Blue Moon

Once in a blue moon is a phrase use to refer to a very rare event.

A blue moon refers to the second full moon which occurs in any calendar month. The full moon will look exactly the same as any other full moon. On average, there will be 41 months that have two full moons in every century. By that calculation ‘once in a blue moon’ means once every two-and-a-half years.

This year we will have blue moons in month of May, June or July depending on where you live. The Americans will have the blue moon end of this month, 31 May. The British, Europeans, Asians and Australians will have theirs end of next month, 30 June while the Kiwis will have theirs end of July.

However, there were situations where the moon appears to be bluish to the casual observer which is a rare event. This effect can be caused by smoke or dust particles in the atmosphere. For example after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years, after forest fires in Sweden in 1950 and Canada in 1951, and massive eruption of Mount Pinatubo in Philippines in 1991.