September 2007

by Deane Morrison

As the summer sun recedes, it makes way for the autumn stars sweeping in. Many belong to water constellations like Pisces, Capricornus, and Aquarius, which are now chasing the Milky Way westward. Late evenings are graced by a furiously brightening Mars, and approaching morning sees Venus soar to a lofty throne, there to preside until the break of day.

Mars rises shortly before midnight and is high in the south at dawn. The Red Planet glides between the horns of Taurus in mid-month, then heads into Gemini. As Mars and Earth draw closer, Mars continues to brighten; by the end of September it holds its own with the brilliant winter stars in whose company the planet will reach its pinnacle of brightness in December.

Venus takes a giant leap into the morning sky, rising 90 minutes before the sun at the beginning of September but three-and-a-half hours ahead of it by the end. If you're up early on the 9th, look for a thin crescent moon below Venus, accenting the planet's loveliness. Also catapulting into the morning sky are Saturn and the bright star Regulus in Leo. The planet and star separate as they climb higher and end the month with Regulus between Venus and Saturn.

Summer ends promptly at 4:51 a.m. CDT on the 23rd, when the autumnal equinox arrives. At this moment the sun crosses the equator into the southern sky and Earth's sunward face is lit from pole to pole. Day and night are equal--almost. We actually get about 12 hours and eight minutes of daylight, thanks to two facts. One, the atmosphere bends the sun's rays and makes it visible when it is still below the horizon. And two, sunrise is defined as the moment the sun's first rays clear the horizon but sunset is the moment the last rays disappear. Sunrise and sunset are closest to 12 hours apart on the 26th.

September's biggest event comes near the end of the month, when the harvest moon lights the sky for farmers and romantics throughout the Northern Hemisphere. It rises the evening of the 26th, just four hours after becoming full.

The harvest moon is famous for its habit of rising earlier than usual from night to night around the time of fullness. At the latitude of the Twin Cities, it appears only 23 minutes later on the 27th (7:14 p.m.) than on the 26th (6:51 p.m.). The average interval over the course of a year is about 50 minutes. In Europe, a more northern land, the harvest moon may come up 20 or even 10 minutes later from one night to the next.

The harvest moon phenomenon comes about because full moons that occur around the time of the fall equinox follow a sharply northward course through the sky, just as the sun does at the spring equinox. The spring sun rises earlier each day, and the fullish harvest moon rises relatively earlier each night. But the two phenomena occur six months apart because full moons are opposite the sun in the sky. Thus, Earth's Northern Hemisphere tilts more and more toward the sun in spring, but more and more away from it--and, therefore, toward the full moon--in fall. Watch where the harvest moon comes up for several nights before and after the full phase; you'll see that it moves quite a distance north along the eastern horizon.

Chief among the fall stars is the Great Square of Pegasus, now well up in the east after nightfall. Below it is the Circlet of Pisces, and to the lower right of the Circlet is the compact, Y-shaped Water Jar of Aquarius. The bright star in the south is Fomalhaut, marking the dim constellation Piscis Austrinus, the southern fish.

Although it's late in the season, the Summer Triangle of bright stars still rides high after sunset. Altair and Deneb, the stars at the southern and northeastern points of the Triangle, respectively, lie within the Milky Way's faint ribbon of light; Vega, at the northwestern point, is just outside. Deneb and some of its closest stars form the Northern Cross, a star grouping in Cygnus, the swan. Altair is part of Aquila, the eagle, and Vega reigns in Lyra, the lyre.

If skies are dark enough to trace all these constellations, you can probably find a little-known treasure within the borders of the Triangle. Using binoculars, sight along a line from Altair to Vega. Approximately halfway between the stars you should find a star grouping that looks like its name: the Coat Hanger. It will be a line of six stars with four stars hanging down in a hook. Astronomers have debated whether the Coat Hanger is a star cluster, in which the stars are physically related and moving together, or an asterism, which is a chance grouping that doesn't have constellation status. Whatever its true nature, this is one sight that stands out clearly and beautifully. So grab your binoculars and give your eyes a treat.