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Moon and Spica

August 7, 2016

Until the invention of the telescope, the science of astronomy consisted of little more than counting objects in the sky and measuring their positions. Yet those basic observations revealed much about how the universe works.

Consider Spica, the brightest star of the constellation Virgo, which stands to the left of the crescent Moon tonight.

Because it’s bright, skywatchers carefully noted Spica’s position in the sky. Some cultures even built temples that aimed toward its rising or setting points at certain times of year — feats that required detailed knowledge of the star’s movements.

In the second-century B.C., Greek astronomer Hipparchus noticed a change in Spica’s position. He compared his measurements of Spica to those made centuries earlier. He found that the star had moved a couple of degrees relative to the Sun’s position in the sky at the autumnal equinox.

Eventually, others discovered that all of the stars undergo this same motion, known today as the precession of the equinoxes. It has nothing to do with the stars, though. Instead, it happens because Earth wobbles like a gyroscope that’s winding down. Over the millennia, that gives us different pole stars. It also causes the entire starry background to shift position relative to the Sun. So thousands of years ago, the Sun was in Libra at the time of the autumnal equinox. But today, it’s moved one constellation over — into Virgo, well to the west of its brightest star, Spica.

Script by Damond Benningfield

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