Dropping temperatures and long nights — combined with this week’s new moon — make for some of the best sky-watching. The sun still sets well before 5pm, and within another 90 minutes the sky is truly dark. Coupled with the darkness, the cold weather knocks any humidity from the air, providing crisp, undistorted views of the stars and planets.
As evening twilight deepens, December’s sky is filled with familiar figures. To the west are the lingering constellations of summer, Aquila the eagle, Lyra the harp and Cygnus the swan, the brightest stars from each making the familiar asterism of the Summer Triangle. Cygnus is also known as the Northern Cross, as this time of year it stands upright above the horizon.
To the north is the familiar shape of the Big Dipper, which is itself a part of Ursa Major, the great bear. Below it is the Little Dipper, host of the North Star Polaris, around which the entire celestial sphere revolves.
High overhead is W-shaped Cassiopeia the queen and Cepheus the king, which looks more like a bishop’s miter than a king’s crown. Beneath them is their daughter Andromeda, flanked by the square of Pegasus to the west and the hero Perseus to the east.
Rising in the east in the evening are the constellations of winter. There’s Taurus the bull, pursued by Orion the hunter, his hourglass shape one of the easiest to make out. Trailing Orion is Canis Minor, the Little Dog, and Canis Major, the Big Dog, home to Sirius, the brightest star in the Northern Hemisphere. Standing high over the east horizon by 10pm are the Gemini twins.
Keep your eyes on those two this week, as they are the apparent source for the consistently best meteor shower of the year, the Geminids. There’s no interfering moonlight, and unlike other meteor showers, you don’t have to wait until the wee hours for the best of this show. This year’s peak, in the dark hours Sunday to Tuesday, can produce more than 100 meteors in an hour. The meteors can appear anywhere in the sky, but traced backward they all appear to radiate from Castor and Pollux in Gemini. By midnight this radiant is almost directly overhead.
The Geminids are unique, in that their parent is the asteroid 3200 Phaethon, while all other meteor showers are spawned by comets. In either case, as these frozen interlopers near the sun in their orbit around the solar system, some of their matter melts off into bits of stellar debris that burst aflame as they carom into earth’s atmosphere.
As daybreak approaches, Gemini is high in the west. By that time Jupiter, now rising around midnight, is high in the south, trailed by Mars and much closer to the horizon Venus in brilliant glory