Moon and Spica

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Moon and Spica

Those GPS numbers on your smartphone pinpoint your latitude and longitude — your position on Earth’s surface. Latitude is your position north or south of the equator, usually expressed in degrees, minutes, and seconds. And longitude is your position east or west, in hours, minutes, and seconds.

Astronomers use a projection of those coordinates to define locations in the sky. Instead of latitude, though, the north-south position is known as declination. And the east-west position is called right ascension.

There’s a difference in the land-based and sky-based coordinates, though — the celestial ones change position. Earth wobbles on its axis, which causes the stars to shift around the sky. So when astronomers use a star’s coordinates, they have to specify time as well.

As an example, consider Spica, the brightest star in Virgo. Today, it’s at right ascension 13 hours, 26 minutes, 12 seconds, and a declination of minus 11 degrees, 17 minutes, and four seconds. But astronomers frequently use the location from the year 2000 to provide a fixed time. In that system, Spica is 13 hours, 25 minutes, 12 seconds, by 11 degrees, nine minutes, and 42 seconds — a slight but well-understood difference.

Tonight, there’s an easy way for most of us to spot Spica: It’s quite close to the Moon at nightfall, and stays close throughout the night.

Script by Damond Benningfield

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