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Moon and Aldebaran

January 1, 2015

Like a cat stalking a mouse, the Moon creeps up on the star Aldebaran on this New Year’s night. The two are in the east as night falls, with bright orange Aldebaran close to the lower left of the Moon. As the hours roll by, though, the Moon will move ever closer to the star. By the time they set in the wee hours of the morning, they’ll look like they’re almost touching each other — especially from the west coast, where they’ll be separated by about the width of the Moon itself.

The gap between them closes because the Moon is orbiting Earth. It takes about four weeks to complete one full turn with respect to the distant stars. So each night, the Moon stands against a slightly different starry background.

And even within a given night, it moves against that background from hour to hour. It covers a distance equal to its own diameter in a little less than an hour. It’s tough to notice that change when the Moon is far away from any bright stars. But when it’s close to a bright one, as it is tonight, the Moon’s motion is pretty noticeable.

By nightfall tomorrow, the Moon will have moved a good bit farther eastward against the starry background, so Aldebaran will stand to the upper right of the Moon. And over the following weeks, the Moon will loop all the way around the sky. It’ll stalk Aldebaran once again on the 28th — continuing a cat-and-mouse game in the night sky.

Tomorrow: reminders of a forgotten constellation.


Script by Damond Benningfield, Copyright 2014

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