Blue Moon Friday 31 July 2015

This Friday 31 July 2015 there is an event which happens only “once in a blue moon”. Literally. This month there is a Blue Moon.

The occurrence of a Blue Moon doesn’t mean that the Moon will in fact turn blue. Instead a Blue Moon refers to a second Full Moon occurring within a fixed amount of time.

A Full Moon, definitely not Blue

There are two widely accepted definitions of a Blue Moon: either it is an additional Full Moon within a season, or an additional Full Moon within a calendar month.

Moon Phases

Normally there are twelve Full Moons in a year, with one occurring every month. In fact the word “month” is derived from “Moon”. However the phases of the Moon don’t cooperate and divide the year perfectly into twelve with no left overs.

The Moon orbits the Earth every 27.32166 days, known as a sidereal month. As it does so we see different fractions of the lit half of the Moon, creating different phases. However during these 27.32166 days the Earth also orbits the Sun, and so the rate at which the phases change and repeat themselves is slowed down. Looking at the Moon from down here on Earth we see the pattern of phases repeating every 29.53059 days, known as a synodic month.

This is roughly one calendar month, but not exactly. It’s because of this “not exactly” that we don’t get a round number of Full Moons occurring every year, and don’t get exactly one occurring every calendar month.

In fact there are 12.37 Full Moons every year, and for this reason, every so often, we get 13 Full Moons in a year, which means an extra one in a season or in a calendar month.

The Maine Farmers’ Almanac Blue Moon (Type 1)

The original definition of the Blue Moon came from the Maine Farmers’ Almanac which defined a Blue Moon as the third Full Moon within a quarter-year season that has four Full Moons. Confused? You’re not alone. Normally a quarter-year season will have three Full Moons in it, as normally there are 12 Full Moons in a year. But due to that extra Full Moon that we sometimes get, every so often there are 13 Full Moons in a year. This extra Full Moon will occur in one specific season, and in that season the third of the four Full Moons is known as the Blue Moon.

Additional confusion arises due to the fact that the Maine Farmers’ Almanac uses a different definition of a season from the one astronomers use. Astronomers define the start and end points of the four seasons by the position of the Sun in the sky, or put another way the position of the Earth in its orbit. Because the Earth moves at different speeds at different points in its orbit the astronomical seasons are different lengths. Agricultural seasons in the Maine Farmers’ Almanac were all the same length.

This leads to the situation where a Blue Moon (as defined by the Maine Farmers’ Almanac) might occur in an agricultural season but not within an astronomical season. In order to avoid this additional confusion, seasonal Blue Moons are calculated with respect to the astronomical seasons these days.

For decades this definition of a Blue Moon held and was the only one. However now we have an alternative definition, thanks to a mistake in a prominent astronomy magazine.

The Sky and Telescope Blue Moon (Type 2)

In 1946 the astronomy magazine Sky and Telescope published an article by James Hugh Pruett in which he mistakenly interpreted the Maine Farmers’ Almanac. He correctly stated that due to the 12.37 Full Moons per year, we get an extra (thirteenth) Full Moon in seven years out of every 19. He then went on to state that the extra Full Moon that occurs in these seven years must occur in a specific month (correct) and that the second Full Moon in a calendar month is known as the Blue Moon (incorrect, according to the original definition).

Despite the fact that this definition of a Blue Moon was a mistake at the time, it was widely adopted, probably in large part due to its relative simplicity, and is the one that most people use these days.

This Month’s Blue Moon

This Friday’s Blue Moon is an example of a Type 2 Blue Moon, the second Full Moon within a calendar month (July 2015). The first Full Moon this month occurred on 2 July, leaving ample time for the second Full Moon to sneak in at the end of the month, on Friday 31 July 2015.

A Type 2 Blue Moon occurs on average once every 2.7 years. Most type 2 Blue Moons occur within months of 31 days, but they can occur in 30-day months. Because February is only 28 or 29 days long (shorter than the 29.53059 days of the synodic month) February can never have a Blue Moon (jn fact sometimes February has no Full Moons in it at all! The last time this happened was February 1999; the next time it will happen is February 2018).

Within any given century you can expect 37 Blue Moons, around 33 of which will occur in a 31-day month, and around seven of which will occur in a 30-day month.

Future Blue Moons

After this week’s Blue Moon the next one won’t occur until 2018, but then we get two that year! The first occurs on 31 January 2018 (Full Moons on 2 and 31 January 2018) and the second on 31 March 2018 (Full Moons on 2 and 31 March 2018).

After that we have to wait until 31 October 2020.

The next Blue Moon to occur in a 30-day month happens on 30 September 2031.