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Moon and Spica

May 21, 2013

The Moon takes aim at one of the brightest stars in the night sky tonight: Spica, the leading light of the constellation Virgo. The star is close to the lower left of the Moon as night falls, and even closer to the Moon as they set in the wee hours of the morning. They’ll be at their closest for skywatchers on the West Coast, where they’ll be separated by the width of your finger held at arm’s length.

The gap between them gets smaller because of the Moon’s orbital motion around Earth. It takes 27-and-a-third days for the Moon to make one full loop against the background of stars. At that rate, it covers a distance equal to its own diameter in about an hour.

Keep in mind, of course, that the gap between the Moon and Spica is only a projection effect — the two bodies just happen to line up in the same direction in the sky.

In reality, they’re far apart. The Moon is a bit more than one light-second away — the distance that light covers in a little more than one second, or about a quarter-of-a-million miles. The distance to Spica, on the other hand, is 250 light-years — about six billion times farther than the Moon.

To look at it another way, it took the Apollo astronauts about three days to reach the Moon. At that same average speed, it would take about 50 million years to reach Spica.

So keep an eye on Spica — a star that looks like it’s close to the Moon — throughout the evening and into the early morning.


Script by Damond Benningfield, Copyright 2013

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