Overnight, stargazers witnessed a rare trifecta in the sky: a supermoon, this year’s harvest moon, and a partial lunar eclipse.
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Those who don’t look at the moon often may not have noticed that “the supermoon is suddenly so much bigger,” says Noah Petro, NASA project scientist for Artemis III.
On the other hand, if you happened to notice that the supermoon looked particularly large when it rose, you may have fallen victim to a “well-known but little-studied optical illusion,” he said.
People watch the rise of the full moon ahead of the lunar eclipse at the Samalayuca Dunes on the outskirts of Ciudad Juarez, Mexico, September 17, 2024. By Jose Luis Gonzalez/Reuters
Here’s the scientific explanation of what you saw, plus a cool experiment you can try next time.
What is a supermoon?
Put simply, a supermoon occurs when the moon is closest to Earth in its orbit and is full at the same time, making it appear larger and brighter than usual to us viewers, says Petro.
Because the moon does not orbit the earth in a perfect circle but in an oval orbit, called an ellipse, the moon is closer to the earth at certain times of the year than at others.
The point at which the moon is closest to the earth is called perigee. The farthest point is apogee. Graphic provided by NOAA
When the moon is close to the horizon, it appears even closer than it actually is, Petro said, deceiving the human eye.
“Basically, it’s a kind of trick of human evolution: we assume that things that are deep on the horizon are close,” Petro said.
To debunk the illusion, he recommends trying these two strategies:
- Take a cardboard tube, like an empty kitchen roll, and hold it up so that you can see the moon right through it. Compare the size of the moon to something small like your thumb. Repeat this process several times throughout the night. You will see that the moon stays the same size even 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 looked at the moon upside down through my legs and thought, ‘Yep, oh wow, it’s not that unusually big,'” Petro said, laughing.
How rare are supermoons?
There are typically about three to four supermoons each year, with the largest occurring when the moon is at its closest distance from Earth, according to Petro.
The biggest supermoon of the year will rise next month, on October 17. Petro predicts that the biggest supermoon of 2025 will rise in November.
A harvest supermoon rises over a lighthouse in Scituate, Massachusetts, on September 17, 2024. Photo by Lauren Owens Lambert/Reuters
What is a lunar eclipse?
The combination of supermoon and lunar eclipse is more unique.
According to NASA, a normal lunar eclipse occurs when the moon passes through the Earth’s shadow about twice a year. From Earth, we can see the moon partially or completely darken during a lunar eclipse. During a total lunar eclipse, the moon turns a rusty red color, also known as a “blood moon.”
REGARD: Learn more about blood moon myths from around the world
The last time a supermoon and a total lunar eclipse overlapped was in 2018, and Petro predicts that the next partial lunar eclipse and supermoon will occur on January 12, 2028.
A partial lunar eclipse is seen over the residential building in Moscow, Russia, September 18, 2024. Photo by Maxim Shemetov/Reuters
The simultaneous occurrence of a supermoon and a lunar eclipse is rare, but not considered rare, Petro said. For a lunar eclipse to occur, the moon must be full, so “if there’s one, there’s a chance of the other.”
What is a harvest moon?
Unlike a supermoon, the harvest moon is not a celestial phenomenon, but rather a social and cultural construct whose roots, according to almanac traditions, go back to the harvest season in agriculture.
The Harvest Moon generally refers to the full moon closest to the autumn equinox (September 22 this year). It gets its name from farmers who enjoyed a few days of early moonrise during this full moon, which meant they could harvest their summer crops late into the evening.
The Harvest Moon plays an important role in many cultures as it marks the beginning of celebrations, such as the Mid-Autumn Festival in Chinese culture, Chuseok in Korean culture, Madhu Purnima in Buddhism, and Pitru Paksha in Hindu culture.
A Bedouin rides a horse as a harvest supermoon is seen in the background, in Rahat, southern Israel, September 18, 2024. Photo by Amir Cohen/Reuters
What happens next?
In addition to the next supermoon on October 17, an annular solar eclipse will be visible from the Pacific and southern parts of South America on October 2.
Next March there will be a total lunar eclipse in Western Europe, West Africa and the Americas.
Petro said NASA has several upcoming projects related to lunar exploration, such as a flyby mission to the moon in September 2025 and the launch of a manned lunar 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 enter a whole new world of really exciting exploration of the Moon and its surroundings, and my goodness, it’s a great time to be exploring the Moon,” Petro said.
For the more occasional, earthbound explorer, Petro said the supermoon is a “reward for the regular moon watcher.” He encouraged everyone to use this celestial event as a reminder to pay more attention to the moon, its eclipses, and “how magnificent they are.”
“I think interest in the moon is in our blood. We just need to remind ourselves of it every now and then,” Petro said.
