On December 2 and 3, 2020, the bright waning gibbous moon shines in front of the constellation Gemini the Twins. Watch for them not at nightfall, but shortly afterwards, when the sky has gotten good and dark. They’ll climb over your eastern horizon in early evening, and, once the moon and Gemini are up, they’ll be out for rest of the night.
The lunar glare will probably wash out the stick figure of the Gemini Twins, but the constellation’s bright stars, Castor and Pollux, should be able to withstand the moon-drenched skies. If you can’t see these stars otherwise, place a finger over the moon and take another look.
Gemini contains the radiant point for the Geminid meteor shower. The Geminid meteors – December’s best meteor shower – streak the sky in any and all directions. But, if you trace the paths of these meteors backward, they all appear to emanate from the constellation Gemini, from near its bright star Castor. It’s only a chance alignment, of course, as the Geminid meteors are just bits of comet-dust, which vaporize or burn up about 60 miles (100 km) above the Earth’s surface. Meanwhile, Castor is a mighty star, residing some 34 light-years distant. The Geminids originate right here in our own solar system, in a mysterious blue comet-asteroid hybrid object known as 3200 Phaethon. Expect the Geminids’ peak on and around the night of December 13-14, 2020.
That’s from mid-to-late evening Sunday night (December 13) until dawn on Monday (December 14), with the greatest numbers coming around 2 a.m. (at all time zones).
If you can’t stay up late on December 13-14 (Sunday-Monday), try the weekend nights instead: December 11-12, and December 12-13. You might still see a good spattering of meteors.
How can you find Gemini when the moon has moved away? Maybe you know you don’t need to find a meteor shower’s radiant point to enjoy meteors. But maybe you’ll want to find Gemini, anyway, when the moon has moved on. There are several ways to do this. If you live at a northerly latitude, and are familiar with the famous Big Dipper asterism, try star-hopping to Gemini, as we show below.
Another way to star-hop to the constellation Gemini is by way of the constellation Orion the Hunter.
Draw an imaginary line between Orion’s two brightest stars – Rigel and Betelgeuse – to fly to Castor and Pollux, Gemini’s two brightest stars.
Bottom line: We can use the moon to find the Gemini stars Castor and Pollux on December 2 and 3, 2020. Then, when the moon is long gone – some week and a half from now – we can rely on the Big Dipper or Orion to escort us to Castor and Pollux when the Geminid meteor shower is in full swing.