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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.

Look east In September 2011 – 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 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 2011 – 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 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.

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.

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