Observatory telescopes

Around the time of the September equinox, the days shorten at their fastest clip and the night reasserts its dominance of the northern sky. This year the equinox arrives at 2:50 a.m. on the 23rd. At that moment the sun will shine directly over the equator, lighting the Earth from pole to pole.

September’s full moon rises the evening of the 13th. As the closest full moon to the equinox, it qualifies as the harvest moon. The harvest moon acquired its name by rising less than 30 minutes later from night to night for several days around full moon. This relative abundance of early moonlight helps farmers working into the night to harvest their crops before spoilage or an early frost sets in.

At nightfall, face south to see Saturn on the left and Jupiter on the right, both quite low in the sky. A waxing crescent moon visits Jupiter on the 5th; if you’re a night owl, you may want to stay up and watch the planet and the moon redden as they descend to the western horizon together. The moon visits Saturn on the 7th and 8th.

Above the planets, the Summer Triangle of bright stars reaches its highest and most favorable position for evening viewing this month. To the east, the autumn “water” constellations are entering the sky. If you like challenges, see if you can find the westernmost and dimmest of them: chevron-shaped Capricornus, the sea goat. Capricornus can be particularly hard to find because it has no bright stars. To make things easier, first have a look on the 10th, when the moon will be right in the middle of the constellation. Then wait for a moonless evening and try your luck. If you have a star map, this is a good time to use it.

In the west, Bootes, the herdsman, is dropping toward the horizon. At the base of the kite-shaped constellation is Arcturus, the brightest star in the northern hemisphere of sky. As Bootes falls, the kite stands upright, as though the weight of Arcturus were dragging it down. And speaking of downwardly mobile stars, Arcturus is actually falling through our galaxy. Instead of circulating in the disk of the Milky Way like the sun, it’s slicing rapidly down through the disk. Taking the plunge with Arcturus are at least 52 stellar traveling companions, collectively known as the Arcturus Stream.

Deane Morrison, with the University of Minnesota, can be contacted at morri029@umn.edu. Find U of M astronomers and links to the world of astronomy at www.astro.umn.edu.

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