What a Blood Moon Is—and Isn't

Separating Fact from Fiction

May 21, 2021
Lunar Eclipse

Total lunar eclipse


A total eclipse of the Moon is on May 26. Some folks are calling it a “Blood Moon.” There’s boatloads of celestial hype about this phenomenon. We’ll help you separate fact from fiction.

What is a Blood Moon?

May 26, 2021 brings a total lunar eclipse (and a supermoon!)—only partially visible from North America. The full Moon of May is also this year’s closest, biggest, and brightest Moon: a supermoon. See the Almanac’s May Moon Guide to learn more about the Full Flower Moon.

A total eclipse of the Moon is sometimes called a “Blood Moon” because the Moon turns reddish when it’s completely submerged in the Earth’s shadow.

We’ve heard all kind of strange theories about a “Blood Moon.” Back in 2014–2015, there was a series of four consecutive total lunar eclipses (Blood Moons) and the media hyped end-of-the-world prophesies. Clearly, the world did not end.

“Blood Moon” is not a technical term used in astronomy. It’s more of a popular phrase, perhaps because it sounds so dramatic. It simply refers to a “total lunar eclipse.” Yep, that’s it. 

During a lunar eclipse, the Earth is passing between the Sun and the Moon, blocking the Sun’s rays. However, the Moon isn’t completely dark. What we see from Earth is the Moon slowly darkening and changing color over a few hours to an orange-red.

Image: NASA

While most of the sunlight is indeed blocked, some rays bent around the edge of Earth and reach the Moon’s surface. Earth’s atmosphere scatters the blue/green colors (short wavelengths), but the orange/red colors (long wavelengths) reach our eyes. It’s similar to a sunset. 

So, don’t let the term “Blood Moon” throw you. Plus, a fully-eclipsed Moon actually becomes orange or coppery like a penny, not red like blood.

More About Eclipses

Lunar eclipses shouldn’t be confused with solar eclipses. Both involve the Moon, but are different events.

A total solar eclipse is the greatest celestial event the human eye can behold. It’s when the Moon passes between the Sun and the Earth, blocking out the Sun for a short period of time. See the next total solar eclipse date.

By the way, an animated aurora borealis might hold second place. An exploding meteor, called a bolide, or a brilliant colorful one called a fireball might hold third place. And a brilliant comet like Hale-Bopp that came around 20 years ago might be the fourth greatest spectacle.

A lunar eclipse is interesting, but it doesn’t quite have that make-you-gasp, pedal-to-the-metal glory. Moreover, the thrill is the solar totality. That’s when flames (“prominences”) shoot off the Sun’s edge, and its corona leaps far across the sky, and stars come out, and many people weep. A partial solar eclipse, which requires eye protection, offers none of those things. That’s why “total” is the critical eclipse adjective.

But lunar eclipses are different. When the Moon is 99% eclipsed it’s quite fascinating. Nothing extra happens when the Moon plunges into the final one percent of Earth’s shadow. In fact, some might argue that 99% is more visually spectacular because there’s then one final spot of white on the Moon’s edge, which makes its overall coppery color more dramatic.

Bottom line: Totality is far less critical when it comes to lunar eclipses. And you may enjoy a lunar eclipse from anywhere on the planet where it’s nighttime! 

See all of the upcoming solar and lunar eclipe dates

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s hub for everything stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe