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On the Move

September 6, 2015

The Sun is on the move. It’s a slow journey, though — it’ll take about 26,000 years to complete. And when it’s done, it’ll start all over again.

The journey is known as precession. It happens because Earth “wobbles” a bit, like a toy gyroscope that’s beginning to spin down.

The wobble has a couple of well-known effects. For one, it causes Earth’s axis to draw circles on the sky. Over the centuries, that gives us different pole stars. Right now, Polaris marks the north celestial pole. In about 8,000 years, though, the pole will swing all the way over to Deneb, the tail of Cygnus, the swan.

Another effect is that it causes the Sun to move westward through the zodiac. To see how it works, consider Capricornus, the sea-goat. The wide triangle of faint stars is in the southeast at nightfall, and wheels across the south during the night.

When Capricornus was first drawn, several thousand years ago, the Sun appeared against it at the December solstice — the shortest day of the year in the northern hemisphere. That gave the constellation special appeal to many cultures.

Thanks to precession, though, today the Sun appears in Sagittarius at the solstice — one constellation over from Capricornus. And in less than three centuries, it’ll move into Ophiuchus, which isn’t even one of the classical constellations of the zodiac.

Eventually, precession will bring the Sun back to Capricornus for the solstice — in about 22,000 years.

Script by Damond Benningfield, Copyright 2015

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