Few people take the time to give thanks to American astrologer Richard Noelle. Astrology as a whole may not have contributed much to the advancement of science, but that doesn’t mean that an astrologer’s ideas can’t have a very big impact. In 1979, Noelle had a good idea indeed, when he coined the now-ubiquitous term “supermoon.”
Prior to Noelle’s brainstorm, the common descriptor for a full moon that occurs at the low point, or perigee, in its orbit around the Earth was a “perigean full moon,” a turgid bit of phrasing not remotely in keeping with the loveliness of the phenomenon. This year has been a good one for supermoons, with three occurring already—on July 3, Aug. 1, Aug. 30—and a fourth one teed up to appear in the eastern sky just after sunset on Sept. 28. September’s supermoon will reach its brightest illumination at 5:58 a.m. ET on Sept. 29, and set shortly after.
During the time the supermoon is riding across the sky there will be plenty to see. For one, the moon will not make its appearance alone. About an hour before it becomes visible, Saturn will rise and precede the moon across the sky throughout the night. Jupiter will then appear, about 90 minutes after the moon does, and similarly fly in tandem with it.
Then there’s the spectacular sight of the moon itself. The average distance between the Earth and the moon is 384,400 km (238,855 mi.). But that figure has a lot of wiggle room. The moon’s path around the Earth is elliptical; at its most distant point, or apogee, it is 405,500 km (253,000 mi.) away; at its perigee, it is just 363,300 km (226,000 mi.) from us.
A full moon during such close approaches appears to be as much as 14% larger than an ordinary full moon. Not everyone is impressed by this. In a post on X (formerly Twitter), in 2017, astrophysicist, author, and television personality Neil deGrasse Tyson wrote: “If last month’s Full Moon were a 16.0 inch pizza, then this month’s ‘Super’ moon would be 16.1 inches. Just saying.” In a follow-up post, he added: “If a 16.1 inch pizza is ‘super’ to you, compared with a 16.0 inch pizza, then we have an issue of vocabulary.”
But the visual impact a supermoon can make is powerful—especially when it has just risen or is about to set, thanks to the straightforwardly named “moon illusion.” When the moon is in the sky but hugging the horizon, there are lots of relatively small surface features like trees and houses in the foreground. The farther away from us those objects are, the smaller they seem to be. The occipital lobe and other sight-processing centers of the brain act in error on this information, comparing the apparent size of the moon to the apparent size of the little objects and concluding that the moon must be huge to dwarf them so easily. It then serves up a perception in keeping with that conclusion. That makes the 14% edge a supermoon has over a standard issue full moon seem even larger. As Tyson points out, the illusion is especially pronounced in cities, because skyscrapers provide the same point of reference as houses and trees, and they do so even when the moon is well above the horizon.
(In case you want to shatter the moon illusion, NASA has a way. When the full moon is near the horizon, extend your arm and raise your index finger. The moon will appear to be about the same size as your fingernail. Now repeat the comparison when the moon is high in the sky; the fingernail and the moon will still be of equivalent sizes.)
If you feel like you’ve been hearing more about supermoons lately, you’re not wrong. Supermoons have become more common than they used to be—but not because the moon has gotten any closer to the Earth. Rather, it’s that astronomers, as well as NASA, have, since at least 2001, changed the standards of what constitutes a supermoon, defining it as a full moon that comes within 90% of perigee, rather than requiring it to reach that closest 226,000 mi. approach. That new standard has slowly caught on over the years with more and more moons earning the “super” label.
September’s supermoon, like all supermoons, will go by a nickname. A full moon in early July was dubbed a Buck Moon by Native Americans because it appears at the same time male, adult deer begin to grow new antlers. Early August brought us the Sturgeon Moon, another legacy from Native Americans, who chose the name because August is the peak fishing month for sturgeon in the Great Lakes. The supermoon that appeared in late August was known as the Blue Supermoon. The “blue” part was a gift not from Native Americans, but from amateur astronomer Hugh Pruett, who in 1946 wrote an article for Sky & Telescope magazine, and, for reasons never quite clear, chose the color blue to describe the second full moon in a month. Supermoons occur every three or four months, but a Super Blue Moon is much less common—happening, on average, just once a decade. The upcoming super moon on Sept. 29 is known as the Harvest Moon, a name conferred by both Native American and Colonial planters because that was the month in which they reaped their crops. The first use of “Harvest Moon” has been traced to 1706, when the Oxford English Dictionary published the name.
After this Harvest Moon passes below the horizon, the curtain will come down on all supermoons in 2023. The sky shows will resume next year, with three supermoons in succession: on Feb. 9, March 10, and April 8. They’re worth waiting for. Supermoons are stunning, fun, and entirely free—a little cosmic dance the Earth and moon have been performing for more than four billion years now.
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Write to Jeffrey Kluger at firstname.lastname@example.org