# Happy Blue Moon!

2024-08-19 - [53] 6:2

This full moon today is a blue moon! I celebrated today and this last weekend by drinking blue moon beer!

This is the second August in a row where some news and popsci sites mention that the full moon is a blue moon. The thing is though, they are using 2 different definitions. My local news station was even a bit smug about how the definition of this full moon is the correct definition of the blue moon, even though they also used the other definition last year. So many sites will also start off by stating something along the lines of "A blue moon is not actually blue in color".

## What is a Blue Moon?

There are usually 12 full moons in a year, about 29.5 days apart from each other, but because a tropical year is about 365 days long and 12 moon cycles are about 354 days in length, we get a difference of about 11 days a year. This means the moon cycle and tropical year are not synchronized, so the first full moon of the year starts about 11 days earlier than the year before. Once a full moon happens early enough in a year, a 13th full moon is able to squeeze in.

The term blue moon to mean a 13th full moon seems to have originated in the Maine Farmer's Almanac of 1937. The definition it used was essentially "In a tropical season, if there are 4 full moons, the 3rd of those full moons is a blue moon". Basically a moon is squeezed into a tropical season because the first full moon of the tropical season happened shortly after the season started.

In the case of this year, the June Solstice (Summer in the Northern Hemisphere, Winter in the Southern Hemisphere) occurred at about June 20th 2024 20:52:23 UTC and the first full moon after the June Solstice occurred at about June 22th 2024 01:08:58 UTC. With the September Equinox occurring at about September 22th 2024 12:44:53 UTC, there is enough room for 4 full moons to occur this Summer in the Northern Hemisphere, Winter in the Southern Hemisphere. The 4 full moons happened or will happen at about June 22th 2024 01:08:58 UTC, July 21st 2024 10:18:21 UTC, August 19th 2024 18:27:05 UTC (today), and September 18th, 2024 02:35:44 UTC.

There is another definition that quite a few people use, which is the second full moon in a calendar month that has 2 full moons. This definition would exclude February as a potential month because the moon cycle is longer than Februrary is. An issue with this definition is that people in different longitudes might not experience the full moon as a blue moon. It's possible for a full moon to occur, for instance, at August 31st 23:30:00 in California, but it would be September 1st 00:30:00 in New Mexico, giving enough time for a second full moon to occur in September for New Mexico. California would experience a blue moon by this definition on August 31st, but New Mexico wouldn't experience a blue moon until September 30th.

The first definition avoids the issue of the second definition by not needing to worry about local timezones. The entire Earth essentially experiences the same moon phase at the same time, even if we can't see the moon (light taking time to travel makes this technically untrue, but at the time differences are incredibly tiny and don't matter in this case anyways), and all that is needed is 4 full moons between a solstice and equinox or an equinox and solstice, which doesn't care about timezones.

Both definitions of blue moon involve 13 full moons in a tropical year. It's just a matter of where you slice the year to start counting full moons. In the case of the first definition, the year is sliced at the solstices and equinoxes, where in the case of the section definition, the year is sliced at midnight of January 1st.

To see when blue moons of the first definition occur, I have calculated the times for years 1901-2200, separated to different pages by century, at the following link.

Calculated Blue Moon Times

## Black Moon

There is an equivalent of the blue moon called a black moon. The difference between the blue moon and black moon is that instead of counting full moons, you count new moons. The 3rd new moon in a tropical season that has 4 new moons is a black moon. The next black moon is at about August 23 2025 06:07:30 UTC. You might see something in the end of December 2024 about December 30th/31st being a black moon, but that's using the equivalent of the second definition of the blue moon.

I also calculated the black moon times for years 1901-2200, separated to different pages by century, at the following link.

Calculated Black Moon Times

Maybe someday I'll add a {blue moon} and {black moon} wiki page to my site.

## Contact/Reply

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