Moon near Castor and Pollux, morning of October 17

Moon near Castor and Pollux

On the morning of October 17, 2022, the last quarter moon passes by the bright stars Castor and Pollux in the constellation Gemini the Twins. You’ll easily notice the bright stars Castor and Pollux because they are close together in the sky. Also, you may notice a brilliant star on the other side of the moon. That’s Procyon, also called the Little Dog Star. It’s the brightest star in the constellation Canis Minor and the 8th brightest star in the sky.

To get a precise view from your location, try Stellarium online.

The ‘twin’ stars of Gemini

Gemini is the constellation of the Twins, referring to the twins Castor and Pollux from Greek and Roman mythology.

But these two stars aren’t really twins. In fact, they’re quite different. Although Castor and Pollux appear close to each other, they’re not physically related or close to each other in space. Pollux is the closer star, and lies about 34 light-years away. Conversely, Castor resides about 51 light-years distant.

More differences in the ‘twins’

When viewing the twin stars, you might notice Castor and Pollux aren’t the same brightness or color. Castor is white and not as bright as golden Pollux. Strangely, Castor is listed as the alpha star in Gemini – a designation usually reserved for a constellation’s brightest star – even though it’s only 2nd-brightest.

Meanwhile, Pollux shines more brightly than Castor. And it’s categorized as an orange star (although your eye might see it as golden). By the way, a star’s color indicates its surface temperature. A white star is a relatively hot, young star. But an orange star is often a cool star.

The orange color of Pollux reveals that it’s a giant star. Pollux has a diameter of about 10 times that of our sun and is the closest giant star to Earth. It’s also one of the very few giant stars known to have an exoplanet.

Meanwhile, the single point of light we see as Castor is really six stars. It’s a star system, consisting of three pairs of binary stars, all revolving around a common center of mass. You can see Castor as two stars in a small telescope.

Bottom line: On the morning of October 17, 2022, look for the last quarter moon near Castor and Pollux, the twin stars of Gemini.

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