September 2006

by Deane Morrison

The sun has been dropping through the sky since the June solstice, but it really picks up speed in September. This month we lose 90 minutes of daylight, compared to only 49 minutes during July. The shorter days may be bad news for sunbathers, but they mean more time to watch the stars and planets.

In the early evening, Jupiter commands the western sky. Low in the south, Scorpius extends its claws toward the giant planet, but it remains well out of reach-for now-in Libra. Because they occupy such a southerly position, Scorpius and its faithful follower, the Teapot of Sagittarius, aren't up for very long. In September they have a last hurrah before disappearing over the western horizon.

The autumn stars are sweeping in from the east. Among them are the water constellations Aquarius, the water carrier; Pisces, the fishes; Capricornus, the sea goat; Delphinus, the dolphin; Cetus, the whale; and Piscis Austrinus, the southern fish. Low in the southeast, the bright star Fomalhaut marks the otherwise unremarkable Piscis Austrinus. Above it, the Y-shaped Water Jar of Aquarius should be easy to find.

Early risers will see a somewhat faded Saturn high in the east at dawn. Its brightness has been dimming as its rings approach the edge-on position, exposing less of their expanse. Below the ringed planet, the bright star Regulus, in Leo, climbs higher as the month goes on. Below Regulus, usually brilliant Venus sinks close to the horizon but doesn't disappear; the queen of planets has scheduled her exit for October.

Venus, Jupiter and Saturn--not to mention Mars and Mercury, which are largely hidden behind the sun this month--aren't at their best right now, but planets as a whole have been drawing a lot of attention lately. Last month, astronomers gathered at the International Union of Astronomers meeting in Prague voted to redefine the word "planet." The new definition describes a planet as a celestial body whose gravity is strong enough for it to be nearly round in shape, which is in orbit around a star but is itself neither a star nor a satellite of a planet, and which has swept up the debris in its orbital neighborhood. The IUA definition excludes Pluto, which was reclassified as a "dwarf planet," along with Ceres, the largest asteroid; and a distant body called 2003 UB313, nicknamed Xena.

"The astronomers wanted to define 'planet' so that it has a physical basis," said University of Minnesota astronomy professor Terry Jones. "And that basis is that it has to be massive enough to be round." The definition gives a nod to the diversity of celestial objects orbiting the sun. Their variety reveals the richness of environments where stellar systems form, an area of active research by astrophysicists at the University of Minnesota and elsewhere.

The full moon of September, called the fruit moon, arrives the afternoon of the 7th. This will be the closest full moon of the year, only 221,938 miles away--about seven percent closer than the moon's average distance of 238,900 miles. Its proximity will cause higher tides than usual in coastal areas. Two weeks later, on the 22nd, we'll have the most distant new moon of the year, 252,587 miles away. A waning lunar crescent will appear between Saturn and Regulus the morning of the 19th, and two days later its very thin, aged sliver will crowd Venus close to the predawn horizon.

Fall arrives at 11:03 p.m. CDT on the 22nd, the moment the sun crosses the equator to take up residence in the southern sky. Day and night will be theoretically equal, but we'll actually experience a little more than 12 hours of daylight. That's because the atmosphere bends sunlight so as to allow us to see the sun when it's just below the horizon. Another factor is that sunrise is defined as the moment when the first solar rays shoot over the horizon, and sunset when the last rays are extinguished.