A Guide to the Autumn Constellations — The Great Square and the Tale of Andromeda
Using the Great Square of Pegasus as your landmark, star-hop all the way to the Andromeda Galaxy, the most distant object visible to the naked eye. This is a journey through the autumn sky, where Cassiopeia and Perseus meet the myth of a royal family hung together among the stars.
The autumn sky's landmark — the Great Square of Pegasus
The autumn sky has few bright first-magnitude stars and can look dull at first glance, but in their place a single clear landmark hangs high overhead: the large square traced by the four stars forming the body of Pegasus, the "Great Square of Pegasus (the Autumn Square)." Around 9 p.m. from September to October it rises in the southeast, and when the neat square is drawn large across the sky, that is the starting point of your autumn tour.
This square is the body of the winged horse Pegasus, shown upside down, and from the upper-left corner (Alpheratz) two lines of stars stretch out to form the neighboring constellation Andromeda. The inside of the square is nearly empty of stars and looks blank, so once you find the square you have your reference for star-hopping across the rest of the autumn constellations.
The most distant object visible to the naked eye — the Andromeda Galaxy
From Alpheratz at the upper left of the square, hop two stars along Andromeda, then rise a little upward, and you will see a faint, oval patch of light. This is our galactic neighbor, the "Andromeda Galaxy (M31)." About 2.5 million light-years away, it is the most distant object visible to the naked eye — the light reaching our eyes now left this galaxy 2.5 million years ago.
Under a dark sky it shows even to the naked eye as a hazy smudge, and with binoculars the shape of an oval galaxy — bright at the center, spreading toward the edges — becomes clear. If star-hopping to it proves difficult, it helps to remember that the sharper vertex of Cassiopeia's W roughly acts as an arrow pointing toward the Andromeda Galaxy.
The W of Cassiopeia and the Double Cluster in Perseus
In the northern sky, Cassiopeia rises against the Milky Way, its five stars tracing a clear W (or M) shape. In autumn and winter, as the summer Big Dipper tips westward, this W becomes the go-to guidepost for finding the North Star. Cassiopeia is circumpolar, never setting the whole year round, so you can meet it in the northern sky at any time.
Between Cassiopeia and neighboring Perseus, where the Milky Way runs thick, hides the "Double Cluster (h and χ Persei)." It appears even to the naked eye as two hazy points, and with binoculars it opens into a spectacle of hundreds of stars packed into two knots — reckoned the finest binocular target of the autumn sky. Perseus also holds the famous "Demon Star," Algol, whose brightness varies at regular intervals.
A royal family hung among the stars
The autumn constellations are woven together by a single Greek myth. When Cassiopeia, queen of Ethiopia, boasted that her daughter was more beautiful than the sea nymphs, the enraged god of the sea sent a monstrous whale (Cetus) to threaten the kingdom. Following an oracle, the princess Andromeda is chained to a rock as a sacrifice — but the hero Perseus, passing by on the winged horse Pegasus having just slain Medusa, defeats the monster and rescues her.
The characters of this tale — Cassiopeia, King Cepheus, Princess Andromeda, the hero Perseus, the winged horse Pegasus, and the monster Cetus — all hang side by side as constellations in the autumn sky. That is why they are called the "Royal Family," and by star-hopping from one to the next around the Great Square you can read out an entire myth spread across the sky.
Best times and directions to observe
The autumn constellations climb high from the southeast into the southern sky between 8 and 11 p.m. from September through November. Around 10 p.m. in October, the Great Square of Pegasus stands high due south and the Andromeda Galaxy hangs near overhead — the best time to observe. The Andromeda Galaxy is hard to see in the city, so to see it properly, seek out a dark sky on a moonless night.
Autumn air is dry and steady, making it one of the seasons when stars appear most transparent all year. Clear nights come often now that the rains and typhoons have passed, giving excellent observing conditions — and as the night air turns quite cool, bring a warm jacket and take your time gazing up at the sky.