The first “supermoon” of 2021 arrives Monday evening.

While this may sound like a plot point from Disney’s Marvel Cinematic Universe, it’s not. A supermoon is an astronomical phenomenon that occurs when a full moon coincides with the moon’s closest approach to Earth.

Full moons, known for their brightly illuminated faces, typically happen once per month. They take place when Earth passes between the sun and the moon. (Sometimes full moons occur twice in a month, the second of which is called a “blue moon.”)

April’s full moon will last about three days, from Sunday evening to Wednesday morning. The moon will be biggest and brightest at 11:32 p.m. EDT, or 8:32 p.m. PT, on Monday night, weather conditions notwithstanding.

What is a “supermoon”?

Supermoon isn’t a scientific term.

Richard Nolle, an astrologer—that is, a practitioner of pseudoscience, not real science—coined the word in a horoscope-related magazine in 1979. The term caught on because, well, it’s catchy—certainly more so than the Ph.D. jargon: “perigee-syzygy.”

Let’s parse that phrase. “Perigee” refers to when the moon’s orbit brings it closest to Earth. “Syzygy” is the alignment of three or more celestial bodies in a straight line; in this case, the sun, Earth, and moon. When both conditions are met, it means there’s a supermoon.

“Different publications use slightly different thresholds for deciding which full moons qualify as supermoons, but for 2021 all agree the two full moons in April and May are supermoons,” notes Gary Johnston, a program executive at NASA.

Some publications count this year’s March and June full moons as supermoons too.

Pink moon at night, spectator’s delight

What makes this full moon a pink moon?

Not the color, sadly. April’s full moon is known colloquially as the pink moon because it arises in springtime when many flowers—like Phlox subulata, or “moss pink”—are in bloom. The Old Farmer’s Almanac, America’s oldest continuously published periodical, popularized the term as part of its popular astronomical reference tables.

The April full moon has other nicknames, many of which derive from Native American sources. These include the “sprouting grass moon,” the “egg moon,” and the “breaking ice moon.”

This full moon also holds religious significance. It’s known as the paschal moon in Eastern Christianity, and it occurs just before that faith’s Easter Sunday. The full moon marks Hindus’ Hanuman Jayanti, a deity’s birthday celebration, and Buddhists’ Bak Poya, an anniversary of the Buddha’s visit to Sri Lanka, too.

This supermoon also happens to be arriving about midway through Islam’s holy month of Ramadan.

Blood moon rising

The next supermoon should appear exactly one month from today on May 26, 2021, making it a full flower supermoon in Almanac-speak. That supermoon will be the year’s last and biggest one, coming about 100 miles closer to Earth.

A supermoon appears about 7% bigger than the moon typically looks at its average distance of 239,000 miles from Earth, although the enlargement is barely detectable to the human eye.

The mind is known to plays tricks, however. When the moon appears near the horizon, an optical illusion can cause it to look much bigger. In comparison to trees, buildings, and mountains, the moon looks far bigger than it does when set higher up against the vastness of space.

The May full moon, in addition to being a supermoon, is set to coincide with a total lunar eclipse. To lucky observers in western North America, where the eclipse will be visible, the moon will take on a reddish hue as it passes through Earth’s shadow. People call this a “blood moon.”

Boldly going

In a few years, humans plan to have an even better close-up view of their orbiting neighbor.

In October 2024, NASA is teaming up with rocket-maker SpaceX to launch Artemis 3. The crewed lunar landing mission is set to be the space agency’s first touchdown on the moon since Apollo 17 in 1972.

In the meantime, the U.S. is searching for possible traces of ancient life on Mars with its Perseverance rover. The space agency has already tested new interplanetary technologies on the Red Planet as part of the mission, including a drone helicopter called Ingenuity.