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.

Night Sky Photography -- 05-19-2019 -- 2019 Seasonal Blue Flower Moon & Jupiter

Good evening from upstate South Carolina, my fellow skywatchers! 

This evening, yours truly stayed up really later -- over an hour after midnight in point of fact! -- to capture a really outstanding set of images showing the Seasonal Blue Moon of 2019, along with the giant planet Jupiter close by. 

I captured these shots of the May full moon -- the Flower Moon --  near some large power lines in the foreground, including a close up of both Jupiter and the full moon. 

Now for all of y'all who are familiar with the normal term "Blue Moon" y'all know that the moon does not turn blue in point of fact. By normal term, I refer to the classic definition of a Blue Moon: the second full moon of a calendar month. Since a lunar cycle (or the time from a new moon, to a full moon, and back) lasts about 29.5 days, and most months have 30 or 31 days, inevitably there will be overlap, with the occasional calendar month including two full moons.

This is the definition that most of us are the most familiar with from the old expression, “Once in a blue moon,” which refers to the fact that this rare occurrence usually only happens every two to three years (though two of them actually happened in January & March of 2018). 

Also this definition applies to the 13th full moon in a calendar year. Since 12 lunar months only have 354 days, some years will also have 13 full moons.

However, this evening's full moon is neither of those Blue Moons. 

A Seasonal Blue Moon refers to the third full moon in an astronomical season that has four full moons. 

An astronomical season is divided by the solstices and the equinoxes -- summer solstice beginning in June, winter solstice beginning in December, spring equinox beginning in March, and fall equinox beginning in September. An astronomical season includes three months, so it would normally encompass three full moons. But since our seasons are divided by a solar calendar and not a lunar calendar, there can sometimes be overlap between lunar cycles and astronomical seasons. When this happens, and there are four full moons in an astronomical season, the third one of these is called a Seasonal Blue Moon.

It's a good thing this blogger was awake to capture it this time around since the next Seasonal Blue Moon is not due again until August 22, 2021. 

By tomorrow evening, a beautiful nearly still full Luna will move closer to Jupiter -- at least from our relative perspective here on Earth.  

Well folks, I hope y'all enjoyed my photos once again. I am headed to bed. Have a good evening and remember to keep looking to the night skies, y'all!