blue moon

A blue moon is the second full moon in a single month. The term does not refer to the color of the moon.

A blue moon occurs, on average, every 2.72 years. Since 29.53 days pass between full moons, there can never a blue moon in February because this month only contains 28 days. On rare occasions, a blue moon can be seen twice in one year, but only in certain parts of the world. The next pair of blue moons will occur in 1999, during the months of January and March.

Sometimes the moon does appear bluish; however, this effect is caused by the way moonlight is scattered as it enters the Earth's atmosphere. A bluish moon was observed was on September 26, 1950, due to Canadian forest fires which had released dust and soot high into the atmosphere.