What’s a supermoon? We’re about to have 4 in a row
Deborah Byrd and EarthSkyVoices
The coming full supermoon will be on August 19, 2024. This full moon is also a Blue Moon.
The full supermoon of September 18, 2024, will be a Super Harvest Moon. Plus, it’ll undergo a partial lunar eclipse.
The full supermoons in 2024 will be:
Aug 19: 224,917 miles (361,969 kilometers)
Sep 18: 222,131 miles (357,485 kilometers)
Oct 17: 222,055 miles (357,363 kilometers)
Nov 15: 224,853 miles (361,866 kilometers)
What’s a supermoon?
2024 has a lot of supermoons! There were five new supermoons earlier this year, and now we will have four full supermoons in a row, for the coming four months. So … what’s a supermoon?
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. And 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.
What’s our source?
Note that different people define supermoons slightly differently. We use Fred Espenak’s full supermoon table to give us dates and moon distances for full supermoons in 2024. Contrast the moon distances above – for 2024’s four full supermoons – to the moon’s average distance of 238,900 miles (384,472 km).
Also note that Fred’s dates and times are in UTC. So some supermoons may fall on the previous date your local time.
Supermoons came from popular culture
Where did the term supermoon originate? 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.
As mentioned above, 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.
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 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.
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.
Most of us can’t tell any difference in the size of a supermoon and an ordinary full moon.
Is a supermoon brighter?
But … supermoons do look brighter than ordinary full moons! 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 15%. And then, it exceeds the disk size of a micromoon (a year’s most distant and therefore smallest full moon) up to 14% and the brightness of a micromoon 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 micromoon 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.
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. According to Fred Espenak, the gravity from the closest supermoon is only 4% greater than the gravity from the moon at its average distance.
How often do we have a supermoon?
Often! But it also depends on your definition of supermoon.