Moon by Castor and Pollux September 20 and 21

Moon by Castor and Pollux: 2 dots labeled Castor and Pollux with two crescent moons, one close and one farther down.
Look for the waning crescent moon by Castor and Pollux, the 2 bright stars of Gemini the Twins, on the mornings of September 20 and 21, 2022. Also nearby is Procyon, the brightest star in Canis Minor. Chart via John Jardine Goss/ EarthSky.

The moon by Castor and Pollux September 20 and 21

On the mornings of September 20 and 21, 2022, the waning crescent moon slides by the bright stars Castor and Pollux in the constellation Gemini the Twins. Castor and Pollux are bright and easily noticed since they are close together in the sky. If you notice a brilliant star on the other side of the moon, that’s Procyon, or the Little Dog Star. It’s the brightest light in the constellation Canis Minor and the 8th brightest star in the sky.

Castor and Pollux: The twin stars of Gemini

Gemini is the constellation of the Twins, referring to the twins Castor and Pollux of Greek and Roman myth. But these two stars aren’t twins; they’re quite different. Although Castor and Pollux appear close to each other, they’re not physically related or close together in space. Pollux, the closer star, lies about 34 light-years away, while Castor resides at about 51 light-years.

If you study Castor and Pollux, you might notice a difference in color. Castor appears white in contrast to the orange light of Pollux. A white star is a relatively hot, young star, while an orange star is a cool star moving toward the end of its life.

Moreover, Pollux’s orange color reveals that it’s a giant star.

Pollux is the nearest giant star to Earth and has a diameter of about 10 of our suns. It’s also one of the very few giant stars known to harbor a planet.

Meanwhile, Castor is six stars in one, consisting of three pairs of binary stars, all revolving around a common center of mass.

Bottom line: On the mornings of September 20 and 21, 2022, look for the moon close to Castor and Pollux, the twin stars of Gemini.

For more great observing events in the coming weeks, visit EarthSky’s night sky guide

September 18, 2022

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