Moon in Balance

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

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

But thousands of years ago, the claws were pulled 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 autumn equinox. Day and night are roughly equal then — a sort of “balance” in the heavens.

Thanks to an effect known as precession, though, the Sun is no longer in Libra at the equinox. 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 Zubenelgenubi not far to the left of the Moon as night falls, with Zubeneschamali farther to the Moon’s upper left — the former claws of the scorpion that are now in cosmic “balance.”

Tomorrow: un-balanced planets in the evening sky.
 

Script by Damond Benningfield