Overnight, stargazers witnessed a rare celestial trifecta: a supermoon, this year's Harvest Moon and a partial lunar eclipse.
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If you don't frequently look at the moon, you may not have noticed that "the supermoon all of a sudden is so much bigger," said Noah Petro, NASA's project scientist for Artemis III.
People watch the full moon rise ahead of the lunar eclipse at the Samalayuca Dunes on the outskirts of Ciudad Juarez, Mexico, Sept. 17, 2024. By Jose Luis Gonzalez/Reuters
Then again, if you happened to notice the supermoon looking especially large as it was rising, you may have fallen victim to a "well-known, poorly studied optical illusion," he said.
Here's the scientific explanation of what you were seeing, plus a cool experiment to try next time.
What is a supermoon?
A supermoon, in simplest terms, occurs when the moon's orbit is closest to Earth at the same time the moon is full, making it appear larger and brighter than average to us spectators, according to Petro.
Because the moon orbits the Earth in an oval-shaped path called an ellipse rather than a perfect circle, there are certain parts of the year where the moon is closer to Earth than at other times.
The closest point the moon comes to Earth is called its perigee. The farthest point is the apogee. Graphic provided by NOAA
When the moon is near the horizon, it appears even closer than it actually is, Petro said, deceiving the human eye.
"It's basically kind of a human evolution trick where things that are low on the horizon, we assume are close," Petro said.
To debunk the illusion, he recommends trying out these two strategies:
Take a cardboard tube like an empty paper towel roll and hold it up to center your view of the moon through it. Compare the size of the moon to something small like your thumb. Repeat this process several times throughout the night. You'll see that the moon stays the same size as its position in the sky changes. Turn your back to the moon and look at it upside down through your legs.
"Believe it or not, yes, I have looked at the moon upside down through my legs and went, 'Yup, oh wow, it's not quite as unusually large,'" Petro said, laughing.
How rare are supermoons?
There are usually about three to four supermoons every year, with the largest one making an appearance when the moon reaches its closest distance to Earth, according to Petro.
This year, the superest supermoon to watch will occur next month on Oct. 17. Petro predicts that the largest supermoon of 2025 will rise in November.
A harvest supermoon rises over a lighthouse in Scituate, Massachusetts, Sept. 17, 2024. Photo by Lauren Owens Lambert/Reuters
What is a lunar eclipse?
The combination of both a supermoon and a lunar eclipse is more unique.
A regular lunar eclipse occurs when the moon passes through the Earth's shadow about twice a year, according to NASA. From Earth, we can see the moon partially or totally darken during a lunar eclipse. During a total lunar eclipse, the moon turns into a rusty red color, otherwise known as the "blood moon."
WATCH: Learn about blood moon myths from around the world
The last time a supermoon and total lunar eclipse overlapped was in 2018, and Petro forecasts the next partial lunar eclipse and supermoon to take place on Jan. 12, 2028.
A partial lunar eclipse is seen over the residential building in Moscow, Russia, Sept. 18, 2024. Photo by Maxim Shemetov/Reuters
The joint appearance of a supermoon and lunar eclipse occurs infrequently, but is not considered rare, Petro said. In order for a lunar eclipse to happen, the moon must be full, so "when you have one, you have a chance for the other."
What is a Harvest Moon?
Unlike a supermoon, the Harvest Moon is not a celestial phenomena but rather a social and cultural construct that traces its roots back to agricultural harvesting seasons, according to almanac lore.
The Harvest Moon typically refers to the full moon closest to the autumnal equinox (this year on Sept. 22) and was given its name by farmers who enjoyed a few days of early moonrise that come with this full moon, allowing them to harvest their summer crops later into the evening.
The Harvest Moon is significant in many cultures, signaling the start to celebrations such as the Mid-Autumn Festival in Chinese culture, Chuseok in Korean culture, Madhu Purnima in Buddhist communities and Pitru Paksha in Hindu culture.
A Bedouin rides a horse as a harvest supermoon is pictured in the background, in Rahat, southern Israel, Sept. 18, 2024. Photo by Amir Cohen/Reuters
What's next?
In addition to the next supermoon on Oct. 17, an annular solar eclipse will be visible from the Pacific Ocean and southern parts of South America on Oct. 2.
Next March, a total lunar eclipse will appear across Western Europe, Western Africa and the Americas.
Petro said NASA has several upcoming projects related to lunar exploration, such as a flyby mission to the moon in Sept. 2025 and the launch of a crewed moon landing mission, Artemis III, in 2026.
READ MORE: Astronauts get first look at the Artemis II spacecraft that will fly them around the moon
"We're about to get into this completely new world of really exciting exploration of the moon and its environment, and my goodness gracious, what a great time it is to be studying the moon," Petro said.
For the more casual, Earth-bound explorer, Petro said the supermoon is really a "reward to the regular lunar observer." He encouraged everyone to use this celestial event as a reminder to pay greater attention to the moon, its eclipses and "how awesome they are."
"I think it's baked into our DNA to be interested in the moon. We just have to remind ourselves of that every once in a while," Petro said.