SKYLIGHTS

Astronomy news for the week starting Friday, September 17, 1999.

We begin the week with the Moon passing its first quarter on Friday the 17th shortly after noon, around the time of moonrise. As the gibbous phase grows toward full, the Moon will pass Neptune on Monday the 20th, then Uranus on Tuesday the 21st, both planets within Capricornus.

The major event of the week is the passage of the Sun across the autumnal equinox in Virgo on Thursday the 23rd. At 6:31 AM Central Daylight Time, about the time of sunrise, the Sun will cross the celestial meridian and enter the southern celestial hemisphere to mark the beginning of northern hemisphere astronomical autumn (and southern hemisphere spring). On that day the Sun will rise due east, set due west, will be up for 12 hours and down for 12, giving us equal days and equal nights (hence the term "equinox.") The Sun is now shining lower and lower in the sky, and as a result sunlight is spread over a larger area and the ground chills, the southern solar progression finally stopped when the Sun hits the winter solstice in Sagittarius on December 22. On the day of the equinox passage the Earth's axis stands vertically to the direction to the Sun, the Sun will pass overhead at the equator, will set at the north pole to begin six months of darkness, and will rise at the south pole.

Some technical details alter these numbers a bit. Days and nights (even discounting twilight) are actually not exactly equal on the equinox. The Sun appears as a disk half a degree across, and sunrise and sunset are counted from the disk's first and last appearance, which extends the day a bit. Moreover, sunlight is refracted, bent, when it enters the Earth's atmosphere. As a result, when the Sun is at the horizon, its image is lofted up by another half a degree, so we first see the Sun when in fact it is actually slightly below the horizon. The total effect in mid- latitudes is to extend the day by about eight minutes. At the north pole the Sun is also still visible, real sunset not taking place for another two days.

With the Sun near the equinox, the first quarter Moon, 90 degrees to the east of the Sun, will be near the winter solstice in Sagittarius, giving us the most southerly first quarter of the year.

The planets welcome us to autumn as well, Mars hanging in the evening southwest above Antares, Jupiter and Saturn climbing the eastern sky in late evening, and Venus gloriously bright to the east in morning dawn.