Astronomy Essentials

What is a supermoon? And when, in 2023?

New moon happens when the moon (in its monthly orbit of Earth) goes more or less between the sun and Earth.

Full moon happens when the moon (in its monthly orbit) is on the opposite side of Earth from the sun.

Perigee – from the root words peri meaning near and geo meaning Earth – is the moon’s closest point to Earth in a month.

So the new or full moon closely coincides with perigee several times each year. When that happens, in the language of popular culture, we have a supermoon.

Supermoon: Bright full moon with a woman and a girl watching it.
View at EarthSky Community Photos. | Radu Anghel in Parjol, România, caught what he called “a family photo with the supermoon of July 13, 2022.” Thank you, Radu! While supermoons don’t appear bigger to the eye than other full moons, they do appear brighter!

What is a supermoon?

As it’s used today, the word supermoon was coined by astrologer Richard Nolle in 1979. And according to Nolle’s definition, a full moon or new moon is a supermoon when it’s also within 90% of its closest point to Earth. However, different websites calculate supermoons differently. EarthSky uses supermoon dates as determined by astronomer Fred Espenak. He’s best known for his time at the Goddard Space Flight Center, where he became a world expert on eclipse predictions. Additionally, his method of calculating supermoons takes into account changes in the moon’s orbit during each lunar cycle.

New supermoons in 2023

Fred Espenak’s new supermoon table gives us these values – dates and moon distances – for new supermoons in 2023. Contrast these moon distances to the average moon distance of 238,900 miles (384,472 km).

Note dates are based on UTC time so some supermoons may fall on the previous date.

Jan 21: 221,562 miles (356,571 km)
Feb 20: 223,112 miles (359,065 km)
Mar 21: 227,522 miles (366,161 km)

The supermoon of January 21, 2023, was the closest new supermoon for 2023.

Read more: The 1st of 5 new supermoons in a row

Full supermoons in 2023

Fred Espenak’s full supermoon table gives us these values – dates and moon distances – for full supermoons in 2023. Contrast these moon distances to the average moon distance of 238,900 miles (384,472 km).

Note: Fred’s dates are based on UTC time, so some supermoons may fall on the previous date. Those supermoons are listed below with the overnight dates when they’ll appear most full.

Jul 2-3: 224,895 miles (361,934 km)
Aug 1: 222,158 miles (357,530 km)
Aug 30-31: 222,043 miles (357,344 km)
Sep 28-29: 224,658 miles (361,552 km)

Note dates are based on UTC time so some supermoons may fall on the previous date.

The supermoon of August 30-31, 2023, will be the closest full supermoon for 2023 and it’s also a monthly Blue Moon. That is because it’s the second full moon in a calendar month. By the way, the next monthly Blue Moon is not until May 31, 2026.

Read more: 4 full supermoons in a row in 2023

2 photos of half the moon facing each other. The one on the left is bigger.
Eliot Herman in Tucson, Arizona, compared the June 2017 full moon with the November 2016 supermoon. He wrote: “A comparison of the closest supermoon since 1948 on November 14, 2016, with farthest micro-moon of 2017. Both images captured with a Questar telescope and a Nikon D800 camera. The images were combined with Photoshop.” Thank you, Eliot![

Supermoon hype?

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. In fact, we even notice some diehards are starting to use it now. Such is the power of folklore.

The hype aspect of supermoons probably stemmed 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. But it doesn’t. Nowadays, most people seem to realize that supermoons don’t look bigger to the eye than ordinary full moons.

While it’s true experienced observers do say they can detect a difference. But you’d have to be a very keen observer to notice it. Truly, most of us can’t tell any difference in the size of a supermoon and an ordinary full moon.

Is a supermoon brighter?

But … do supermoons look brighter than ordinary full moons? Yes! By a noticeable amount. By a noticeable amount. That’s because a supermoon exceeds the disk size of an average-sized moon by up to 8% and the brightness of an average-sized full moon by some 16%. And then, it exceeds the disk size of a micro-moon (a year’s most distant and therefore smallest full moon) up to 14% and the brightness of a micro-moon by some 30%. So go outside on the night of a full supermoon. Even if you’re a casual observer of the moon, there’s the potential you’ll notice the supermoon is exceptionally bright!

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. Again, that difference isn’t noticeable to the eye at the moon’s distance. But the brightness difference is noticeable.

By the way, before we called them supermoons, we in astronomy called these moons perigean full moons, or perigean new moons. No doubt about it. Supermoon is catchier.

Bright golden moon rising over dark trees.
View at EarthSky Community Photos. | Nathaniel Adam Cruz in Cagayan de Oro City, Philippines, captured the July supermoon on July 14, 2022, and said: “After July’s full moon reached its peak at 2:38 a.m., the biggest supermoon of 2022 sets on the hillside ridge of Carmen Hill in Cagayan de Oro City, Philippines.” Thank you, Nathaniel.

High tides from new and full supermoons

What’s more, all supermoons have the potential to cause higher-than-usual tides. High tides during the full or new moon are called spring tides. High tides during a full or new moon at perigee are called perigean spring tides, or, sometimes, king tides. And nowadays you sometimes hear them called supermoon tides.

These perigean or king or supermoon spring tides tend to follow the date of new or full moon by a day or so. These especially high tides are highly dependent on the shapes of local coastlines and on local weather conditions.

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.

2 diagrams: the sun, moon and Earth, and their positions during new moon and full moon.
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 a supermoon?

Often! But it also depends on your definition of supermoon.

Here’s a list of each year’s closest full supermoon perigees from 2015 to 2025 (from Espenak’s full supermoon table):

September 28, 2015 (356,878 km or 221,754 miles)
November 14, 2016 (356,523 km or 221,533 miles)
December 3, 2017 (357,987 km or 222,443 miles)
January 2, 2018 (356,604 km or 221,583 miles)
February 19, 2019 (356,846 km or 221,734 miles)
April 8, 2020 (357,035 km or 221,851 miles)
May 26, 2021 (357,462 km or 222,117 miles)
July 13, 2022 (357,418 km or 222,089 miles)
August 31, 2023 (357,344 km or 222,043 miles)
October 17, 2024 (357,364 km or 222,055 miles)
November 5, 2025 (356,980 km or 221,817 miles)

Orange orb, darker on the bottom, peeking out from between high-rise buildings in deep twilight.
View at EarthSky Community Photos. | Kannan A in Singapore captured this photo of the setting supermoon in the early morning of April 27, 2021. He wrote: “The April 2021 ‘Pink’ supermoon descending in the northwest of Singapore this morning before the sunrise in the east. The moon was 99.9% full and was about 357,615 km (222,212 miles) away from Earth. The moon was really super huge when it was seen in contrast with the high rise public housing standing at 30 stories high. The sky was favorable throughout the night, and the moon was clearly visible from the rise to the setting.” Thank you, Kannan A!

The recurring cycle of supermoons

The full moon supermoon series of 2023 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

So given that supermoons recur in cycles of 413 days (about one year, one month and 18 days), we can expect the full moon supermoons to come about one month and 18 days later next year, in 2024.

Bottom line: In 2023, new moon supermoons were the new moons of January, February and March. Then the full moons of July, two in August and one in September are full supermoons. The supermoon of August 31, 2023, is the closest and brightest full supermoon of the year and also a Blue Moon.

Read more: Does a supermoon have a super effect on us?

June 5, 2023
Astronomy Essentials

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Deborah Byrd

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