once in a blue moon*

* Very occasionally, the moon actually does appear to be blue. This sometimes occurs after a volcanic eruption, like that of Krakatao in 1883. Dust particles in the atmosphere are normally of a size to diffract blue light, making the moon appear reddish at sunset. Larger volcanic dust particles diffract red light, making the moon appear bluish. Or it could occur from atmospheric effects such as a big forest fire on Earth causing a lot of haze.

* The definition of a Blue Moon has changed over the years. However, the commonly accepted definition of a Blue Moon is simple: it is a full moon that rises twice in one month. Due to our calendar days, sometimes the days of the month are less than the cycle of the moon, which is 29.5 days. This can cause the moon to rise twice in one month. It is said that blue moons happen every 33 months or about every 3 years. This relatively rare occurrence has spawned the saying "once in a blue moon".

Comments