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.