SKYLIGHTS

Astronomy news for the week starting Friday, September 22, 2000.

The Moon, just having passed its third quarter, heads toward its new phase, which will be reached on Wednesday, the 27th. As the waning crescent descends the morning sky, it moves against the dim stars of Cancer and then through Leo, passing north of Mars the morning of Monday, the 25th. The day before Martian passage, the Moon goes through the perigee point of its orbit, where it is closest to the Earth for the month.

The Moon, however, is a detail this week, as the big event involves the Earth, for which astronomical autumn begins in the northern hemisphere, spring in the southern. Today, Friday the 22nd, at 12:27 PM CDT (1:27 EST, 10:27 AM PST), the Sun crosses the autumnal equinox in Virgo. On that day, the Sun will rise due east, set due west, will be up for (roughly) 12 hours and down for 12 hours, rendering days and nights (sans twilight) of equal length everywhere in the world. On the day of the equinox, the rotation axis of the Earth is exactly perpendicular to the line to the Sun, causing the Sun to shine overhead at the Earth's equator. The Sun will also set at the Earth's north pole and rise at the Earth's south pole.

That said, reality is a bit different. The Sun is an extended body half a degree across, and sunrise and sunset are counted from the appearance and disappearance of the upper edge. Moreover, the Earth's atmosphere is a refractive (light-bending) medium, and everything in the sky is slightly lifted upward from its actual position (at the horizon by half a degree, the solar angular diameter). As a result, the duration of daylight on equinox-day is a few minutes longer than 12 hours, the Sun will actually set a couple days late at the north pole, and will already have risen at the south pole.

Venus is now making a fine appearance in evening twilight, even though you still need a clear western horizon to see it. Brilliant, it is unmistakable. Last week, Venus passed the star Spica in Virgo, toward which the Sun is heading. This week it is Mercury's turn to pass Spica, the star and planet in line on Saturday the 23rd. Well down and to the right of Venus, the planet is difficult to find in bright twilight.

As the sky darkens during equinox week, the two great birds of the northern hemisphere, Cygnus the Swan and Aquila the Eagle, both fly along the celestial meridian to the south, the first magnitude star Deneb in Cygnus close to overhead in the central latitudes of North America. Below it, ten or so degrees north of the celestial equator, Aquila's Altair twinkle, its two fainter companions Alshain and Tarazed by its side.