Blue Moon in December, 2009
Be sure to look up and take in the “Blue Moon” on Friday night.
While the moon actually won’t be colored blue, it is an unusual sight—but more for reasons of timing than appearance.
Blue moons occur when there’s a second full moon in the same calendar month. While they’re quite predictable, we only see them every year or three. After Friday’s, we won’t see another until January 2018.
It’s All In The Timing
Blue moons arise because a lunar month, the time it takes for the moon to orbit the Earth, is a bit shorter than most of our terrestrial calendar months—29 days 12 hours 44 minutes and 3 seconds to be exact. So most years have twelve full moons that occur approximately once per month. But those extra days accumulate, so every two or three years, there is a 13th full moon.