A supermoon is a new or full moon closely coinciding with perigee, the moon’s closest point to Earth in its monthly orbit.
According to the original definition of supermoon – coined by Richard Nolle in 1979 – a full moon or new moon has to come within 90 percent of its closest approach to Earth to be dubbed a supermoon. In other words, any full moon or new moon that comes to within 224,775 miles or 361,740 km (or less) of our planet, as measured from the centers of the moon and Earth, is considered a supermoon.
This year, in 2019, the moon’s distance at its closest point to Earth is 221,681 miles or 356,761 km (100 percent of closest approach) and at its farthest point swings out to 252,622 miles or 406,555 km (0 percent of closest approach). At its closest point, the moon is approximately 30,000 miles or 50,000 km closer than at its farthest.
So, by Nolle’s definition, when is the next supermoon?
The next supermoon comes with the January 21 full moon, which, moreover, is to stage a total lunar eclipse . This supermoon on January 21 ushers in the first in a series of three full supermoons falling on January 21, February 19 and March 21, 2019. Of these, the February 19th full moon will showcase the closest and largest full supermoon of 2019.
Full moon distance (2019 Jan 21): 357,715 km
Full moon distance (2019 Feb 19): 356,846 km
Full moon distance (2019 Mar 21): 360,772 km
Still to come in 2019 … a series of new moon supermoons will occur when the new moon closely pairs up with perigee on August 1 and 30 plus September 28.
The second of these three new moon supermoons on August 30, 2019, will present the closest new moon supermoon of 2019. Of course, these new moons will be invisible to the eye, but they will bring about larger than usual perigean spring tides.
The full moon supermoon series will recur after 14 lunar months (14 returns to full moon, a period of about 1 year 1 month and 18 days):
Full moon distance (2020 Mar 09): 357,404 km
Full moon distance (2020 Mar 09): 357,035 km
Full moon distance (2020 Apr 07): 361,184 km
The new moon supermoon series will also recur after 14 lunar months (14 returns to new moon). Thus we’ll have new supermoons on September 17, October 16 and November 15, 2020.
Some astronomers have complained about the name supermoon … but we notice even the diehards are starting to use it now. Such is the power of folklore.
Before we called them supermoons, we in astronomy called these moons perigean full moons, or perigean new moons. Perigee just means near Earth.
The moon is full, or opposite Earth from the sun, once each month. It’s new, or more or less between the Earth and sun, once each month.
And, every month, as it orbits Earth, the moon comes closest to Earth, or to perigee. The moon naturally swings farthest away once each month, too; that point is called apogee.
No doubt about it. Supermoon is a catchier term than perigean new moon or perigean full moon.
Supermoons don’t look bigger to the eye than ordinary full moons, although experienced observers say they can detect a difference.
But supermoons do look brighter than ordinary full moons! The diameter of a supermoon is about 7 percent greater than the diameter of the average-size full moon and 14 percent greater than the diameter of a micro-moon (year’s smallest moon). Yet, a supermoon exceeds the disk size and brightness of an average-size full moon by some 15 percent, and that of a micro-moon by 30 greater percent. For a visual reference, the size difference between a supermoon and micro-moon is proportionally similar to that of a U.S. quarter versus a U.S. nickel.
What’s more, Earth’s oceans feel the extra pull of supermoons. All full moons (and new moons) combine with the sun to create larger-than-usual tides, called spring tides. But closer-than-average full moons (or closer-than-average new moons) – that is, supermoons – elevate the tides even more. These extra high spring tides are wide ranging. High tides climb up especially high, and, on the same day, low tides plunge especially low. Experts call these perigean spring tides, in honor of the moon’s nearness. If you live along an ocean coastline, watch for them! They typically follow the supermoon by a day or two.
Do extra-high supermoon tides cause flooding? Maybe yes, and maybe no. Flooding typically occurs when a strong weather system accompanies an especially high spring tide.
Extra-close supermoons from 2010 to 2020
2010 Jan 30 (356,593 km)
2011 Mar 19 (356,575 km)
2012 May 06 (356,955 km)
2013 Jun 23 (356,991 km)
2014 Aug 10 (356,896 km)
2015 Sep 28 (356,877 km)
2016 Nov 14 (356,509 km)
2018 Jan 02 (356,565 km)
2019 Feb 19 (356,761 km)
2020 Apr 08 (356,907 km)
There wasn’t an extra-close perigee full moon – a closest full supermoon – in 2017 (by “extra-close,” we’re considering moons less than 357,000 kilometers from Earth). After November 14, 2016, the extra-close coincidence of full moon and perigee didn’t happen again until January 1-2, 2018.
Looking further into the future, the perigee full moon will come closer than 356,500 kilometers for the first time in the 21st century (2001-2100) on November 25, 2034 (356,446 km). The closest full moon of the 21st century will fall on December 6, 2052 (356,425 km).
By the way, some astronomers will call all the full moons listed above proxigee full moons. The word proxigee just means an extra-close perigee.
But, like many of you, we’ll have fun just calling ’em supermoons.
Bottom line: The first three full moons of 2019 are supermoons. The next supermoons of 2018 will be the new moons of August 1 and 30 plus September 28.
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.