Astronomically speaking, "there is no such thing as a 'blood moon,'" Dr. Jay Pasachoff, an astronomer at Williams College in Williamstown, Mass., told The Huffington Post in an email. People who use that term are either "misguided or malicious," he added.
The moon will look reddish for a brief time on the night of Sept. 27-28. But that's only because the moon will be cast into shadow as the Earth passes between it and the sun--in other words, we're set to see a lunar eclipse.
In the U.S., the eclipse will begin at 9:07 p.m. E.T. and will last for more than three hours. What astronomers call "totality"--when the moon is fully enveloped within the Earth's shadow--will begin at 10:11 p.m. and last for 72 minutes.
Fair enough. But why exactly will the moon take on that crimson cast?
In a new video (above) posted on its website, NASA suggests a simple thought experiment can make it all clear:
Using your imagination, fly to the Moon and stand inside a dusty lunar crater. Look up. Overhead hangs Earth, nightside facing you, completely hiding the sun behind it. The eclipse is underway. You might suppose that the Earth overhead would be completely dark. After all, you're looking at the nightside of our planet. Instead, something amazing happens. When the sun is located directly behind Earth, the rim of the planet seems to catch fire! The darkened terrestrial disk is ringed by every sunrise and every sunset in the world, all at once. This light filters into the heart of Earth's shadow, suffusing it with a coppery glow. Back on Earth, the shadowed Moon becomes a great red orb.
In more scientific terms, the moon looks red during a lunar eclipse because of Rayleigh scattering. That's a phenomenon in which sunlight--essentially a mash-up of all the different colors of light--is scattered by Earth's atmosphere. Since blue light is scattered more than red light, it's essentially "filtered out" as it passes through the atmosphere--leaving red light to reach the moon.
You don't need any special equipment to see a lunar eclipse. But watching the action unfold with a telescope or binoculars will add to the fun. As astronomer Phil Plait wrote recently on Slate, "The moon can take on an odd three-dimensional appearance when you use binoculars during an eclipse, and it's pretty cool to see."
This is the last total lunar eclipse until 2018, according to Sky & Telescope. So enjoy the show!
Correction: A previous version of this story misstated the relative positions of the Earth, moon, and sun that occur during a lunar eclipse.