Some of the big constellations of autumn begin to push their way into the evening sky this month. Pegasus, the flying horse, is well up in the east at nightfall by month’s end, with Andromeda, the prin- cess, to its left. Under especially dark skies, you should be able to pick out M31, the Andromeda Galaxy. The planet Jupiter is ending its run in the evening sky, and will disappear in the Sun’s glare next month. Another giant planet, Neptune, is at its best for the year, although still far too faint to see with the eye alone.
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In the Sky This Month
September 16: BL Lacertae
BL Lacertae, a galaxy a billion light-years away, is firing a beam of energy in our direction — a “jet” of particles powered by a supermassive black hole. The galaxy is in the faint constellation Lacerta, which is in the northeast at nightfall.
September 17: Cepheus
Cepheus, the king, is a faint constellation not far from the Pole Star, Polaris. Look for it high in the north this evening. Its brightest stars look like a child’s drawing of a house, with a box of stars topped by a tall triangle.
September 18: Big Bear
The Big Dipper stands in the northwest this evening. The bowl is roughly parallel to the horizon and the handle extends skyward. The dipper is part of Ursa Major, the big bear. The bowl represents the bear’s rump and the handle is its tail.
September 19: Moon and Aldebaran
A bright star follows the Moon across the sky tonight. That seems appropriate, since its name, Aldebaran, means “the Follower.” The star will stand to the lower left of the Moon as they climb into view, around midnight, and closer to the Moon at dawn.
September 20: Big Galaxy
If you have nice, dark skies, look for the Milky Way crossing the sky this evening. The subtle glow of its myriad stars arcs high across the sky as darkness falls, and drops down the western sky later on.
September 21: Last-Quarter Moon
The Moon is at last quarter tonight. It is three-quarters of the way through its monthly cycle of phases, so it lines up at a right angle to the Earth-Sun line. At that angle, sunlight illuminates half of the lunar hemisphere that faces our way.
September 22: Autumnal Equinox
Day and night will be just about equal the next few days for the entire world. That’s because fall arrives in the northern hemisphere late tonight at the September equinox, which is the moment the Sun crosses the equator from north to south.