As we travel September skies we’ll move from the age of mythology to the age of science.
First, the age of mythology. Had you been born a few hundred – or even a few thousand – years ago, the eastern sky in September shortly after sunset would look something like this to your imaginative eye.
The tale is easy to remember. The king (Cepheus) and queen (Cassiopeia) felt their kingdom was threatened by a sea monster, so as a sacrifice to the monster they tied their daughter, Andromeda, to a coastal rock. But don’t worry, our hero Perseus, fresh from slaying Medusa, appears to rescue Andromeda, and they ride off across the starry heavens on his faithful steed, Pegasus, the flying horse. Really – today the king and queen would be tried for child abuse!
Of course as usual with the ancient constellations, the figures bear only the crudest relationship to the pattern of bright stars, so a lot of imagination is required to see them. But that said, I do find this myth an easy way to remember these five constellations. In modern times we’ve drawn complex boundaries around each constellation and used these imaginary celestial boundaries to name and locate stars. But more importantly, we’ve developed a celestial coordinates system much along the lines of Earthly longitude and latitude.
If you imagine the Earth’s latitude lines projected onto the dome of the sky, they become circles indicating declination – how far in degrees a point is from the celestial equator. The celestial equator itself is a projection onto the sky dome of Earth’s equator. Longitude is projected and marked in 24 hours of “right ascension” so the whole celestial clock appears to pass overhead in the course of a day. I found it difficult remembering where these hours begin until I learned about the “Three Guides.” These are three bright stars – indicated by arrows in the chart below – that fall very close to the zero hour line of right ascension. Above them (think of these as preceding them, for they rise first) the hours count backward from 24. Below them – think of these as following them, since they rise afterwards – are the hours counting up from 0.
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 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.”
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. 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 – which is the “celetsial meridian” as defined in the equatorial coordinate system.
As mentioned, the equatorial coordinate system 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” – the one which rises first and leads the rest. (Remember – all stars appear to move westward as the earth turns.) 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.
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.