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Moon and Regulus

January 21, 2011

The star Regulus arcs over the top of the Moon tonight. The leading light of Leo is to the upper left of the Moon as they climb into view by around 8:30 or 9, and to the right of the Moon at first light tomorrow.

To veteran skywatchers, Leo is known as a spring constellation. By the spring equinox in March, the entire lion is in view as darkness falls, and he springs high across the sky during the night.

Leo and all the other constellations have their own "seasons" because of Earth's orbit around the Sun.

As Earth turns on its axis, each star returns to the same position in the sky exactly 23 hours and 56 minutes later. There's one exception, though: the Sun. Over that same period, Earth moves a million and a half miles in its orbit around the Sun, so it takes a little extra time for the Sun to return to its same position in the sky: four minutes -- hence the 24-hour day.

This slight difference means that every other star rises four minutes earlier each night. So if Regulus rises at 8 o'clock tonight from your location, it'll rise at 7:56 tomorrow night, and 7:52 the night after that.

Unless you're a very careful skywatcher, though, it's hard to see that difference from one night to the next. But it adds up in a hurry -- an average of two hours a month. So over the course of a year, each star and constellation cycles through the sky, giving each its own "season" in the spotlight.

Script by Damond Benningfield, Copyright 2010

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