Once In A Blue Moon

As I was closing the kitchen blind to go to bed last night, about 10pm, I had a look out (image from Heavens Above):

The Moon was shining in through the window, about 95% full, and was approaching full Moon.

There is the saying "once in a blue Moon". The historic definition is that if there are 4, rather than the usual 3, full Moons in a season, then the third of them is called a blue moon.

This year summer runs from 6.04am Central European Time/British Summer Time on 21 June to 9.44pm on 22 September. And in that time, there are indeed, four full Moons:

• 12.32pm on 23 June
• 7.16pm on 22 July
• 2.45am on 21 August
• 12.13pm on 19 September

The September full Moon this year is the Harvest Moon - the one nearest to the autumnal equinox.

With there being 4 full Moons, the August one satisfies the historical definition of a blue Moon.

A modern definition has come in - namely that when there are two full Moons in a month, the second one is a blue Moon. We have to wait until July 2015 for this to happen.

How common are these? When I looked at the date of Easter, I noted that there were 235 full Moons in a 19-year period.

19 years is 76 seasons. On average, 69 seasons will have 3 full Moons, while 7 of them will have 4. So we should expect a blue Moon every 2 or 3 years.

With the modern definition, we note that 19 years is 228 months. So, again we should expect 7 blue Moons in a 19-year period.