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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.

In September 2012 – let Venus be your guide to the Beehive, crescent Moon, and Zodaical Light

Once again, Venus steals the show in the morning sky this September, while Mars and Saturn dance low in the southwest in the early evening. Jupiter crosses over into the evening sky, but just barely – it is still better seen during the early morning hours. In my book, the most fascinating and attractive naked eye challenge of the month will be seeing Venus  in the midst of the Zodaical Light – those minute solar system dust particles that in their own special way and time can mimic the display of the Milky Way.

Check out this wonderful photo of the Zodaical Light – and keep in mind that it was taken through the thin air and superbly dark skies of a mountain observatory and benefits from the camera’s ability to do a better job of capturing faint displays than our eyes. Still, it gives us a good idea of the shape and size of what we are looking for when we seek this elusive glow in our skies.

Zodiacal Light Seen from Paranal, European Southern Observatory.

Now I know the predawn hours are not for everyone, so let’s deal first with the continuing show in the southwest where Mars and  Saturn are still visible low in the sky shortly after sunset – and they still team up with Spica to make an interesting combination. What’s more interesting, however, is as the month goes on Saturn and Spica head  for the horizon pretty quickly while Mars will hold its own for the next several months, hanging out near the horizon and letting the background stars slide behind it.

On September 1, 2012 Mars, Saturn, and Spica will be near the horizon – but visible – 45 minutes after sunset in the west-southwest.

Prepared from Starry Nights Pro screen shot.

For the rests of the month Mars will remain at roughly the same altitude – betwen 11 and 13 degrees above the horizon about 45 minutes after sunset. However, Saturn and Spica swiftly fall out of sight. By mid-month Spica is a mere two degrees above the horizon and Staurn – barely visible – at about 7 degrees high. (Using binoculars will help locate it.)  By the end of September Spica has set at this point (45 minutes after sunset) and Saturn is a mere two degrees above the horizon – most likely too difficult to find.  But Mars’ rapid orbital motion carries it eastward as seen against the background stars which all appear to be moving westward – towards the horizon – because of the orbital motion of Earth.

Venus and the Beehive

Prepared from Starry Nights Pro screen shot.

From about September 9-18, 2012  you can watch Venus pass close enough to the Beehive (M44) star cluster for both to appear in the same low power binocular view. The most interesting view will come on the morning  of September 12 when they are joined by  a crescent Moon. Center Venus in your binoculars, the put it in the right side of the field of view and you should be able to see the Beehive. Put it in the left side of the field of view and you’ll see the Moon.

Oh – and on October 3, 2012 Venus will have an incredibly close visit with first magnitude guidepost star, Regulus. In fact they will be so close for those in mid-northern latitudes that  I doubt you’ll be able to separate them with the naked eye, though they should make a nice binocular – or telescopic – double! They should be about 8-minutes of arc apart – which means they’re closer together than Mizar and Alcor, the famous test of eye sight in the handle of the Big Dipper. I think you will separate them with binoculars, but the large difference in magnitude – Venus is -4.1 and Regulus about 1.3 = could make this a serious challenge. I should add, however, that Venus is moving quickly and  exactly how far apart the two  appear on this particular morning will depend on your location. If I move to the West Coast i get a larger separation. 

Basking in the Zodiacal Light

Prepared from Starry Nights Pro ecreen shot.

The second half of September 2012 will be a good time to start looking for that most elusive of Solar System sights, the Zodiacal Light – and Venus will help!  You actually have a brief window when it’s visible starting about two hours before sunrise and going to about  80 minutes before sunrise. After that the twilight will drown it out. Draw a line between first magnitude Regulus – near the horizon – and Venus. This line will tilt to the right (at least from mid-northern latitudes) and the Zdaical Light will be located along it since that is pretty much the line the ecliptic takes and the ecliptic marks the plane of our Solar System.  The ecliptic marks the general area where you are going to find most Solar System bodies – planets, moons, asteroids – and yes, tiny specks of dust that make up the Zodaical Light.

You don’t need a totally clear horizon to see the zodiacal light, or binoculars, but you do need total darkness and that means little-to-no light pollution and no  – or very little – Moon. In Septmeber 2012 the last two weeks fit the bill – from about Sept. 14-28. I feel I have a good shot at it from my favorite ocean-front observing point where I have a clear horizon to the east with no cities to create light domes there. Mornings in September and October –  or evenings in February and March – are the best times for folks at mid-northern latitudes to look for the Zodaical Light.

The Zodiacal Light is roughly the same intensity as the Milky Way, so if you can see the Milky Way from your chosen location, then you should be able to pick up this faint glow. Like the Milky Way, it stretches over a good deal of sky. It is widest near the horizon and gets narrower as it rises towards the zenith. You want to look for this starting 120 minutes before sunrise, but I advise you also allow at least 15 minutes to half an hour for your eyes to dark adapt. (For projects like this I frequently keep a red flashlight near my bed and use it to preserve my night vision when I get up.)  If you try to look for this later, you may confuse it with twilight. What we are seeing is sunlight reflecting off interplanetary dust particles – dust particles that orbit in the same plane as the planets – the area we call the zodiac – and thus the name for this phenomena, Zodiacal Light.

How high? How bright? How wide? All this depends on your conditions – and even in an area where theere is little or no light pollution, it will vary. All nights are not equally transparent.  But you do want to avoid the Moon and it will come back into the equation by the 28th – but don’t worry. If you miss it this month, October and November are also good months to see it. And if you wish to see it in the evening sky, March and April are good.

If you see it, reflect on this explanation from Wikipedia:

The material producing the Zodiacal Light is located in a lens-shaped volume of space centered on the Sun and extending well out beyond the orbit of Earth. This material is known as the interplanetary dust cloud. Since most of the material is located near the plane of the Solar System, the zodiacal light is seen along the ecliptic. The amount of material needed to produce the observed zodiacal light is amazingly small. If it were in the form of 1 mm particles, each with the same albedo (reflecting power) as Earth’s Moon, each particle would be 8 km from its neighbors.

For the metrically-challenged (that includes me) that means one dust particle every five miles! And each particle is tinier than a bird shot – way tinnier than a BB. And that causes all that light?! Awesome!

Look east In September 2012 – 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.)

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 gaps 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 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 the poles and 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.

As you move upward from this zero line in the general direction of the Summer triangle, the hours count backwards counting the Zero Hour as 24. Move downward, towards the horizon and the hours count forward from zero. This sequence is marked on our chart around Polaris.

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 2011. 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 another 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.

Look North in September 2012 – 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. With them we can figure out how far away a Cepheid variable star is based on how bright it appears to be.

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 parallax 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 really 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.

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