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By Tom Howarth
Science Reporter (Nature)
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A rare astronomical event is happening tomorrow night as the second supermoon in a series of four this year will light up the sky across the U.S.
But that's not all—tomorrow's September full moon, known as the Harvest Moon, will be joined by a partial lunar eclipse, where part of the moon dips into Earth's shadow.
The next supermoons will occur in October and November.
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How to see the September supermoon and partial lunar eclipse
The Moon will reach its fullest point at 10:34 p.m. EDT. If you're on the East Coast, it will rise in the east around 7 p.m. and remain visible until nearly 6 a.m. the following morning.
For those on the West Coast, the Moon will hit peak fullness at 7:34 p.m. PDT. There, it will rise above the eastern horizon about half an hour before reaching its full phase and set in the west around 6 a.m.
The partial lunar eclipse will start on the East Coast at 8:41 p.m. EDT on September 17 and conclude at 12:47 a.m. EDT on September 18. The eclipse will reach its peak at 10:44 p.m., just 10 minutes after the full moon appears.
On the West Coast, the eclipse begins at 6:55 p.m. PDT and ends at 9:47 p.m., with the maximum occurring at 7:44 p.m.
As long as the weather cooperates, you'll have a clear view of the supermoon without any special equipment. For the best experience, consider heading out of the city to avoid light pollution from buildings and streetlights.
Thanks to an optical illusion, the supermoon will appear most impressive at moonrise or moonset, when it's closer to the horizon.
"During the twilight, the residual solar light scattered all around by our atmosphere allows us to admire the scenery, while the full moon rises or sets," Gianluca Masi, an astronomer from the Virtual Telescope Project, previously told Newsweek.
"Furthermore, at its rise and set the moon appears behind monuments and elements of the landscape, generating the feeling that its disk is larger than usual. But this is just an optical illusion resulting from the presence of those terrestrial elements in the line of sight that our brain uses for comparison."
What is a supermoon?
"Supermoon" isn't a scientific term but a popular one, coined by astrologer Richard Nolle in 1979 to describe full moons that occur when the moon is closest to Earth.
The moon's orbit around Earth is slightly elliptical, not perfectly circular.
Supermoons happen when a full moon coincides with the moon's closest point to Earth, called perigee, which is approximately 226,000 miles away.
At its farthest point, known as apogee, the moon is roughly 253,000 miles from Earth.
"The term 'Supermoon' is a vague, non-scientific term, and so different people define it in different ways," Darren Baskill, a physics & astronomy lecturer at the University of Sussex, UK, told Newsweek.
"One common definition is to say that we have a supermoon when the Moon is within 90 percent of its average distance away from the Earth—but which average distance? Some say within 90 percent of the average Earth-Moon separation for that particular month, while others say within 90 percent of the average Earth-Moon separation for that entire year.
"There is no overall consensus. Depending on which of those definitions you use, you can have three or four supermoons per year."
During a supermoon, the moon can appear up to 14 percent larger and 30 percent brighter than when it's at apogee—a difference similar to comparing a nickel to a quarter. Compared to an average full moon, it's about 8 percent larger and 15 percent brighter.
Supermoons occur multiple times a year because the moon's orbit and phases frequently align. However, some are more striking than others, depending on how closely the full moon coincides with perigee. The distance of perigee itself can also vary slightly due to the shape of the moon's orbit at the time.
"The scientific term for Supermoon is perigee syzygy of the Earth–Moon–Sun system, which to me sounds much more exciting," Baskill said.
What is a partial lunar eclipse?
A partial lunar eclipse happens when the Earth moves between the sun and the moon, but the alignment is slightly off.
This causes only part of the moon to pass into Earth's shadow, called the umbra, darkening a portion of the moon while the rest stays lit by sunlight.
"This year's September full moon will just pass through the shadow of the Earth, resulting in a partial eclipse that will be visible from the UK and Europe, Africa and across the Americas," Baskill previously told Newsweek.
"Only 10 percent of the Moon will pass through the dark umbral shadow of the Earth, however, and so during the middle of the eclipse we will see just a small bite missing from the Moon as it hides in the shadow of the Earth."
What is the Harvest Moon?
The name "Harvest Moon" isn't about the moon itself, but its timing, as it marks the start of autumn.
During this season, farmers in the Northern Hemisphere harvest their crops ahead of winter. Before the advent of electricity, they relied on the bright light of the full moon to work late into the night, according to NASA.
Other full moons have seasonal names as well: January's is the Wolf Moon, March's the Worm Moon, and last month's was called the Sturgeon Moon.
Correction 09/16/24, 03:25 a.m. ET: This story has been updated to change a reference to solar eclipse in the second paragraph to lunar eclipse.
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