Astronomy EssentialsHuman World

What is a supermoon?

Large bright full moon with two children standing in a field reaching toward it.
Happy supermoons, y’all! This great moon photo comes from EarthSky Facebook friend Rebecca Lacey in Cambridge, Idaho.

We have a supermoon on April 26-27, 2021. The crest of the full moon falls on April 27 at 3:31 UTC (evening of April 26 for us in the U.S.). The moon’s perigee will also fall on April 27, at 15 UTC. Click here for more.

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 astrologer Richard Nolle in 1979 – a full moon or new moon has to come within 90% of its closest approach to Earth to be dubbed a supermoon. In other words, any full moon or new moon that comes to within 224,791 miles or 361,766 km (or less) of our planet, as measured from the centers of the moon and Earth, can be called a supermoon, according to Nolle’s original and rather generous definition. That’s why you might hear about a number of supermoons in any given year.

We figure out “90% of the moon’s closest approach to Earth” by the year’s closest perigee (moon’s closest point to Earth for the year) and the year’s farthest apogee (moon’s farthest point from Earth for the year). In 2021, the closest perigee is 221,702 miles (356,794 km). The farthest apogee is 252,595 miles (406,512 km). So – reckoning it this way – any full moon or new moon coming closer than 224,791 miles (361,766 km), as measured from the centers of the Earth and moon, counts as a supermoon in 2021.

Full moon supermoons:

April 27, 2021: 222,212 miles (357,615 km)

May 26, 2021: 222,117 miles (357,462 km)

June 24, 2021: 224,662 miles (361,558 km)

The second of these full moon supermoons on May 26, 2021, presents the closest full moon supermoon of 2021. This May 2021 full moon will more closely align with lunar perigee – the moon’s closest point to Earth in its monthly orbit – than any other full moon in 2021.

Lunar perigee on May 26, 2021, at 1:52 UTC: 222,023 miles (357,311 km)

Full moon on May 26, 2021, at 11:14 UTC: 221,117 miles (357,462 km)

Of course, the closest and therefore the biggest full moon of the year will cause larger-than-usual perigean spring tides, which people living near the coast will surely see and discuss.

Read more: Tides and the pull of the moon and sun

By the way, the most distant and smallest full moon of the year will fall on December 19, 2021. Sometimes called a micromoon, it’ll be 252,235 miles (405,932 km) away. That’s a whopping 30,118 miles (48,470 km) farther away than the year’s closest and biggest full moon on May 26, 2021.

New moon supermoons:

November 4, 2021: 223,604 miles (359,856 km)

December 4, 2021: 221,708 miles (356,804 km)

The second of these new moon supermoons on December 4, 2021, will present the closest supermoon of 2021. This December 4 new moon will more closely align with lunar perigee – the moon’s closest point to Earth in its monthly orbit – than any other supermoon in 2021. In fact, it’ll be the only time this year that the moon will come closer than 357,000 km (221,830 miles).

New moon on December 4, 2021 at 7:34 UTC: 221,708 miles (356,804 km)

Lunar perigee on December 4, 2021 at 10:01 UTC: 221,702 miles (356,794 km)

The closest new moon of the year (like the year’s closest full moon) will cause larger-than-usual perigean spring tides.

Read more: Tides and the pull of the moon and sun

Extremely thin, threadlike crescent against blue background.
View larger. | You can’t see a new moon in the sky. It’s more or less between the sun and Earth for that monthly orbit and crosses the sky with the sun during the day. Here’s a cool photo taken at the instant of new moon – 07:14 UTC on July 8, 2013 – by Thierry Legault. Read more about this image.

Some astronomers complain about the name supermoon. They like to call supermoons hype. But supermoons aren’t hype. They’re special. Many people now know and use the word supermoon. We notice even some 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. That’s probably why the term supermoon has 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.

The hype aspect of supermoons probably stems from an erroneous impression people had when the word supermoon came into popular usage … maybe a few decades ago? Some people mistakenly believed a full supermoon would look much, much bigger to the eye. It doesn’t. Full 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 angular diameter of a supermoon is about 7% greater than that of the average-size full moon and 14% greater than the angular diameter of a micro-moon (year’s farthest and smallest full moon). Yet, a supermoon exceeds the area (disk size) and brightness of an average-size full moon by some 15%, and the micro-moon by some 30%. 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.

So go outside on the night of a full supermoon, and – if you’re a regular observer of nature – you’ll surely notice the supermoon is exceptionally bright!

A smaller full moon superimposed on a larger crescent moon.
Above, Peter Lowenstein superimposed a mini-moon (full moon at apogee, its farthest from Earth for that month) on a young crescent moon (covered over in earthshine) near perigee, its closest to Earth for that month. The size difference 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.

