Unless you’ve been living in outer space, you know that a new moon will pass in front of the sun on August 21, 2017, giving the mainland United States its first total solar eclipse since 1979. What you might not know is that some people will call this particular new moon a Black Moon. By one definition of the term, the Black Moon refers to the third of four new moons to take place in one season, with a season being the period of time between a solstice and an equinox (or vice versa).
Thus the August 21 eclipse can – and doubtless will, by some – be called a Black Moon eclipse.
Most of the time, there are only three new moons in one season. But if the first new moon comes early enough in the season, it’s possible for a fourth new moon to sneak in before that season comes to an end. That’s exactly what happens during the Northern Hemisphere summer (Southern Hemisphere winter) of 2017:
2017 Jun 21: June solstice
2017 Jun 24: new moon
2017 Jul 23: new moon
2017 Aug 21: new moon
2017 Sep 20: new moon
2017 Sep 22: Sept equinox
A Black Moon by the seasonal defintion occurs 7 times in 19 years. Every 19 years, the phases of the moon recur on or near the same calendar dates. For instance, let’s take a look 19 years from now, in the year 2036 (2017 + 19 = 2036):
2036 Jun 20: June solstice
2036 jun 24: new moon
2036 Jul 23: new moon
2036 Aug 21 new moon
2036 Sep 20 new moon
2036 Sep 22: Sept equinox
There are 235 lunar months (235 returns to new moon) in one 19-year Metonic cycle. On the other hand, there are only 228 solar months in this same period of time. The 76 seasons (19 years x 4 seasons = 76 seasons) consist of three solar months each, giving rise to a total of 228 solar months (76 seasons x 3 solar months = 228 solar months) in the 19-year Metonic cycle.
In short, there are 235 lunar months yet only 228 solar months in 19 calendar years. Therefore, it’s inevitable that 7 of these left-over new moons (235 – 228 = 7) must fall within the confines of 7 different seasons. Therefore, 7 of these 76 seasons have to harbor 4 new moons.
We list the 7 Black Moons (3rd of 4 new moons in one season) in the upcoming 19-year Metonic cycle:
2020 Aug 19
2023 May 19
2025 Aug 23
2028 Aug 20
2031 May 21
2034 Feb 18
2036 Aug 21
By the way, there will be a Black Moon eclipse on 2036 August 21, too, though it’ll only be a partial eclipse of the sun.
Meanwhile, the Black Moon total solar eclipse on 2017 August 21 will be total and, perhaps, the most-viewed total solar eclipse in history.
Bottom line: Black Moon is one name for the third of four new moons in a season. The August 21 new moon is the third of four new moons between the June 2017 solstice and September 2017 equinox. On August 21, the new moon will pass directly in front of the sun. Voila! Black Moon eclipse.
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.