In the Sky This Month

Eridanus, the river, meanders across the southern sky at this time of year. It is one of the largest constellations, stretching almost 60 degrees. Its northern end is northwest of Rigel, the brightest star in Orion.

The Milky Way arches high overhead on December evenings. It outlines the disk of our home galaxy. At nightfall, it passes from the Northern Cross, in the west, to W-shaped Cassiopeia high in the northeast, to near the face of Taurus, the bull, in the east.

The planet Saturn is near the Moon tonight. They are quite low in the southwest at sunset, so there’s little time to look for them. Saturn looks like a bright star close to the upper left of the Moon. It will stand to the lower right of the Moon tomorrow night.

Saturn is disappearing from view. The giant planet perches quite low in the southwest not long after sunset, so even though it is fairly bright, you need a clear horizon to spot it.

Canis Major, the big dog, will trot across the southern sky tonight. It’s marked by Sirius, the Dog Star — the brightest star in the night sky. Sirius is preceded by Procyon, the Little Dog Star, which is to the upper left of the big dog in late evening.

Bright orange Mars is in the south at nightfall. Tonight, Neptune is to its upper left by less than the width of a finger held at arm’s length. Through a telescope, it looks like a small star. The planets will move past each other over the next few nights.

Although the shortest day of the year, the winter solstice, is more than two weeks away, most of the United States is seeing the earliest sunsets of the year about now. The date of earliest sunset varies with latitude, with the date getting later as you go north.

Venus lines up to the upper right of the Moon at first light tomorrow, with Spica farther along the same line. Venus is the brilliant “morning star,” so it’s impossible to miss.

A bright triangle will greet early risers tomorrow: the Moon, the planet Venus, and the star Spica. Venus, the “morning star,” will stand almost directly below the Moon, while fainter Spica, the leading light of Virgo, will be to the lower right of the Moon.

The brilliant planet Venus is brightest for its current “morning star” appearance. It is in the east-southeast at first light, with the star Spica to its upper right. The Moon stands high above them.