Full moon is called a blue moon. The term "blue moon" used in this sense may date to as early as the 16th century, but it became well known in the United States due to the "Farmers' Almanac" (published since 1818). According to the "Farmers' Almanac", a "blue moon" is the third full moon in any period between either solstice and equinox, or between equinox and solstice, (calculated using the mean tropical year), which contains four full moons. These seasons are equal in length, unlike the astronomical ones, which vary in length depending on the Earth's speed in its elliptical orbit round
The Blue Equinox The Blue Equinox The Blue Equinox, officially known as The Equinox: Volume III, Number I, is a book written by the English occultist Aleister Crowley, the founder of Thelema. First published in 1919, it details the principles and aims of the secret society O.T.O. and its ally the A∴A∴, both of which were under Crowley's control at the time. It includes such topics as "The Law of Liberty", The Gnostic Mass, and Crowley's "Hymn to Pan". In 1904, Crowley was staying in Cairo, Egypt with his wife Rose when he claimed to undergo a spiritual experience. "The Blue Equinox" opens