How often do we have a seasonal Blue Moon?

Most likely, the August 2013 full moon won’t appear blue in color, although it’s a Blue Moon all the same – at least by one definition of the term. This year – August 21, 2013 – gives us a seasonal Blue Moon: the third of four full moons to occur in a single season. A season is defined as the period of time between a solstice and an equinox, or vice versa. Most often, a season only has three full moons, so a Blue Moon by this definition only comes 7 times in 19 calendar years.

However, the next Blue Moon by the most well-known definition of the term will happen on July 31, 2015. By popular acclaim, the second of two full moons to come in the same calendar month is also called a Blue Moon. A Blue Moon by this definition comes 7 or 8 times in 19 calendar years.


Hmm. That seems like a lot of Blue Moons. And doesn’t the phrase “once in a Blue Moon” indicate something precious and rare? Now that there are two well-known definitions for the term Blue Moon, perhaps they’re not that rare after all. So how often do we have a Blue Moon by the seasonal definition – the third of four full moons in one season? In a period of 19 years, there are 235 full moons but only 228 calendar months (76 three-month seasons). Given that there are 76 seasons and 235 full moons in 19 calendar years, it’s inevitable that at 7 of these 76 seasons should harbor four full moons. In other words, if you’re defining a Blue Moon as the third of four full moons in the same season, you’ll have a Blue Moon once every two to three years. The 7 seasonal Blue Moons in the next 19-year lunar cycle:

 1) May 21, 2016 

2) May 18, 2019 

3) August 22, 2021

 4) August 19, 2024

 5) May 20, 2027 

6) August 24, 2029

 7) August 21, 2032 

 By the way, we should mention that this August 2013 Blue Moon falls on August 21 for the world’s Eastern Hemisphere (Europe, Africa, Asia, Australia and New Zealand) but on August 20 in the Americas. Astronomical almanacs usually list the time of full moon by Universal Time (UT). The August 2013 full moon comes on August 21 at 1:45 Universal Time. For the U.S. time zones, that translates to August 20, at 9:45 p.m. EDT, 8:45 p.m. CDT, 7:45 p.m. MDT or 6:45 p.m. PDT.

 If you wish to find out the full moon time for your time zone, you must convert Universal Time to your time zone. Here’s how to translate Universal Time into your time zone Bottom line: For all of the world, the August 21, 2013 full moon is the third of four full moons to occur between the June solstice and the September equinox. That’s why it’ll be called by the name Blue Moon!

source and credit a earthsky