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September’s mild nights and darkening skies make for some of the year’s best star watching.
In the west, brilliant Arcturus is slowly sinking, dragging its kite-shaped constellation, Bootes the herdsman, with it. Just east of Bootes hangs the semicircular Corona Borealis, or northern crown; its sole bright star is called Gemma or Alphecca. Moving east again, an hourglass of stars defines the torso of upside-down Hercules.
Beside Hercules shines Vega—the beacon of Lyra, the lyre, and the brightest of the large Summer Triangle of stars. This month, the Triangle will be high in the south at nightfall—that is, in prime viewing position at the prime viewing hour. Below Vega, a small parallelogram of stars outlines the lyre. Moving east again, somewhat dimmer Deneb, also a Triangle star, marks the tail of Cygnus, the swan, and the head of the Northern Cross. To the south, Altair, in Aquila, the eagle, forms the sharpest point in the Triangle. And slightly above and east of Altair, diminutive Delphinus, the dolphin, leaps into a dark sea.
September’s full moon rises the evening of the 9th, and crosses the night sky between Saturn, to the west, and Jupiter, to the east. On the morning of the 17th, Mars will be sandwiched between Aldebaran—the eye of Taurus, the bull—and a very high last quarter moon. Last quarter moons near the fall equinox always ride high, because the Northern Hemisphere tilts toward their position.
The equinox arrives at 8:03 p.m. Thursday, the 22nd, when the sun passes over the equator into the southern sky and an observer from space would see Earth lighted from pole to pole.
Earth laps Jupiter in the orbital race on the 26th. That event places Jupiter opposite the sun in the sky, so it will be up all night long.
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