Resulting from the exact right alignment of the sun, Earth and moon, lunar eclipses are relatively common, but “Blood Moons” are much rarer –– and more striking.
This week, part of the world will be treated to a rare kind of lunar eclipse, as the moon will turn red and become a so-called Blood Moon.
This month’s full moon –– the Worm Moon –– will completely enter Earth’s shadow on the night of March 13 or early morning of March 14. Unlike a solar eclipse that is only visible within a narrow stretch of a few hundred miles, the “Blood Worm Moon” will be visible in the entire Western Hemisphere, including all of North and South America and the far western parts of Africa and Europe.
But what is a lunar eclipse and what makes this month’s total lunar eclipse so special?
The answer to both questions comes down to “a little bit of luck and a little bit of geometry,” says Jacqueline McCleary, an assistant professor of physics at Northeastern University.
Both lunar and solar eclipses are “essentially a geometric effect,” she says. The orbit of Earth around the sun and the orbit of the moon around Earth are offset by about five degrees, which means that most of the time they are not perfectly aligned. However, every so often the sun, Earth and moon fall into perfect alignment and create either a lunar or solar eclipse based on how the three are overlapping.
When the Earth sits directly between the moon and the sun, leaving the moon in the Earth’s shadow, we get a lunar eclipse, McCleary explains. After about two weeks, the moon orbits to the other side of Earth, covering the sun and creating a solar eclipse.
The striking appearance of a lunar eclipse –– an Earth-darkened moon –– also comes down to a remarkable coincidence, McCleary says.
“The reason they are as dramatic as they are on Earth is because, by a remarkable coincidence, the sun has a size that is about 400 times larger in the sky than the moon, but it is also about 400 times further away than the Earth,” McCleary says. “So, the moon and the sun appear to have about the same size in the sky even though, of course, the sun is much, much bigger in reality.”
Eclipse cycles are relatively common –– they happen every six months or so –– but the Blood Moon that will appear in the sky this week is much rarer because it requires “a more careful alignment of the Earth, sun and moon,” McCleary adds.
Earth casts a wide shadow shaped like a cone, and where the moon sits in that cone determines the color of a lunar eclipse. For a partial eclipse, the moon will appear a little bit darker than usual, but when the moon is directly behind Earth in a total lunar eclipse, something fairly unique happens.
“All of the light from the sun gets bent by the Earth’s atmosphere,” McCleary says. “Blue light gets scattered, but red light still makes its way onto the moon’s surface. It’s sort of like as if every sunset and every sunrise on Earth were happening on the face of the moon.”
For those standing in this week’s eclipse zone, there’s also no need to worry about hurting your eyes as you stare up into the night sky.
“Unlike a solar eclipse, where you’re still staring at the sun and it’s very bright, lunar eclipses are no brighter than any other full moon,” McCleary says. “In fact, because it’s in the Earth’s shadow, they’re a little less bright, so it’s perfectly safe to look at with the naked eye.”
McCleary adds that if you’re worried about missing the lunar eclipse, the next eclipse is not far behind: A partial solar eclipse will follow on March 29, which will be visible in the northeastern United States and Canada and northern Europe. Just don’t forget to wear your sunglasses for this one.