Thursday, November 29, 2007

The Great Square of Pegasus

Once upon a time in ancient Greece, a handsome, high-spirited young man named Bellerophon, son of the sea god Poseidon, coveted a marvelous horse named Pegasus. Pegasus was a winged horse who had sprung from the blood of Medusa after the hero Perseus killed her. Medusa, you may recall, was a winged monster with a scale-covered body and hair of live snakes, so dreadful that anyone who looked upon her was instantly turned to stone.

The Pegasus Vase by the British Museum

With a golden bridle given him by the goddess Athena, Bellerophon tamed Pegasus. Together they soared through many thrilling adventures and feats of derring-do. Eventually hubris got the best of Bellerophon, when he tried to fly up to Olympus, the realm of the gods, to hobnob with the immortals. Pegasus knew better and threw his rider back down to earth where he wandered alone, abandoned and despised by the gods, until his death. Pegasus, however, was given shelter in the heavenly stable of the supreme ruler Zeus, where he served his master by bringing him thunder and lightning whenever “Father Sky” got the urge to wield his awful thunderbolt.

Pegasus the Winged Horse is a defining constellation of the autumn sky. When seasoned summer stargazers see the winged steed’s nose nuzzling the eastern horizon, they know that Pegasus is rising and autumn’s shiver is not far behind.

Amateur astronomers commonly talk about constellations, planets, and the Moon “rising” and/or “setting” during an observing session. What they really mean, of course, is that the Earth’s 24/7 rotation on its axis--toward the east--makes it appear as though objects are rising in the east, crossing the night sky, and setting in the west. To put another spin on it, our planet’s spin is the source of the night sky’s apparent motion.

Autumn sky chart by Hawaiian Astronomical Society

The easiest way to spot Pegasus is to look for its most prominent asterism, the “Great Square,” four stars that mark the corners of a large square. Asterism is a fancy word for any recognizable star pattern; think of an asterism as a sort of celestial landmark that tells you whose constellation ‘hood you’re in. For example, the Big Dipper is an asterism--probably the most famous--and when we find it, it tells us we are on the Big Bear’s turf. Provided the light pollution is not too bad and you don’t have unshielded light fixtures in your line of sight, you can even spot the Great Square of Pegasus in the city or suburban sky.

Saddle up, stargazers!

Pegasus star map by James B. Kaler

1) You will need to know where south is. Use a compass or make note of where the Sun sets at your viewing location. The spot where it touches the horizon is approximately west, and if you stand with your right shoulder to the west, you will be facing roughly south.

2) Wait at least one hour after sunset to begin observing, so that it’s good and dark. The Moon is currently waning (shrinking), so you will have several hours of dark before it rises and washes out the sky. This period of dark will increase each night over the next week as the Moon rises later and later.

3) Face south and then tilt your head all the way back until you are looking at the zenith, the point in the sky that is directly above your head. An hour after sunset, the Great Square is slightly to the left (east) of the zenith and may also be down a bit toward the southern horizon if you‘re in the upper half of the U.S. As the night goes on, the Square will progressively move to the right, toward the western horizon.

Each side of the Square is about two fists wide, if you hold your fist at arm’s length against the sky and measure across the widest part. Have you found it? Excellent! You have lassoed the torso of the legendary sky-horse. Sadly, when the constellations known to the ancients were reorganized by professional astronomers in 1930, Pegasus lost his hindquarters. He now prances through eternity with just a head, torso, and two front legs. Of course, he does still have his wings, so I suppose that is some consolation.

4) Now let’s find his head. From the southwestern corner of the Great Square, you should see a four-star asterism shaped like an “L” stretching out to the right (west). This is our noble steed’s neck and head. You can see from this old pictorial star atlas that the acrobatic Pegasus is flying upside down in the sky (if you’re facing south in the Northern Hemisphere, as we are).

The star at the end of the “L” is Enif (pronounced ENN if), which appropriately means nose in Arabic. Enif is the brightest star in Pegasus, an orange supergiant with a diameter 150 times that of our Sun. The star marking the corner of the Great Square where the “L” begins is called Markab (pronounced MARK ahb), Arabic for saddle. Markab is a hot, blue, rather average star that, like our Sun, is expected to end its life as a swollen red giant star ultimately dwindling to a white dwarf.

Next week we will explore a special use for the Great Square, as well as some binocular and telescope targets in Pegasus. Until then, happy gazing!