You may have read that the Full Moon of yesterday, July 31, was a Blue Moon, because there was also a Full Moon on July 2. I return from a two-day journey to Land’s End (having been last month at Finisterra, the Spanish Land’s End) to tell you that this was not so, or was only so in one of two senses.
The probably original sense applied to times when four Full Moons, instead of the usual three, fell in one quarter. “Quarters” here means the seasonal quarters of the year, divided by the solstices and equinoxes. Here is the listing of quarters and Moons from the Astronomical Calendar 2015 section on “Special Moons,” which explains all this more – well, there’s no other word – fully.
Full Jan 5
Full Feb 3
Full Mar 5
– equinox Mar 20
Full Apr 4
Full May 2
Full Jun 2
– solstice Jun 21
Full Jul 2
Full Jul 31
Full Aug 29
– equinox Sep 23
Full Sep 28
Full Oct 27
Full Nov 25
– solstice Dec 22
Full Dec 25
So in this sense 2015 contains no Blue Moon.
The meaning now popular – a second Full Moon inside a calendar month – has been traced to a 1946 article in Sky & Telescope that misunderstood the earlier meaning. In this sense, 2015 does have a Blue Moon.
2016 will have only 12 Full Moons, therefore no Blue Moon in the two-in-a-month sense. But it will have a Blue Moon in the four-in-a-season sense, since that of June 20 will squeeze in just 11 and a half hours before the solstice.
The next year with a Blue Moon in the second-in-month sense will be 2018. In fact it will be one of the years in which (because of February’s shortness) there will be Blue Moons pf this kind in both January and March.
In a third and literal sense, the Moon has occasionally looked blue, or even green, for atmospheric reasons, but far more often yellow or orange. And in a fourth and vague, and possibly oldest, sense you may get a stroke of luck “once in a blue moon.”