Tonight – March 31, 2018 – that full moon you’ll see in the sky all night carries the name Blue Moon. In fact, it’s the second and final Blue Moon of this year. By popular acclaim, a Blue Moon is defined as the second of two full moons to occur in one calendar month. January had two full moons. Now March does, too.
Two Blue Moons in one year seem to belie the idea that once in a Blue Moon indicates something rare. It’s true that, in recent years – with more than one definition for Blue Moon – Blue Moons seem to happen pretty often. Yet it’s indeed quite rare to have two Blue Moons in a single calendar year. It last happened in 1999 and won’t happen again until 2037.
A calendar year only embraces two Blue Moons if there are 13 full moons in one calendar year – and, in addition, February has no full moon at all. That’s exactly what happens in 2018. February didn’t have a full moon, while January and March both have two full moons. Thus – in 2018 – the full moons on January 31 and March 31 both count as Blue Moons.
February is the only calendar month that can go without a full moon. Prior to 2018, we last had a February with no full moon 19 years ago, in 1999. And – as you might expect – we also had two Blue Moons in 1999, on January 31 and March 31. Nineteen years from now – in the year 2037 – February again will have no full moon, and the months of January and March will each feature two full moons.
Is there something special about the period of 19 years, with respect to Earth’s moon? Yes. In periods of 19 years, the phases of the moon fall on or near the same calendar dates because 19 calendar years are nearly commensurate with 235 lunar months (235 returns to the same phase). There are 235 full moons yet only 228 calendar months (19 x 12 = 228) in this 19-year cycle. Therefore, 7 of these 228 calendar months have to harbor an extra full moon (235 – 228 = 7 extra full moons).
Moreover, if there is a February with no full moon, we can count on an 8th full moon to fall into the lap of another month. Therefore, we have a total of 8 Blue Moons in the upcoming 19-year Metonic cycle:
1. October 31, 2020
2. August 31, 2023
3. May 31, 2026
4. December 31, 2028
5. September 30, 2031
6. July 31, 2034
7. January 31, 2037
8. March 31, 2037
Nineteen years, and 38 years after 2037 – the years 2056 and 2075 – won’t present a February with no full moon. It’ll be a near-miss both times, with the full moon falling on February 1 in both of these years. For this reason, there will be only one Blue Moon in 2056 (March 31) and one Blue Moon in 2075 (April 30).
Finally, 57 years (19 + 19 + 19 = 57) after 2037 – the year 2094 – will deliver a February with no full moon and a year with two Blue Moons (January 31, 2094, and April 30, 2094). Then 19 years after that – in the year 2113 – will feature two Blue Moons (January 31, 2113, and May 30, 2113) and a February with no full moon at all.
Of course, we must emphasize that all these dates are based on Coordinated Universal Time (UTC), and may not necessarily apply to your time zone. For instance, at North American time zones, the year 2075 will actually showcase two Blue Moons but no full moon in February 2075.
Based on UTC, the 21st century (2001 to 2100) presents three years with no February full moon and two Blue Moons: 2018, 2037 and 2094.
Bottom line: Enjoy the second Blue Moon of the year on March 31, 2018. The second of two Blue Moons in one calendar year won’t happen again until March 31, 2037.
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.