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Moon in Balance

May 31, 2015

The bright gibbous Moon is in “balance” tonight. That’s because it’s passing through the constellation Libra, the balance scales. In fact, Libra’s two brightest stars are close above the Moon this evening.

Originally, those two stars formed the claws of Scorpius, the scorpion, which spreads out to the lower left of the Moon. And even today, the names of those stars still represent that heritage: Zubeneschamali and Zubenelgenubi, the northern and southern claws.

But thousands of years ago, the claws were ripped away from the scorpion and assigned to a new constellation: Libra. At least in part, that may be because, at the time, the Sun passed across that part of the sky at the autumnal equinox. Day and night are roughly equal then — a sort of “balance” in the heavens.

Thanks to an effect known as precession, though, that’s no longer the case. Earth wobbles on its axis like a spinning gyroscope that’s running down a little bit. As it wobbles, our perspective on the stars changes, so the Sun appears against different constellations. Today, it passes through Virgo at the equinox, and doesn’t cross through Libra until about a month later.

Look for Zubeneschamali a little above the Moon as night falls, with Zubenelgenubi to the Moon’s upper right. An even brighter pinpoint of light is to the lower left of the Moon — the planet Saturn. More about that tomorrow.


Script by Damond Benningfield, Copyright 2015

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