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 larger-than-usual tides. According to the definition of supermoon coined by Nolle, the year 2017 has a total of four supermoons. The new moons of April, May and June and the full moon of December all qualify as supermoons. Follow the links below to learn more about supermoons.
What is a supermoon? We confess: before a few years ago, we in astronomy had never heard that term. To the best of our knowledge, astrologer Richard Nolle coined the term supermoon over 30 years ago. The term has only recently come into popular usage. Nolle has 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 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.
When are the supermoons of 2017? By Nolle’s definition, the new moon or full moon has to come within 362,146 kilometers (225,027 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 year 2017 has a total of four supermoons. The first supermoon, for 2017, comes with the April 26 new moon. The new moons on May 25 and June 24 are also considered supermoons, according to Nolle’s definition, and that same definition dictates that the full moon of December 2017 – plus the full moons on January 2 and 31, 2018 – will be supermoons, too. Thus, three successive full moon supermoons (aka near-perigee full moons) come in December 2017 and January 2018:
Full moon of 2017 December 3 at 15:47 UTC
Full moon of 2018 January 2 at 2:24 UTC
Full moon of 2018 January 31 at 13:27 UTC
The new moon on May 25, 2017 will present the closest supermoon of the year (357,265 kilometers or 221,994 miles). What’s more, this will be the first time since the year 2009 that the new moon (instead of the full moon) ushers in the year’s closest supermoon. It’ll also be the first time since 2009 that the closest supermoon didn’t come closer than 357,000 km.
Want more detail? Okay. In 2017, the moon comes closest to Earth on May 26 (357,207 kilometers), and swings farthest away on December 19 (406,603 kilometers). That’s a difference of 49,396 kilometers (406,603 – 357,207 = 49,396). Ninety percent of this 49,396-figure equals 44,456.4 kilometers (0.9 x 49,396 = 44,456.4). Presumably, any new or full moon coming closer than 362,146.6 kilometers (406,603 – 44,456.4 = 362,146.6) would be “at or near (within 90% of) its closest approach to Earth.”
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
2011 Mar 19
2012 May 06
2013 Jun 23
2014 Aug 10
2015 Sep 28
2016 Nov 14
2018 Jan 02
2019 Feb 19
2020 Apr 08
There won’t be 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: The term supermoon doesn’t come from astronomy. It comes from astrology, and the definition is pretty generous so that there are about 6 supermoons each year. This post explains what a supermoon is, how many will occur in 2017, which moon is the most “super” of all the 2017 supermoons, and gives a list of upcoming full supermoons for the years ahead.
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.