This is Dudley Observatory’s Skywatch Line for Friday, December 4, through Sunday, December 6, written by Alan French.
Reaching last quarter early Thursday, a waning crescent Moon will rise after midnight this weekend, leaving much of the night moonless and dark. The Moon rises at 1:20 am Saturday, 2:17 am Sunday, and 3:14 am Monday.
We’re in a time of transition, moving from the stars of summer toward the winter skies. You can get a preview of the stars of winter, low toward the east to east southeast, by 9:30 pm, and this will likely be offered under much more moderate temperatures.
First look for a very bright star low toward the south southeast, just seven degrees above the horizon. This is Sirius, the Dog Star. It is the brightest star in the night sky and lies only 8.7 light years away. Sirius makes its home in the constellation Canis Major, the Big Dog.
East of Sirius and higher in the sky is Procyon, the luminary of Canis Minor, the Little Dog. It is also bright largely because it is a nearby neighbor, lying 11.4 light years from us.
Moving farther left or northward along the horizon and looking higher still you should spot a pair of star of very similar brightness. The lowest is Pollux, and the upper is Castor, and they mark the heads of the Gemini twins. Pollux, lying at 33.8 light years distance, is closer than Castor at 50.9 light years.
Well above and perhaps a bit right of Castor and Pollux you’ll find bright Capella, which we wrote about last week. To its right and lower in the sky look for the bright reddish star Aldebaran. This is the brightest star in Taurus, the Bull, and marks his eye. If you have found the right star, you should see that it is the top of the lower half of a “V” of stars, lying on its side and open top to the left. Aldebaran is almost 70 light years away.
Below Aldebaran you should easily spot the distinctive and bright star pattern of Orion, the Hunter, one of the most recognizable constellations in the night sky. The three stars of Orion’s belt are vertical in the sky, with the two stars marking his shoulder to their upper left. Brighter, reddish Betelgeuse marks the hunter’s right shoulder. Now look below and right of the belt to two stars mark Orion’s knees. The brightest, marking his left knee, is Rigel.
Compared to the other stars we’ve visited, Orion’s luminaries are distant neighbors and must be intrinsically bright stars. Rigel lies at a distance of 860 light years and Betelgeuse is 500 light years away. As we look at stars farther way from our Earth, our measurements of distance grow more uncertain.
Revisit our winter stars regularly. They rise four minutes earlier each night. That may not seem like much, but it amounts to two hours in a month, so our winter friends will rapidly move higher in the evening sky as the nights pass. In two months they will be due south at 9:30 pm.