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What’s a seasonal Blue Moon?

Image credit: Robert Couse-Baker

Tonight for May 20, 2016

Most likely, the May 2016 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. The moon will probably appear full to the eye tonight (May 20-21), but it’s really a full-looking waxing gibbous moon. The moon turns full on May 21, at 21:14 Universal Time.

That brilliant “star” following the moon across the sky all night long tonight is actually the planet Mars, shining at its brilliant best for the year! You’ll see the moon and Mars even closer together as darkness falls tomorrow, on May 21.

Look for the moon to pair up with Mars on May 21, and Saturn on May 22. The green line depicts the ecliptic - Earth's orbital plane projected onto the dome of sky.

Look for the moon to pair up with Mars on May 21, and Saturn on May 22. The green line depicts the ecliptic – Earth’s orbital plane projected onto the dome of sky.

This year – May 21, 2016 – 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. This May 2016 full moon counts the third of four to fall between the March 2016 equinox and June 2016 solstice.

The first full moon of the season comes soon enough after the March 2016 equinox to allow for a fourth full moon to take place shortly before the June 2016 solstice:

Equinox: March 20, 2016

Full moon: Mar 23, 2016
Full moon: Apr 22, 2016
Full Moon: May 21, 2016 (Blue Moon)
Full moon: Jun 20, 2016 (11:02 Universal Time)

Solstice: June 20, 2016 (23:34 Universal Time)

Possible to have only two full moons in a single season?

However, the next Blue Moon by the most well-known definition of the term won’t happen until January 31, 2018. 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.

Click here to learn the history of Blue Moons

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 18, 2019
2) August 22, 2021
3) August 19, 2024
4) May 20, 2027
5) August 24, 2029
6) August 21, 2032
7) May 22, 2035

Astronomical almanacs usually list the time of full moon by Universal Time (UT). The May 2016 full moon comes on May 21 at 21:14 Universal Time. For the U.S. time zones, that translates to 5:14 p.m. EDT, 4:14 p.m. CDT, 3:14 p.m. MDT or 2:14 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 May 21, 2016 full moon is the third of four full moons to occur between the March equinox and the June solstice. That’s why it’ll be called by the name Blue Moon!

Image credit: Robert Couse-Baker

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