The phrase “Once in a Blue Moon” is used to describe an exceedingly rare event, but the lunar event on which the phrase is based happens roughly every 2.5 years.
A Blue Moon, which actually does not refer to the color of the moon, is the second full moon of a given month. As there tend to be only 12 full moons a year, ordinarily each month gets one. But, as Phys.org notes, the moon takes about less than a calender month orbit the earth, so the Blue Moon phenomenon is a calender catchup, not unlike a leap year.
MSN.com reports that the moon will reach its fullest at 9:58 a.m. EDT Friday. The previous full moon this month occurred on August 2.
A Blue Moon used to refer to the third full moon of a season, but, over time, it has come to mean the second full moon of a month.
In an unrelated situation, the moon may visibly appear to be blue as a result of volcanic ash or other particles in the atmosphere. That situation is more rare than the officially termed Blue Moon events.
“A blue-tinged moon is a rarity and so maybe that’s where the saying came from. There are reports of blue moons after the Krakatoa eruption in Indonesia in 1883 in which volcanic ash was ejected 80km high, on the edge of the ionosphere,” Dr. Stephen Hughes told Phys.org.
If you missed the last chance to see the so-called Blue Moon in June of 2011, you had better check the sky Friday night. If you miss that one, you’ll have to wait until July of 2015.