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Blue Moon, What is it?

Bruce McClure
July 1, 2004

By popular acclaim, the second Full Moon of a calendar month is called a Blue Moon.

By popular acclaim, the second Full Moon of a calendar month is called a Blue Moon. The first Full Moon of July comes on the 2nd and the second Full Moon falls on the 31st -- a July Blue Moon. We won't have a Blue Moon again -- at least by this definition of "blue moon" -- till the year 2007. Here, in the New World, the Blue Moon will take place on May 31 of 2007; whereas in the Old World, it'll be on June 30 of 2007.

If the first Full Moon of the calendar year happens on or before January 11 (or if it's a leap year, on or before January 12), you can bet that a Blue Moon is in store for that year. When the first Full Moon comes early enough in the year, there will be thirteen full Moons within a calendar year. With an extra Full Moon, it's inevitable that at least one calendar month should have two Full Moons. Sometimes, it's even possible to have two Blue Moons in the same year -- like in 1999, when both January and March had a Blue Moon, while February had no Full Moon at all. Since the phases of the Moon usually repeat themselves on the same dates every nineteen years (the Metonic cycle), expect another double Blue Moon for the year 2018.

So if you look up the date for the first Full Moon for 2004, you'll see that it was January 7. This lets you know right off the bat that the year 2004 is a blue moon year.

Next year, in 2005, the first Full Moon will take place on January 25. That means there can only be twelve Full Moons in 2005. That being the case, there's one Full Moon for each calendar month; but with no thirteenth moon, there's no way for an extra Full Moon to accommodate any calendar month. Hence, next year there will be no Blue Moon.

As suggested earlier, there are other definitions for a Blue Moon. In fact, the popularly accepted definition of Blue Moon -- the second Full Moon of a clendar month -- is thought to be a misinterpretation of the "original" or preceding definition of Blue Moon. (Read about it right here.) This "Old Style" Blue Moon will come next year, on August 19, 2005.

An "Old Style" Blue Moon refers to the third of four Full Moons in one season. In 2005, there's a Full Moon on June 21, July 21, August 19 and September 17. All four of these Full Moons come to pass between the June solstice and the September equinox. The third of these, the Full Moon of August 19, qualifies as an "Old Style" Blue Moon . And since the phases of the Moon repeat themselves on the same dates every nineteen years, August 19, 2024 will also stage an "Old Style" Blue Moon.

There's a surefire way to tell if the upcoming year will have an "Old Style" Blue Moon or not. If the last Full Moon of this calendar year comes after the December solstice but before New Year's Day, then it's inevitable -- the following year will have an "Old Style" Blue Moon. The last Full Moon in 2004 is on December 26, which is definitely after the December 21st solstice and before New Year's Day 2005. So we can count on the year 2005 to feature an "Old Style" Blue Moon.

In the year 2006, the first Full Moon of the year is on January 14. Whenever the first Full Moon falls around middle January, there is neither a "New Style" nor an "Old Style" Blue Moon. In this scenario, only twelve Full Moons happen during the calendar year in 2006; and only twelve Full Moons come to pass between the December solstice of 2005 and the December solstice 2006. There is no extra thirteenth Full Moon to make two Full Moons in a calendar month, or to make four Full Moons in a season -- hence, no "New Style" or "Old Style" Blue Moon.

"New Style" and "Old Style" Blue Moons are very unlikely to happen in the same calendar year. There are always thirteen Full Moons in a calendar year that hosts a "New Style" Blue Moon, and almost always twelve Full Moons during a calendar year that features an "Old Style" Blue Moon. Exceptions to this rule of thumb truly come but once in a blue moon -- like in the year 2048, when there's both a "New Style" and an "Old Style" Blue Moon!

copyright 2004 by Bruce McClure
Re-published on Astronomy Net with permission of the Author

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