If the moon is full when it passes closest to Earth in its elliptical journey around the planet—as it was last week—it’s called a “supermoon,” because the shorter distance makes the shining orb appear larger and brighter than normal.

But to the irritation of the scientific community, the word was coined by an astrologer to describe a phenomenon most scientists contend the human eye can’t detect.

Richard Nolle invented the neologism for an article published in Dell Horoscope magazine in 1979. It captured the imagination of the public and—perhaps adding insult to injury—eclipsed the technical term for the event.

In astronomical language, the phenomenon would be called “perigee syzygy.”

Even professional stargazers admit that’s not as catchy.

“That’s a mouthful,” said Jackie Faherty, an astrophysicist at the American Museum of Natural History. “Supermoon sounds a lot better.”

Mr. Nolle said the name refers to a full or new moon occurring within 90% of its closest approach to Earth.

“I figured nobody’s tongue could handle too many repetitions of perigee syzygy, so I coined a term,” he said.

The inspiration, Mr. Nolle recalled, was Supermon, a type of computer software at the time. “I thought Supermon, Superman, oh, supermoon, there you go.”

A full moon occurs once every 29.5 days. Perigee is the point at which the moon passes closest to Earth during its oval-shaped orbit; apogee is the point at which it’s farthest away; and syzygy is when the moon, Earth and sun perfectly align, something that happens with both a full and new moon.

There are only three or four supermoons each year, and because they typically don’t happen precisely at perigee, their distance from Earth varies.

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Last week, perigee occurred at 9:50 p.m. ET on May 25, when the moon was 222,023 miles from Earth.

But the moon wasn’t completely full until nearly 7:19 a.m. on May 26.

“It missed perigee by nine hours,” said Kelly Beatty, senior editor of Sky & Telescope magazine, who noted it was the closest a full moon will come to Earth this year.

Still, it wasn’t the superest of supermoons.

On Nov. 14, 2016, a full moon came within 221,524 miles of Earth and was the closest since Jan. 26, 1948, when one passed within 221,494 miles.

In the future, some will come even closer.

On Nov. 25, 2034, a full moon and perigee will occur within a half-hour of each other, for a supermoon that will be 221,485 miles away.

The nearest supermoon of this century will occur on Dec. 6, 2052, when the full moon will be just 221,470 miles away.

“ ‘I figured nobody’s tongue could handle too many repetitions of perigee syzygy, so I coined a term.’ ”

— Richard Nolle, the astrologer who came up with the word ‘supermoon’

Each of those distances is roughly 17,000 miles closer than average, which might sound like a lot. But the collective assessment of astronomers might be summed up as: big whoop.

A supermoon, they observe, looks so similar in size to a regular moon that no one gazing at the sky can truly distinguish the difference, a point that astrophysicist Neil deGrasse Tyson has made repeatedly, and vehemently.

On Twitter, he called the concept an embarrassment.

On radio, he said, “There is no supermoon.”

On YouTube, he compared two pizzas of nearly the same size to illustrate how inconsequential the variance is. On the scale of pizza, he said, the difference would be about half an inch.

“If that half inch is super to you on a pizza, call the moon whatever the hell you want,” he says in the video. “But in a pizza shop, nobody here’s calling this one super.”

According to NASA, at the moment of perigee, when the moon is closest to Earth, it appears to be about 17% bigger and 30% brighter than the most distant moon of the year.

Borrowing Dr. Tyson’s example, if it were the size of 16-inch pizza, that would make the average moon 15.36 inches and the most distant 14.79 inches.

Because supermoons typically don’t occur exactly at perigee, the difference would be somewhat slighter.

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If last week’s supermoon were the size of a 16-inch pizza, the average moon would measure 15.47 and the most distant 14.99 inches.

“Even I’m like, yeah, I’m not sure I can actually tell,” Dr. Faherty said.

But maybe that’s not the point.

“If you believe it’s a poorly defined word not defined by anything science guides, then you hate it,” Dr. Faherty said. “If you like that it gets the public excited about astronomy, then you love it.”

Or, as Mr. Nolle, the astrologer, put it: “Just because you can’t see it doesn’t mean it isn’t real.”

Write to Jo Craven McGinty at Jo.McGinty@wsj.com