This December is bookended by two full moons, the first occurring tonight and the second on Dec. 31. The second full moon in a calendar month is commonly known as a blue moon. This name may have originated from the rare occasions when atmospheric conditions made the full moon appear bluish. In the early 20th century, the Maine Farmers' Almanac defined a blue moon as the third full moon in an astronomical season that had four full moons in total. However, a 1943 Sky and Telescope article mistakenly interpreted this information, resulting in our current redefinition of “;blue moon.”; In any case, two full moons in a calendar month occur infrequently—about once every 2 1/2 years.

Constellation Spotlight

If you are just beginning to familiarize yourself with the sky, the constellation Orion is a great place to start. Easily identifiable by the three stars in a line that make up his belt, the Greek hunter appears above the eastern horizon after sunset and can be seen traveling westward throughout the night. Look also for the bright red star Betelgeuse, which represents Orion's right shoulder. Betelgeuse is a red supergiant star, so large that our solar system from the sun out to nearly Jupiter would fit inside its diameter. At Orion's left foot is the bluish star Rigel, a very hot star that shines about 40,000 times stronger than our own sun. Although we see Rigel as one star, it is actually part of a multiple star system made up of three separate stars.

In Greek mythology, Orion was a hunter who was stung by a scorpion (the constellation Scorpius) after insulting the goddess Diana. For obvious reasons, Orion wants to stay as far away from the scorpion as possible, which explains why we never see him in the sky at the same time as Scorpius. The ancient Egyptians saw their god Osiris in the same group of stars. In Hawaii this constellation is known as Ka Hei-hei o na Keiki, which refers to a children's string game similar to cat's cradle.

The ancient Maya focused on Orion's lower half—the triangle of stars made up of Rigel, Alnitak (the easternmost star in Orion's belt) and Saiph (Orion's right-foot star). These three stars represent the stones of the hearth out of which the Maya believed the existing universe was created. The smoke from the hearth can be seen in the sky as the Great Orion Nebula. Science and mythology coincide here as the gas clouds of the Great Orion Nebula are forming young stars and infant solar systems—quite literally a place of creation.

Mars Returns

At long last, the planet Mars reappears in the evening sky, rising above the eastern horizon by 11 p.m. early in the month. By the end of December, Mars will be visible by 9 p.m.