A True Blue Moon

Sunday’s full moon shines amid the stars of Taurus the bull. Ten degrees east of the moon you’ll find the red-giant Aldebaran. Half that distance to the moon’s west look for a small, fuzzy patch of light. So close to the moon’s glare, you may need binoculars to discern the stars of the Pleiades cluster.

While this full moon looks much like any other, it has the distinction of being a true blue moon. You may think of a blue moon as the second in a single month, and that’s the colloquial definition.

But to astronomers and Farmers Almanacs, a blue moon is the third of four full moons to fall within a single season. Our current season — autumn — began September 22, the same day as full moon. October’s full moon fell 30 days later on the 22nd, and the fourth of the season falls on December 20, the day before winter solstice.

The full moon washes out all but the brightest stars, leaving the evening sky divided into two seasonal camps, containing some of the brightest stars. The stars of summer hang on stubbornly in the west while the stars of winter advance from the east. The brightest star in the west is Vega of the constellation Lyra. Higher above it is Deneb of Cygnus the swan, while below is Altair of Aquila the eagle. Together the three make up the Summer Triangle.

In the east, the familiar figure of Orion strides over the horizon, with the red-giant Betelgeuse marking one shoulder and blue-white Rigel marking the opposite foot. To the west look for Aldebaran, the red eye of Taurus the bull, and above that golden Capella of Auriga the charioteer. To the northeast shine the Gemini twins Castor and Pollux. To the southeast is brilliant Procyon, the little dog, and below that is the big dog Sirius, the brightest star in all the heavens. All these but Betelgeuse, which is just an added attraction, form the Great Winter Circle.