January 6, 2000

A total eclipse of the moon tops the night of January 20-21. At 10 p.m. the moon first enters the earth’s umbral shadow, with totality lasting from approximately 11 p.m. until 12:30 a.m. Here are some interesting things to watch for.

The moon is always moving in opposite directions at once: from east to west in its apparent motion, due to earth’s rotation, and from west to east, which is its physical motion in orbit around the earth. Normally we are only aware of the former. But during an eclipse, we can vividly observe the latter as the moon crosses the earth’s shadow projected into space. Thus, you will see the left edge of the moon darken first.

The relative sizes of the moon and the earth also become apparent. Consider that the moon’s diameter is 2160 miles, and the earth’s 7928; therefore the moon must travel approximately four times its own width through the disk of the earth’s shadow. Using you knowledge of the moon’s mean distance of 239,000 miles, and simple geometry, you can then roughly calculate the eclipse’s duration, and confirm it with your own eyes.

Because some sunlight still reaches the moon from bending by the earth’s atmosphere, the moon is likely to appear deep red during the totality. It could also have a bright southern tip, making it look like a science fiction image of Mars with its polar cap.

Find the real Mars just to the right of the waxing crescent moon one hour after sunset on the 10th.

The moon passes below Jupiter on the 14th and Saturn on the 15th. A waning crescent moon almost “touches” brilliant Venus at dawn on February 2nd.