Full Moons occur every 29.5 days, so it has always been a way of breaking up the seasons and the year, and is the basis for our calendar months. One full moon per month is the usual, but since every month but February has at least 30 days in it, there is the potential for two full moons in a month.
The term Blue Moon can refer to the second full moon in a given month. When people say “once in a blue moon” they are speaking to the rarity of the occurrence of a second moon happening in the same given month.
This month is one of these unusual months which will have two full moons. There was the full moon on August 2nd, and another full moon appears on Friday, August 31st.
The term Blue Moon is also sometimes used to describe the third of four full moons in a single season.
Are there any moons that actually appear blue in color? Unusual sky conditions can create them, but they are not date-predictable. Particles of dust of a particular size or smoke from large forest fires or volcanic eruptions can cause a moon to look blue in color.
The idea of a Blue Moon as the second full moon in a month appears to have come from a March 1946 issue of Sky and Telescope magazine. In an article called “Once in a Blue Moon” by James Hugh Pruett, he defined the term this way: “Seven times in 19 years there were — and still are — 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon.”
Can there be two blue moons in a single calendar year? Yes and it happened last in 1999. There were two full moons in January and two full moons in March and no full moon in February. The next year of double blue moons will be in 2018.