At dusk on Monday, April 24, 2021 right across the world the year’s second of four “supermoons” will rise. It will be ever-so slightly bigger than most full Moons because of its closeness to Earth in its egg-shaped orbit, but not so much that you’ll notice.
It will still look spectacular at it appears on your eastern horizon at dusk—as all full Moons do—but while the effect on you will be slight, the effects of Monday’s “supermoon” on the natural world will be dramatic.
Routinely derided astronomers they may be, but geographers know only too well that “supermoons” are actual physical phenomenons with consequences for the natural world.
The most recent published research reveals that, according to a 25-year study, “supermoons” cause bigger tidal ranges, higher water levels and more severe erosion.
A “supermoon” is a full moon that appears much larger than a normal full Moon. Technically they’re known as perigee full Moons by astronomers. The Moon’s orbital path around Earth is a slight ellipse, so each month there’s a near-point (perigee) and a far-point (apogee). At perigee it appears a little larger than the average apparent size (a “supermoon”), and at apogee, a little smaller (a “micromoon”).
The second of four “supermoons” or “perigee full Moons” of 2021, April’s full Moon will appear about 6% larger than an average full Moon.
The daily rise and fall of sea levels are called tides. They are caused by the Moon’s gravitational pull on the oceans as it orbits Earth, but also the Sun’s gravitational pull. They combine during a New Moon and a full Moon.
The main physical effect of a supermoon is a king tide, which increases the risk of coastal inundation. A king tide is an unusually high tide that results from a stronger lunar gravitational force than normal.
It’s also known as a perigean spring tide and is an entirely predictable astronomical tide.
Although the effect is magnified the closer the Moon is to the Earth, a supermoon can occur at both New Moon and full Moon. In practice, a supermoon at New Moon is barely mentioned in the media, though its physical effects are just as strong.
That’s because the Moon aligns with the Sun and the Earth every 14 days. At full Moon the Earth is in between the Sun and the Moon, while at New Moon the Moon is between the Sun and the Earth. At both times of the month the resulting alignment causes a tidal force. When the Moon is closer to the Earth than normal during a New Moon or a full Moon—so, during a supermoon—that tidal force is increased.
The distance to the Moon from Earth’s center changes from 406,000 km at apogee to about 357,000 km at perigee.
The research showed a long-term correlation between erosion across the beach and the Moon’s cycles, and suggested that a supermoon increases the risk of more severe beach erosion near the shoreline. These supermoon-driven king tides are more likely to cause coastal disasters when they occur simultaneously with storm surges and high waves.
So as you gaze at the beautiful “supermoon” appearing in the east on Monday evening, bear in mind that its greater gravitational force is what really makes it an important event for our planet. As rising sea levels kick-in, supermoons and the king tides they bring could mean even worse flooding for coastal communities.
Wishing you clear skies and wide eyes.
I’m an experienced science, technology and travel journalist and stargazer writing about exploring the night sky, solar and lunar eclipses, moon-gazing, astro-travel, astronomy and space exploration. I’m the editor of WhenIsTheNextEclipse.com and the author of “A Stargazing Program for Beginners: A Pocket Field Guide” (Springer, 2015), as well as many eclipse-chasing guides.
Bruce McClure of EarthSky:
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!
- What Is a Supermoon?
- The Moon Illusion
- The Moon Phases
- The Moon’s Effect on Tides
- What Is a Micro Moon?
- How Can Full Moon Be in the Daytime?
- Is a Blue Moon Blue?
- The Moon’s Orbit
- The Far Side of the Moon
- What Is a Black Moon?
- What Are Moonbows?
- Full Moon Names
- Taking pictures of the Moon