SKYLIGHTS
Astronomy news for the week starting Friday, January 14, 2000.
The Moon reaches first quarter at the start of the week, Friday
January 14, and then waxes through gibbous to full next Thursday
the 20th. It will be found to the southeast of Jupiter the night
of Friday the 14th and to the southeast of Saturn the following
evening. Less than two days before full, it passes perigee, where
it is closest to the Earth.
The big lunar news, however, is a total lunar eclipse the night of
the full Moon, Thursday the 20th. Since the Earth's long shadow is
directed opposite the Sun, like the Sun it falls on the ecliptic
(the apparent annual solar path). Lunar eclipses can therefore
take place only when the Moon is full and is on or near the
ecliptic. Because of the tilt of the lunar orbit, the full Moon
usually passes above or below the Earth's shadow, but this time it
will plunge right through it. The timing is near-perfect for North
America, allowing everybody to watch. The Moon will enter the dark
part of the Earth's shadow at 9:01 PM CST, and becomes fully
immersed at 10:05 PM. Mid-eclipse, when the Moon is darkest,
occurs at 10:44 PM, the Moon leaves the dark shadow at 11:22, and
the show is over at 12:25 AM. Subtract two hours for PST, add one
for EST. The shadow is not perfectly dark, as it is illuminated by
sunlight playing off the Earth's atmosphere. As a result, the
eclipsed Moon does not disappear, but takes on a dark reddish
color. Its brightness depends on the transparency of the air, and
therefore on cloud cover and particularly on how much
contamination there is from volcanic eruptions. At this eclipse,
the Moon will pass south of the center of the shadow, and at mid-
eclipse you can see a gradation in brightness from south to north.
The changing colors are quite attractive. Though easily seen with
the naked eye, the best view is with binoculars. During the
partial-eclipse stage, note the circular outline of the Earth's
shadow, the first real proof, known since ancient times, that the
Earth is a sphere.
While checking the early evening sky, be sure to note Mars, still
low in the southwest. Venus is still dominating the southeastern
morning sky, while little Mercury is entirely out of sight, passing
through superior conjunction (on the other side of the Sun) on
Saturday the 15th. While the full Moon is darkened during the
eclipse, note its placement midway between Gemini and Cancer,
Gemini's Castor and Pollux pointing right at it. The Moon moves by
its own diameter in an hour, and these background stars make the
motion easy to see.
