What Is A Blue Moon And How Rare Are They?

Later this month, on the night of August 30-31, 2023, there will be a Super Blue Moon -- the second Supermoon of August in fact. Luna (aka Earth's Moon) will be very close to the Earth and appear along with the planet Saturn. It will also be the last Full Moon of the summer season of 2023.

How rare are Blue Moons? Well, the classic phrase "once in a Blue Moon" referring to a rare occurrence aside, Blue Moons take place in years when there are 13 Full Moons in a calendar year. The Full Moon phase occurs every 29.5 Earth days. Because of this Blue Moons occur about once every two to three years on average, although sometimes they can occur in the next year during leap years -- as will be the case in 2024, which will also have a Blue Moon in the month of August.

Although the term Blue Moon nominally refer to the second Full Moon of a single calendar month, there are actually two types of Blue Moons: a monthly Blue Moon and a seasonal Blue Moon.

The monthly Blue Moon meets the traditional definition of the second Full Moon in a single calendar month, while a seasonal Blue Moon is the third Full Moon in a season that has four Full Moons.

The Blue Moon of August 30th will be a monthly Blue Moon, since there will only be three Full Moons in summer of 2023. The next Blue Moon will take place next year -- a leap year -- on the night of August 19-20, 2024, will be a seasonal Blue Moon since there will be four Full Moons during that year's summer season: June 21st (on the summer solstice), July 21st, August 19th, and September 18th.

Finally, the term Blue Moon does not actually refer to the color of the Lunar surface itself. Moons that appear blue (such as the one in the photo above) are incredibly rare and have nothing to do with the calendar or the Moon's phases. Such coloring is usually a result of certain atmospheric conditions: smoke, dust, or even water droplets in the atmosphere from certain types of clouds can all contribute to a Full Moon taking on a blue shade on rare occasions.

Fun Full Moon Fact: The month of February will never experience a monthly Blue Moon as it only has 28 days in a common year and 29 in a leap year. Sometimes February doesn't have a Full Moon at all and this is known as a Black Moon -- which will not happen again till February of 2037.