astronomer, Ole Römer, working at the Paris Observatory. He had made a systematic study of Io, one of the moons of Jupiter, which was eclipsed by Jupiter at regular intervals, as Io went around Jupiter in a circular orbit at a steady rate. Actually, Römer found, for several months the eclipses lagged more and more behind the expected time, but then they began to pick up again. In September 1676, he correctly predicted that an eclipse on November 9 would be 10 minutes behind schedule. This was indeed the case, to the surprise of his skeptical colleagues at the Royal Observatory in Paris. Two weeks later, he told them what was happening: as the Earth and Jupiter moved in their orbits, the distance between them varied. The light from Io (actually reflected sunlight, of course) took time to reach the earth, and took the longest time when the earth was furthest away. When the Earth was furthest from Jupiter, there was an extra distance for light to travel equal to the diameter of the Earth’s orbit compared with the point of closest approach. The observed eclipses were furthest behind the predicted times when the earth was furthest from Jupiter.

From his observations, Römer concluded that light took about twenty-two minutes to cross the earth’s orbit. This was something of an overestimate, and a few years later Newton wrote in the Principia (Book I, section XIV): “For it is now certain from the phenomena of Jupiter’s satellites, confirmed by the observations of different astronomers, that light is propagated in succession (note: I think this means at finite speed) and requires about seven or eight minutes to travel from the sun to the earth.” This is essentially the correct value.

Less successful was an idea they had much later that they could solve the problem of finding one’s longitude – while at sea – by observing the moons of Jupiter. This was very seriously pursued  because knowing longitude is critical to navigation and the accepted method involved using a very precise clock that kept correct time throughout a long sea voyage. It was hard enough to make a very precise clock – but one that retained its precision when subjected to the knocking about and unavoidable moisture that was part of any long voyage by sail? Nearly impossible. (See the wonderful book, “Longitude,” by Dava Sobel, for the story of how they did solve this.)  But that said, as you watch the moons of Jupiter with a small telescope, try to imagine doing this on the heaving deck of a sailing ship with a telescope that is significantly cruder than the one you buy today! Then imagine that your life may depend on the result! Makes you appreciate GPS if nothing else 😉

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