SKYLIGHTS

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

The Moon waxes through its gibbous phase the early part of the week, passing below Jupiter the night of Friday the 17th and below Saturn the night of Saturday the 18th. It reaches full on Wednesday the 22nd. And what a lovely full Moon it will be as a result of a rare coincidence of events.

First, the Sun passes the winter solstice in Sagittarius at 1:44 AM Central Time also on Wednesday the 22nd (2:44 AM EST, but 11:44 PM on Tuesday the 21st Pacific Time and two hours earlier in Hawaii and Alaska). At that moment, astronomical winter begins in the northern hemisphere, summer in the southern, the Earth's axis tilting as much as it can away from the Sun. On this date the Sun becomes circumpolar (not setting) at the antarctic circle, and does not rise at the arctic circle.

The Moon will pass full at 11:31 AM CST on the 22nd, only about 10 hours after the solstice passage, so the full Moon will be near the summer solstice in Gemini, and will therefore be the highest full Moon (at midnight) of the year. Moreover, the Moon will be at perigee, and in fact the closest perigee of the year, at 5 AM CST ALSO on the 22nd, only 6 hours before full. The combination will make for a very bright, high full Moon and a lovely sight at midnight. Offsetting the effect, however, is the tilt of the lunar orbit, which makes this particular full Moon fall somewhat below the summer solstice and several degrees from as high as possible. More important for North America is that the actual phase takes place near NOON. The midnight Moon will therefore be about 7 degrees to the west of the solstice the night of Tuesday the 21st and about the same angle to the east the night of Wednesday the 22nd. That is, North America will not exactly see the Moon as truly full. Since the brightness of the Moon peaks very sharply at full (as a result of the lack of shadowing by craters and the way light is scattered from the surface), the brightness effect will be diminished. Nevertheless, it will be a grand sight on both nights. Adding to the coincidence, the Earth will pass perihelion, when it is closest to the Sun, on Sunday, January 2. Ocean tides are very sensitive to the distances of the Sun and Moon (40% higher at lunar perigee than at apogee). Tides throughout the world will thus be especially high at this full Moon, which in combination with coastal storms could be quite dangerous.

The Moon does not entirely dominate this week's news. Jupiter, now well past opposition with the Sun, ceases its retrograde motion the night of Monday the 20th. Now in eastern Pisces, it once again begins its normal easterly movement, heading towards Aries.