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Moon, Mars, and Spica

July 5, 2014

A beautiful trio highlights this evening’s sky — the Moon, the planet Mars, and the star Spica. They’re in the southwest at nightfall, with Mars quite close to the right or upper right of the Moon, and Spica a little farther in the opposite direction. They don’t set until after midnight.

Despite the tight grouping, the three objects are actually at quite different distances from Earth.

The Moon is our planet’s only natural satellite, so it’s the closest of the bunch — roughly a quarter of a million miles away.

Mars is a good bit farther — about 94 million miles. In fact, today Mars just happens to be exactly the same distance from Earth as the Sun is. Our distance to Mars varies by a couple of hundred million miles, and it’s just a coincidence that it matches the Sun’s distance today.

Another way to think of astronomical distances is in terms of how long it takes light to cross them. It takes a bit more than a second for moonlight to reach Earth — one light-second. And it takes about eight-and-a-half minutes for light to come from Mars — eight-and-a-half light-minutes.

But Spica is much, much farther — about 250 light-years. In other words, the light you see from Spica tonight actually left the star about 250 years ago — well before the United States declared its independence from England.

So take a look at the Moon, Mars, and Spica this evening — three closely bunched objects that aren’t really close at all.


Script by Damond Benningfield, Copyright 2014

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