The next Blue Moon is November 21, 2010
November 15th, 2010 – Astronomy Essentials

There are two Blue Moon definitions in use today. The full moon on November 21 will be called a Blue Moon because it’s the third of four full moons in a season.

When is the next Blue Moon? November 21, 2010. Informed sky watchers know a Blue Moon isn’t really blue in color. So what makes the November 21 full moon a Blue Moon?

In the 21st century, Blue Moon has two meanings. A Blue Moon can be the second full moon in a calendar month. Or it can be the third of four full moons in a single season. The November 21, 2010 Blue Moon is the third of four full moons between the September equinox and December solstice.

Watch for November 21 Blue Moon near Pleiades star cluster

Although certain-sized particles of dust or smoke can cause a moon to look blue in color, the sorts of moons people commonly call Blue Moons aren’t really blue.

Every month has a full moon, and, most of the time, the names coincide with particular months of the year. By either definition, the name Blue Moon accounts for times when there happen to be more full moons than is convenient.

Second full moon in a month. In recent decades, many people have begun using the name Blue Moon to describe the second full moon of a calendar month.

The time between one full moon and the next is close to the length of a calendar month. So the only time one month can have two full moons is when the first full moon happens in the first few days of the month. This happens every 2-3 years, so these sorts of Blue Moons come about that often.

When is the next Blue Moon, according to this first definition? August 31, 2012.

Can there be two blue moons in a single calendar year? Yes. It last happened in 1999. There were two full moons in January and two full moons in March and no full moon in February. So both January and March had Blue Moons.

The next year of double blue moons is coming up in 2018.