A supermoon is a new or full moon closely coinciding with perigee – the moon’s closest point to Earth in its monthly orbit. An astrologer, Richard Nolle, coined the term supermoon over 30 years ago, but now many in astronomy use it as well. Are supermoons hype? In our opinion … gosh, no, just modern folklore. They’ve entered the popular culture (check out Sophie Hunger’s music video in this post, for example). And they can cause real physical effects, such as particularly high and low tides. According to the definition of supermoon coined by Nolle, the full moon of December was 2017’s only full supermoon. January, 2018 will have two full moons, and both will be supermoons. Follow the links below to learn more about supermoons.
When is the next supermoon? By Richard Nolle’s definition, a new moon or full moon has to come within about 361,000 kilometers (224,000 miles) of our planet, as measured from the centers of the moon and Earth, in order to be considered a supermoon.
By that definition, the two full moons in January 2018 are both supermoons. Although the moon turns precisely full at the same instant worldwide (January 2, 2018 at 2:24 Universal Time), the time – and possibly the date – of the full moon varies according to one’s time zone. At North American and U.S. time zones, the fullest moon falls on the evening of January 1 at these times:
22:24 (10:24 p.m.) Atlantic Standard Time (AST)
21:24 (9:24 p.m.) Eastern Standard Time (EST)
20:24 (8:24 p.m.) Central Standard Time (CST)
19:24 (7:24 p.m.) Mountain Standard Time (MST)
18:24 (6:24 p.m.) Pacific Standard Time (PST)
17:24 (5:24 p.m.) Alaska Standard Time (AKST)
16:24 (4:24 p.m.) Hawaiian Standard Time (HST)
Of this series of three supermoons (one in December 2017, two in January 2018), the January 2 supermoon will be the closest and largest. Thus it’ll be the closest and largest full supermoon of 2018.
Full moon distance (2017 Dec 3 at 15:47 UTC): 357,987 km
Lunar perigee distance (2017 Dec 4 at 8:42 UTC): 357,492 km
Full moon distance (2018 Jan 2 at 2:24 UTC): 356,846 km
Lunar perigee distance (2018 Jan 1 at 21:54 UTC): 356,565 km
Full moon distance (2018 Jan 31 at 13:27 UTC): 360,199 km
Lunar perigee distance (2018 Jan 30 at 9:54 UTC): 358,995 km
Some people will call the full supermoon on January 31 a Blue Moon, too. That’s because it’ll be the second of two full moons in one calendar month. Moreover, the January 31 supermoon will stage a total eclipse of the moon.
We can expect each full moon supermoon in this three-part succession to recur after 14 lunar months (14 returns to full moon). By the way, 14 lunar months represents a time period of about 1 year, 1 month and 18 days. In 2019, the second of three full moon supermoons will showcase the closest and largest supermoon of the series:
Full moon supermoons in 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
In 2018, a series of new moon supermoons will occur when the new moon closely pair ups with perigee on June 13, July 13 and August 11, 2018. The second of these three new moon supermoons on July 13, 2018, will present the closest new moon supermoon of 2018. Of course, these new moons will be invisible to the eye, unless you’re in a position to witness the partial solar eclipse on Friday, July 13, 2018.
Once again, the following “season” of three new moon supermoons in 2019 will recur after 14 lunar months (14 returns to new moon): August 1 and 30, plus September 28, 2019. Again, the second of these three new moon supermoons on August 30, 2019, will feature the closest new moon supermoon of 2019.
What is a supermoon? We confess: before a few years ago, we in astronomy had never heard that term. The term has only recently come into popular usage. Richard Nolle defined a supermoon as:
… a new or full moon which occurs with the moon at or near (within 90% of) its closest approach to Earth in a given orbit.
That’s a pretty generous definition, which is why there are so many supermoons. By this definition, according to Nolle:
There are 4-6 supermoons a year on average.
