Tonight – October 11, 2017 – if you look outside at or near the midnight hour, you might spot the rising last quarter moon and the bright Gemini stars, Castor and Pollux, quite low in your eastern sky. Or look outside before sunup on October 12, when Earth’s spin on its axis throughout the wee hours will have placed the moon and the Gemini stars higher in the sky as seen from across Earth.
Want to know exactly when the moon and the star Castor rises into your sky? Try this online tool from the U.S. Naval Observatory.
The moon reaches its last quarter phase on October 12, at 12:25 Universal Time (UTC). At U.S. time zones, that places the time of the last quarter moon on October 12 at 8:25 a.m. EDT, 7:25 a.m. CDT, 6:25 a.m. MDT and 5:25 a.m. PDT.
Castor and Pollux are the two brightest stars in the constellation Gemini the Twins. These two stars aren’t really twin stars, but both are bright and they’re so close together that they must have reminded the early skywatchers of brothers. How can you find them when the moon moves away? If you’re familiar with Orion the Hunter, and up around midnight, look to the north (or left) of Orion to see Castor and Pollux.
At more southerly latitudes, Orion rises before the Gemini stars do. But in either the Northern Hemisphere or Southern Hemisphere, an imaginary line drawn from the brilliant star Rigel through the brilliant star Betelgeuse faithfully escorts you to Castor and Pollux. If you live in the Southern Hemisphere, remember that Castor and Pollux will appear below the moon on the morning of October 12, and the star Procyon will appear above the moon.
Familiar with the Big Dipper? If so, the Big Dipper can also escort you to the Gemini stars. Draw an imaginary line diagonally through the bowl of the Big Dipper, from the star Megrez through the star Merak. You are going in the direction opposite of the Big Dipper handle. This line will point to Castor and Pollux. Of course, the Big Dipper is more easily seen from the Northern Hemisphere.
Bottom line: Let the moon guide you to the brightest stars in the constellation Gemini, Castor and Pollux, on the night of October 11-12, 2017.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.