Above: Moonset over La Paz, capital city of Bolivia, on April 30, 2018, from Max Glaser.
May 29, 2018, ushers in the third of this season’s three full moons. In this case, when we say season, we mean the time period between an equinox and solstice. The March equinox was March 20. We had full moons on March 31 and April 30, and now this May 29 full moon. Most seasons, in fact, do have three full moons, and all full moons have names. Every so often, there’s a season with four full moons, and then a full moon lacks a moon name. In that case, one of those full moons carries the name Blue Moon. More about that below.
In the Northern Hemisphere, we call the May full moon a Flower Moon, Planting Moon or Milk Moon. In the Southern Hemisphere, this same full moon is the Hunter’s Moon, Beaver Moon or Frost Moon.
For the Northern Hemisphere, this May 2018 full moon counts as the final full moon of spring.
For the Southern Hemisphere, this May 2018 full moon is the final full moon of autumn.
What about next year? Checking the moon phase almanac, we find that – a year from now – a full moon happens on May 18, 2019. There’s another full moon on June 17, 2019. Throughout the 21st century (2001 to 2100) the June solstice falls on June 20 or 21. For that reason, we know that the 2019 June full moon will happen before the June 2019 solstice.
As it turns out, the 2019 May full moon will be the third of four full moons in between the March 2019 equinox and June 2019 solstice. The June 2019 full moon will be the fourth of that season’s four full moons. As mentioned above, most seasons have three full moons. But not so between the March equinox and June solstice of 2019:
Equinox: March 20, 2019
Full moon: March 21, 2019
Full moon: April 19, 2019
Full moon: May 18, 2019
Full moon: June 17, 2019
Solstice: June 21, 2019
By tradition, the third of a season’s four full moons is sometimes called a Blue Moon. Four full moons in one season is relatively rare. In a period of 19 calendar years, there are 76 seasons (19 x 4 = 76), yet only 7 of these 76 seasons have four full moons.
Bottom line: In 2018, we have 3 full moons between the March equinox and June solstice. Exactly 12 full moons from now – in May 2019 – we’ll be enjoying the 3rd of 4 full moons of this season. People will call it a Blue Moon.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.