A Blue Moon is when there are two full moons in the same month. It typically takes place every 2.7 years. The phrase “once in a blue moon” means that it rarely happens and you won’t see it again soon.
The Moon’s cycles were not always understood centuries ago, which led to many superstitions. We now know that the Moon keeps the same side facing the Earth – just like our planet does. Even though it is round, we only ever see one face of it as each part rotates at a slightly different rate. This is why new moons and full moons appear. But, in the past, people thought that moons came out of the sky (for example as a result of a battle between two giants) and sometimes even believed that there was only one moon surrounded by many stars.
But what exactly is meant by ‘blue’? A Blue Moon is said to be so called because the Moon looks blue, but this is not correct – it does appear reddish or pink at a full moon. It can be anything from grey to bluish-grey if you look carefully. The Blue Moon of August 1999 was even described as being “cobalt blue” by an observer in Florida!
Full moons themselves are not as bright as you might think. If the Moon was placed in a completely star-less sky, it would be invisible to us at night. Instead we see it because the Sun is shining on it and reflecting its light back towards Earth – just like any other planet. The difference between this moon and all others is that it will rise around sunset.
In fact, the actual Blue Moon of August will not appear until about 11pm on 22nd August. But don’t mistake a “Blue Moon” for other things like a lunar eclipse, which can take place during a full moon and last several hours.
The ‘true’ Blue Moon
The full Moon of Sunday, August 22nd, will be a “Blue Moon” according to the original — but not the most popular — definition of the phrase.
In modern usage, “Blue Moon” has come to refer to the second full Moon in a month (the last of these occurred on October 31, 2020) — but that hasn’t always been the case. This colorful term is actually a calendrical goof that worked its way into the pages of Sky & Telescope in March 1946 and spread around the world from there.
Editors and contributors to Sky & Telescope have traced the traditional astronomical definition to the Maine Farmers’ Almanac in the late 1930s. The Almanac consistently used the term to refer to the third full Moon in a season containing four (rather than the usual three). “Introducing the ‘Blue’ Moon meant that the traditional full Moon names, such as the Wolf Moon and Harvest Moon, stayed in synch with their season,” says Diana Hannikainen (pronounced HUHN-ih-KY-nen), Sky & Telescope’s Observing Editor.
But in 1946, amateur astronomer and frequent contributor to Sky & Telescope James Hugh Pruett (1886-1955) incorrectly interpreted the Almanac’s description, and the second-full-Moon-in-a-month usage was born.
Sky & Telescope admitted to its “Blue Moon blooper” in the March 1999 issue (see “What Is a Blue Moon in Astronomy?” https://skyandtelescope.org/observing/what-is-a-blue-moon). Canadian folklorist Philip Hiscock and Texas astronomer-historian Donald W. Olson worked with the magazine’s editors at the time to figure out the origin of the mistake, and how the two-full-Moons-in-a-month meaning spread into the English language.