Diagram showing sun and moon lined up with ocean stretched toward moon.
Around each new moon (left) and full moon (right) – when the sun, Earth, and moon are located more or less on a line in space – the range between high and low tides is greatest. These are called spring tides. A supermoon – new or full moon at its closest to Earth – accentuates these tides. Image via
Two diagrams: Perigee on sun side of earth (new) and perigee opposite the sun (full).
About 3 or 4 times a year, or more often, a new or full moon coincides with the moon’s closest point to Earth, or perigee. There’s usually only a small difference – typically a couple of inches (or centimeters) – between these “perigean spring tides” and normal tidal ranges. But, at these times, if a storm strikes along a coastline, flooding can occur. Image via NOAA.

How often do we have supermoons? Often! But of course it depends on your definition of supermoon. Here’s a list of each year’s closest supermoon perigees from 2010 to 2021 (they all came closer than 357,000 kilometers or 221,830 miles):

January 30, 2010 (356,593 km or 221,577 mi)

March 19, 2011 (356,575 km or 221,565 miles)

May 6, 2012 (356,955 km or 221,802 miles)

June 23, 2013 (356,991 km or 221,824 miles)

August 10, 2014 (356,896 km or 221,765 miles)

September 28, 2015 (356,877 km or 221,753 miles)

November 14, 2016 (356,509 km or 221,524 miles)

January 2, 2018 (356,565 km or 221,559 miles)

February 19, 2019 (356,761 km or 221,681 miles)

April 8, 2020 (356,907 km or 221,772 miles)

December 4, 2021 (356,794 km or 221,702 miles)

There wasn’t an extra-close perigee in 2017 (by “extra-close,” we’re considering moons less than 357,000 kilometers or 221,830 miles from Earth). After November 14, 2016, the extra-close coincidence of full moon and perigee didn’t happen again until January 2, 2018.

Looking farther into the future, the perigee full moon will come closer than 356,500 kilometers (221,519 miles) for the first time in the 21st century (2001-2100) on November 25, 2034 (356,446 km or 221,485 mi). The closest full moon of the 21st century will fall on December 6, 2052 (356,425 km or 221,472 mi).

By the way, some astronomers call 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.

Line of photos of ascending moon, fading to dark red in middle of line.
View at EarthSky Community Photos. | The first supermoon of 2019 – on January 21 – underwent a total eclipse. Here’s a marvelous time-lapse image of that eclipse from Dennis Schoenfelder in Alamosa, Colorado. One frame every three minutes. Thanks, Dennis!

So, just how much closer are these close full and new moons? This year, 2021, the moon at its closest point to Earth resides 221,702 miles (356,794 km) away. At this juncture, the moon is said to be at 100% of its closest approach for the year.

In 2021, the moon at its farthest point swings out to 252,595 miles (406,512 km) from Earth. At that time, the moon is said to be at 0% of its closest approach.

At its closest point for the year, the moon is roughly 30,000 miles or 50,000 km closer than when the moon is most distant.

The recurring cycle of supermoons

The full moon supermoon series of 2021 will recur after 14 lunar months (14 returns to full moon). That’s because 14 returns to full moon almost exactly equal 15 returns to perigee, a period of about one year, one month, and 18 days.

The mean lunar month (full moon to full moon, or new moon to new moon) = 29.53059 days, whereas the mean anomalistic month (perigee to perigee, or apogee to apogee) = 27.55455 days. Hence:

14 lunar months (14 returns to full moon) x 29.53059 days = 413.428 days
15 anomalistic months (15 returns to lunar perigee) x 27.55455 days = 413.318 days

Given that supermoons recur in cycles of 413 days (about one year, one month and 18 days), we can can expect the full moon supermoons to come about one month and 18 days later next year, in 2022.

Full moon distance (June 14, 2022): 222,238 miles or 357,658 km
Full moon distance (July 13, 2022): 222,117 miles or 357,418 km
Full moon distance (August 12, 2022): 224,569 miles or 361,409 km

The new moon supermoon series will occur on November 4, 2021, and December 4, 2021, and then January 2, 2022.

Large full moon, slightly smaller full moon, split moon with one side large and the other small.
Here’s a comparison between the December 3, 2017, full moon at perigee (closest to Earth for the month) and the year’s farthest full moon in June 2017 at apogee (farthest from Earth for the month) by Muzamir Mazlan at Telok Kemang Observatory, Port Dickson, Malaysia. More photos of the December 2017 supermoon.

Bottom line: The full moons of April, May and June 2021 are supermoons. The new moon supermoons of 2021 will be the new moons of November and December 2021. The supermoon of May 26, 2021, is the closest and brightest full supermoon of the year.

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April 25, 2021
Astronomy Essentials

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Bruce McClure

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