Photo via Tim Geers.
Tonight – March 31, 2018 – presents the second and final Blue Moon of the year. By popular acclaim, the Blue Moon is often defined as the second of two full moons to occur in one calendar month. Tonight’s moon is very unlikely to appear blue in color, though, as exhibited by the playful photographer in the above photo.
Two Blue Moons in one year seem to belie the famous statement: Once in a Blue Moon. Yet it is quite rare to have two Blue Moons in the same 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 sports no full moon, whereas January and March both harbor two full moons. The full moons on January 31, 2018, and March 31, 2018, both count as Blue Moons.
February is the only calendar month whereby it’s possible for no full moon to take place. February with no full moon last happened 19 years ago, in 1999, presenting a Blue Moon on January 31, 1999, and again on March 31, 1999.
Amazingly enough, 19 years later – in the year 2037 – February 2037 again has no full moon, and the months of January and March both feature two full moons. So, after today (March 31, 2018), the second of two Blue Moons in one calendar year will next occur on March 31, 2037.
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 full moon). 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 are 13 full moons in one calendar year – plus 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 give us 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).
The 21st century (2001 to 2100) presents only three years with no February full moon and two Blue Moons: 2018, 2037 and 2094.
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.