SKYLIGHTS

Astronomy news for the week starting Sunday, March 19, 2000.

The next Skylights will appear on Saturday, March 25. The Moon begins Skylight's week at its full phase, which will be reached the night of Sunday the 19th near midnight in North America.

While the three outer naked-eye planets shine in the western evening sky -- Mars setting not long after the close of evening twilight, Jupiter and Saturn glowing brightly high in the west -- the real news is of planet Earth. On Monday the 20th, at 1:35 AM Central Time, the Sun crosses the celestial equator at the vernal equinox in Pisces to begin astronomical spring in the northern hemisphere, fall in the southern. Beginning so close to midnight (2:35 AM) on the east coast, spring will actually arrive on the west coast the night of Sunday, the 19th, at 11:35 PM (and even earlier in Alaska and Hawaii). At that moment, the Earth's axis will stand exactly perpendicular to the line to the Sun, and the Sun will pass overhead for anyone at the Earth's equator. At noon, people at the equator will throw their shadows directly downward. Everywhere else on Earth except at the poles, the Sun will rise due east and set due west, and days and nights will be equal (barring twilight) at 12 hours apiece. At the north pole, the Sun will rise and at the south pole it will set.

Reality is a bit different, as sunrise and sunset are affected both by the Sun's half-degree angular diameter and by refraction (bending of light) in the Earth's atmosphere, which causes things in the sky to appear a bit higher than they really are. As a result, daylight on equinox day is several minutes longer than nighttime, and sunrise and sunset are a bit to the north of due east and west. The Sun also actually rises at the north pole a couple days before the formal first day of spring. Coincidentally, the Moon passes its full phase only three hours before the Sun crosses the equinox, and as a result the full Moon (the "Sap Moon" or "Crow Moon") will be close to the autumnal equinox in Virgo. It too will therefore rise and set very close to due east and west. Because of its orbital tilt, however, the Moon will pass a bit north of the equinox (thus avoiding an eclipse). Shortly after sunset, watch for the rising of Virgo's bright star, Spica.

Two days after the Sun's equinox passage, the largest body of the asteroid belt, Ceres, passes opposition with the Sun, and as a result is also close to the autumnal equinox. Not quite 40 percent Pluto's diameter, and six times fainter than the human eye can see alone, Ceres comes close to planetary status. Chief among tens of thousands on known asteroids, it represents the pinnacle of a failed planet, one that Jupiter's powerful gravity never allowed to form.