SEPTEMBER 2005

by Deane Morrison

The leaves may begin to fall in September, but the real cause of the seasonal change-the southward movement of the sun-has been going on since the June solstice. Even so, we tend to notice the shrinking daylight keenly at this time of year because the sun drops most rapidly around the time of the autumn equinox. At our latitude, we lose about three minutes a day in September. If you think that's bad, consider the Icelanders, who watch a good six minutes of daylight "go south" each day in the middle of September.

The equinox occurs at 5:23 p.m. CDT on the 22nd, the moment the sun crosses the equator and takes up residence in the southern sky. On that day, neither hemisphere of Earth will tilt toward or away from the sun. If you could look at the Earth from space, you would see the border between day and night running everywhere exactly north and south, slicing the planet in two from pole to pole. Except on the equinoxes, the day-night border is slanted compared to the north-south direction.

Of course, Earth isn't a bare sphere, and this complicates the situation a bit. Our atmosphere bends sunlight so that we can see the sun when it is actually just below the horizon. At the poles, the yearly sunrise and sunset and their accompanying twilights are all long drawn-out affairs. Also, sunrise is defined as when the first rays break over the horizon, and sunset is when the last rays disappear, so this biases things in favor of more daylight. Accordingly, we get about 12 hours and 13 minutes of daylight on the equinox; not until the 25th do the days and nights even out at 12 hours each.

Another seasonal phenomenon happens on the 17th. The round harvest moon rises at 9:01 p.m., less than two hours before reaching fullness. Since time immemorial, this moon has lighted fields for farmers working late to bring in the harvest. It works its magic because a moon that's close to full is opposite the sun in the sky. Near the fall equinox, our hemisphere is rapidly tilting away from the sun and toward the fullish moon. Therefore, the moon appears earlier from night to night than it otherwise would. If that's hard to visualize, think of the sun in spring; the more we tilt toward it, the earlier it rises.

The first half of September has its share of delights, too. The moon, Venus, Jupiter and the bright star Spica, in Virgo, perform an intricate dance over the western horizon. Look for them as soon as the sky darkens. The best night will be the 6th, when all four celestial bodies draw closest. Spica will be right under Venus, the brighter planet, and a crescent moon will hang to the lower left of Jupiter. As the days go by, Venus will shine on, but Jupiter and Spica will disappear by the end of the month.

Mercury, Mars and Saturn decorate the morning sky. Mercury is quite low and only visible for the first few days of September, but you may find it by looking to the right of the waning moon on the 2nd. Saturn stays close to the lovely Beehive star cluster all month; check out the pairing with binoculars. Most spectacular of all, though, is Mars. As Earth and the Red Planet approach each other, Mars grows more lustrous and rises earlier each night, appearing by 9 p.m. at month's end. Look for a bright, ruddy orb to the lower right of the Pleiades. Another way to find Mars is to look below the moon the night of the 21st.

The autumn stars include three famous geometrical groupings. The Summer Triangle of Vega, Deneb and Altair still shines high in the south. In the east, the Great Square of Pegasus canters toward center stage. Directly below the Great Square, the Circlet of Pisces is easy to find if skies are dark. The lone bright star in the south is Fomalhaut, the beacon of Pisces Austrinus, the southern fish.

Early risers with good viewing conditions may catch a glimpse of the zodiacal light an hour or two before dawn this month. This faint glow comes from sunlight reflecting off dust in the plane of the solar system. It will look like an oval finger of light pointing up from the eastern horizon along the sun's path through the heavens.