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.)
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.
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.)
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.)
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.
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.
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.)
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.
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.
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