Every calendar year has at least four eclipses – two solar and two lunar. More rarely, we have five, six or even seven eclipses in a single year. But four eclipses per calendar year is the most common number. A solar eclipse always comes within approximately two weeks of a lunar eclipse, and usually in a single pair (one solar and one lunar). Then, generally, another pair of eclipses (one solar and one lunar) comes some six months later. But not so in 2018.
In 2018, we have a total of five eclipses. Beginning a few days from now with the Friday the 13th supermoon solar eclipse, we actually have three eclipses in the span of one lunar month (the time period between successive new moons or full moons):
2018 Jan 31: Total lunar eclipse
2018 Feb 15: Partial solar eclipse
2018 July 13: Partial solar eclipse
2018 July 27: Total lunar eclipse
2018 Aug 11: Partial solar eclipse
So how often do we get three eclipses in one month? Let the investigation begin …
Three eclipses in one calendar month. According to NASA eclipse expert Fred Espenak, three eclipses fall in the same calendar month only 12 times during the five-century span from 1801-2300. Six times there are two solar eclipses and one lunar eclipse in one calendar month. Six times there are two penumbral lunar eclipses and a total (or annular) solar eclipse in one calendar month.
The last time we had three eclipses in a calendar month was in July 2000, when two partial solar eclipses bracketed a total lunar eclipse:
2000 July 01: Partial solar eclipse
2000 July 16: Total lunar eclipse
2000 July 31: Partial solar eclipse
(We wish to state parenthetically that these three eclipses happened exactly one Saros period – or exactly 223 lunar months – before the eclipses of July 13, 27, and August 11, 2018.)
Previous to July 2000, the last time three eclipses took place in one calendar month was in March 1904, when two penumbral lunar eclipses bracketed an annular solar eclipse.
1904 March 02: Penumbral lunar eclipse
1904 March 17: Annular solar eclipse
1904 March 31: Penumbral lunar eclipse
After July 2000, three eclipses will next occur within one calendar month in December 2206:
2206 Dec 01: Partial solar eclipse
2206 Dec 16: Total lunar eclipse
2206 Dec 30: Partial solar eclipse
Three eclipses in one lunar month. Some might argue that the calendar month is an artificial constraint. It might be more appropriate to use a lunar (or synodic) month, which is a natural unit of time. A lunar month refers to time period between successive new moons, or successive full moons.
Although it is rare for three eclipses to happen in the same calendar month, it’s not that uncommon for three eclipses to occur in one lunar month. In fact, from the years 2000-2050, the three-eclipses-in-one-month phenomenon takes place a total of fourteen times. Six times, the lunar month features two solar eclipses and one lunar eclipse (2000, 2011, 2018, 2029, 2036 and 2047). Eight times, the lunar month presents two lunar eclipses and one solar eclipse (2002, 2009, 2013, 2020, 2027, 2031, 2038 and 2049).
Three eclipses last took place in one lunar month in the year 2013:
2013 April 25: Partial lunar eclipse
2013 May 10: Annular solar eclipse
2013 May 25: Penumbral lunar eclipse
Previous to 2013, three eclipses last took place in one lunar month in 2011:
2011 June 01: Partial solar eclipse
2011 June 15: Total lunar eclipse
2011 July 01: Partial solar eclipse
After 2013, three eclipses in one lunar month will next occur in 2018:
Bottom line: In one calendar month, three eclipses are rare. But in one lunar month, three eclipses are are more common. From 2000-2050, it happens 14 times.