Some supermoons are full moons, and some are new moons. In the year 2018, any full moon or new moon coming closer than 361,554.9 km of Earth counts as a supermoon. In 2018, the closest perigee is 356,564 km and the farthest apogee lies 406,464 km distant. This is a difference of 49,899 km, so 90% of this difference is 44,909.1 km (0.9 x 49,899 = 44,909.1). So any full moon or new moon less than 361,554.9 km (406,464 – 44,909.1 = 361,554.9) from Earth enjoys supermoon status.
Some astronomers have complained about the name … but we like it! And it’s entered the popular culture. for example, Supermoon is the title track of Sophie Hunger’s 2015 album. It’s a nice song! Check it out in the video below.
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 the moon orbits Earth, it comes closest to Earth. That point is called perigee. The moon always swings farthest away once each month; that point is called apogee.
No doubt about it. Supermoon is a catchier term than perigee new moon or perigee full moon.
We first became familiar with the supermoon label in the year 2011 when the media used it to describe the full moon of March 19, 2011. On that date, the full moon aligned with proxigee – the closest perigee of the year – to stage the closest, largest full moon of 2011.
Spring tides will accompany the supermoons. Will the tides be larger than usual at the April, May and June 2017 new moons and the December 2017 full moon? Yes, all full moons (and new moons) combine with the sun to create larger-than-usual tides, but closer-than-average full moons (or closer-than-average new moons) elevate the tides even more.
Each month, on the day of the new moon, the Earth, moon and sun are aligned, with the moon in between. This line-up creates wide-ranging tides, known as spring tides. High spring tides climb up especially high, and on the same day low tides plunge especially low.
The closest new moon of the year on May 25 and the year’s closest full moon on December 3 are bound to accentuate the spring tide all the more, giving rise to what’s called a perigean spring tide. If you live along an ocean coastline, watch for high tides caused by these perigean full moons.
Will these high tides cause flooding? Probably not, unless a strong weather system accompanies the perigean spring tide. Still, keep an eye on the weather, because storms do have a large potential to accentuate perigean spring tides.
Dates of closest full supermoons in past and future years. More often than not, the one day of the year that the full moon and perigee align also brings about the year’s closest perigee (also called proxigee). Because the moon has recurring cycles, we can count on the full moon and perigee to come in concert in periods of about one year, one month and 18 days.
A lunar month refers to the time period between successive full moons, a mean period of 29.53059 days. An anomalistic month refers to successive returns to perigee, a period of 27.55455 days. Hence:
14 lunar months x 29.53059 days = 413.428 days
15 anomalistic months x 27.55455 days = 413.318 days
Therefore, the full moon and perigee realign in periods of about 413 days (one year and 48 days). So we can figure the dates of the closest full moons (<357,000 km) in past and future years:
Extra-close perigee full moons 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 in 2017 (<357,000 km) because the extra-close coincidence of full moon and perigee won’t happen again (after November 14, 2016) until January 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.
But, like many of you, we’ll have fun just calling ’em supermoons.
What is a Black Moon? We had never heard the term Black Moon until early 2014. It doesn’t come from astronomy, or skylore, either. Instead, according to David Harper, the term comes from Wiccan culture. It’s the name for the second of two new moons in one calendar month. January 2014, for example, had two new moon supermoons, the second of which was not only a supermoon, but a Black Moon. Does a Black Moon have to be a supermoon in order to be called Black? No. You can read more about Black Moons here.
The next Black moon by the above definition will occur on October 30, 2016. Sten Odenwald at astronomycafe.net lists some other names for the second new moon in a month: Spinner Moon, Finder’s Moon, Secret Moon.
However, we’ve also come across another definition for Black Moon: the third of four new moons in one season. This last happened with the new moon supermoon of February 18, 2015, because this particular new moon was the third of four new moons to take place between the December 2014 solstice and the March 2015 equinox. The next Black Moon by this definition will occur on August 21, 2017, to feature a Black Moon total solar eclipse in the United States.
Bottom line: But that December 3, 2017 full supermoon was the first in a series of three supermoons. The two full moons in January 2018 – on January 2 and 31 – also count as supermoons.
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.