A full harvest moon will break the horizon at 7:32 tonight, foretelling cooler weather and darker days as the autumnal equinox approaches.
It comes with a penumbral eclipse for some parts of the Earth, but whether the September nighttime delight should be called a “supermoon” has ignited a lunar hullabaloo of stellar proportions.
Astronomers have long described a full moon that is closest to Earth in its orbit as a perigee full moon or perigee-syzygy.
Then, astrologer Richard Nolle, a University of Florida graduate, came up with the term supermoon, which he says on his website is “a whole lot easier on the tongue.”
The informal moniker of supermoon, like with the “Godzilla” El Niño, piqued people’s interest.
“As someone who has been doing this for 40 years, I will tell you that until we started calling it a supermoon, it got absolutely no attention,” said Deborah Byrd, editor in chief at the astronomy website Earth and Sky. “I think it’s an awesome descriptive word. But some people in astronomy don’t like it.”
That’s not because the scientific community is averse to kitschy names. There is concern that the very definition of supermoon depends on who is doing the defining.
September’s lunar perigee is technically not until Sunday, so tonight’s full moon will be super close to Earth — about 227,000 miles away — but not its closest this month. While a full moon in perigee does not necessarily appear more rotund to the naked eye, it can often be much brighter.
Under Nolle’s definition of a supermoon, which has to do with proximity to Earth, tonight’s is not a supermoon, but October, November and December will have supermoons.
Fred Espenak, a retired NASA astrophysicist also known as “Mr. Eclipse,” disagrees. He considers tonight’s full moon a supermoon under his own proximity definition. Espenak also includes the full moons of October, November and December as supermoons.
University of Florida associate scientist Francisco Reyes, director of the school’s teaching observatory, sides with Espenak on the definition of supermoon.
But Noah Petro, a research scientist at NASA Goddard Space Flight Center, said NASA is recognizing only one supermoon this year, and that’s on Nov. 14 when the full moon will be about 221,532 miles from Earth.
The November full moon will be the closest full moon to Earth since 1948, Petro said. It won’t be that close again until 2034.
“I don’t think we’ll do a big promotional campaign,” Petro said. “We feel the phrase supermoon may be getting overused. Maybe it has run its course.”
The single supermoon principle has also been adopted by Slooh, an online astronomy website that does live webcasts of astral events from observatories in the Canary Islands and Chile.
But it’s not calling tonight’s a supermoon.
“We have chosen to cover only one per annum — rechristening it the megamoon to differentiate it from the others,” said Paul Cox, a Slooh astronomer. “There are all kinds of arguments going on about the definition of what is a supermoon, so Slooh has now defined the main supermoon each year as the megamoon.”
Regardless of whether it is a supermoon, tonight’s full moon will include a penumbral eclipse — when the moon passes through the Earth’s outer shadow.
The subtle, partial darkening that will occur can be seen only from areas in Europe, Africa, Asia and Australia, but Slooh will broadcast it live from the Canary Islands beginning at 12:45 p.m. The eclipse will last for about three hours.
As far as whether it’s a supermoon, Byrd said just to enjoy it.
“It’s not the most supermoon possible that we can have, but it’s in the realm of supermoon, and I would say many people will call it that and others will say that it’s dumb,” she said. “But it is the harvest moon, and it will be a particularly bright harvest moon.”