As the long days of summer slip away and skies darken earlier each evening, the first stars of autumn come into view. The centerpiece of the autumn sky is a starry square called the Great Square of Pegasus. In mid-October it lies almost overhead from mid-northern latitudes around 10 pm.
Pegasus was the winged horse of Greek mythology, and the Great Square represents the front half of the body of Pegasus. Four stars mark the corners of the Square, boxing in an area of sky so large that you need to hold both hands out at armís length to cover it. Sports fans in North America might think of it as a baseball diamond rather than a square.
You should be able to find the Square without much difficulty, but your star chart can help if you need to orient yourself. (For help, see "Absolute Beginners Ė Start Observing." There is a link at the end of this article.)
Looking at the Square, imagine a line from the right-hand side of the square down towards the southern horizon and youíll come to a bright star called Fomalhaut. Fomalhaut is part of the constellation of Piscis Austrinus, the southern fish. In 2008, astronomers released photographs taken by the Hubble Space Telescope showing a planet in orbit around Fomalhaut.
Returning to the Square, imagine a line upwards from the left side. This will lead you to the W-shaped constellation of Cassiopeia, one of the easiest star patterns to recognize. Continue the line northwards for the same distance to see the north pole star, Polaris.
Although astronomers talk of the Great Square of Pegasus, these days only three of its stars officially belong to that constellation. In ancient times, one star was shared with neighboring Andromeda, marking her head. When, in 1930, astronomers came to define official boundaries for the constellations, they ended the practice of sharing stars. Andromeda kept her head but Pegasus lost a corner of its square.
Andromeda represents the unfortunate daughter of Queen Cassiopeia. She was chained to a rock in sacrifice to a sea monster to atone for the sins of her boastful mother. However, youíll need a good imagination to see Andromeda as more than a couple of crooked lines of stars leading away from the upper left corner of the Great Square.
There is, though, one great treasure to be found in the constellation. Two lines extend from the Square. Look at the more prominent of the two lines. If you count two stars along and then go two stars up, youíll come to it: the Andromeda Galaxy, a spiral of stars 2.5 million light years away. On clear dark nights you can glimpse the Andromeda Galaxy with the unaided eye as a hazy wisp of light. The Andromeda Galaxy is the most distant object you can see without optical aid, but itís more prominent through binoculars and small telescopes.
Beyond the shackled feet of Andromeda lies her heroic rescuer, Perseus. This constellation is shaped rather like a distorted capital A. In the sky he is depicted holding the severed head of Medusa the Gorgon, marked by the star Algol. Click for a diagram of the constellation Perseus. Perseus lies in a rich part of the Milky Way, so if you sweep over this area with binoculars, sparkling star fields will spring into view.
Beyond Perseus the stars of winter lie in wait, but weíll leave these for another time and return again to our starting point, the Square of Pegasus. Near it lie two well-known, but relatively inconspicuous constellations, of the zodiac. They are Aquarius the water carrier and Pisces the fishes.
The stars of Pisces are arranged in a large, faint V-shape below and to the left of the Great Square. The most distinctive feature of Pisces is the Circlet, a ring of stars directly beneath the Square of Pegasus, representing the body of one of the two fish visualized here by ancient astronomers.
Below and to the right of the Circlet of Pisces lies Aquarius. This constellationís most distinctive feature is a trefoil-shaped group of four stars that marks the water carrierís jar. From the jar, a stream of faint stars cascades down towards bright Fomalhaut.
The Monthly Sky Guide, by Ian Ridpath and Wil Tirion