Moon lights up Gemini December 2 and 3

These next few nights – December 2 and 3 – the bright waning gibbous moon shines in front of the constellation Gemini the Twins. But the moon and Gemini won’t be up yet first thing at nightfall. But they’ll climb over your eastern horizon at early evening, and once 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 bright stars Castor and Pollux may be able to withstand the moon-drenched skies. If you can’t see these stars otherwise, try placing a finger over the moon and take another look.

Did you know that this constellation is the radiant for the Geminid meteor shower? The Geminid meteors streak the sky in any number of directions, but if you trace the paths of these Geminid meteor backward, they all appear to radiate from the constellation Gemini (near the star Castor). This is only a chance alignment, however, as the Geminid meteors vaporize or burn up about 60 miles (100 km) above the Earth’s surface, whereas the star Castor resides about 34 light-years distant.

The radiant point for the Geminid meteors is found in front of the constellation Gemini, near the bright star Castor. But you don’t have to find the radiant to watch the Geminid meteor shower.

The Geminid meteor shower is expected to peak on the night of December 13-14, 2020. That’s from mid-to-late evening Sunday night till Monday dawn, with the greatest numbers coming around 2 a.m. (at all time zones). Fortunately, there will be no moon to ruin the show, as the new moon and a total eclipse of the sun will take place on December 14, 2020.

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 still may see a good spattering of meteors.

Although you don’t need to find the radiant to enjoy the Geminid meteor shower, you can still find this constellation when the moon has moved out of this part of the sky. If you live at northerly latitudes, and are familiar with the Big Dipper asterism, try star-hopping to Gemini, as we show below.

A star map showing Gemini and the Big Dipper.

Draw an imaginary line diagonally through the Big Dipper bowl to locate Castor and Pollux. This is around mid-evening in December at mid-northern latitudes.

Another way to star-hop to the constellation Gemini is by way of the constellation Orion the Hunter. Draw an imaginary line from Rigel through Betelgeuse to fly to Castor and Pollux, Gemini’s two brightest stars.

A star map showing Orion and Gemini, with a line from Rigel to Betelgeuse pointing towards Castor and Pollux.

Looking eastward around mid-evening in December from mid-northern latitudes. Draw an imaginary line from Rigel through Betelgeuse to star-hop to Castor and Pollux.

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 one-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.

Bruce McClure