Friday afternoon will see the longest lunar eclipse of the 21st century — the epic blood moon.

David Buckley, Ph.D., East Stroudsburg University professor of physics, says that the celestial event will feature the moon passing directly into the earth’s shadow for a total lunar eclipse. “This one is called a central lunar eclipse because the Earth’s shadow is larger than the moon, and the moon will pass directly through the middle part of the shadow. This tends to make the total part of the eclipse longer and, on average, darker,” Buckley said.

And as the name suggests, for a limited time, the moon will take on a somewhat sinister-looking red hue.

There is no need to worry about the ominous appearance, though. In fact, the science behind the peculiar coloration is quite interesting.

Buckley said that the moon will take on the orange-red shade for the same reason that the western sky appears red just after sunset. Dust in the atmosphere acts as a color filter for the spectrum of the sun’s light, and those dust particles tend to let red and orange colors through while scattering away blue, green and violet hues.

“So if you were standing on the moon during this Friday’s eclipse — in a protective spacesuit hopefully — you would see the Earth eclipsing the sun in the lunar sky. But, you would see a bright red ring of light around the Earth’s silhouette — you would be seeing all the sunrises and sunsets of the Earth at the same time! It’s that red ring of sunlight channeled by the Earth’s atmosphere onto the moon’s surface that allows us on Earth to see the eclipsed moon, even when it’s in the darkest part of the Earth’s shadow,” Buckley said.

The blood moon’s total eclipse phase will last about 1 hour and 43 minutes, and the entire event will go on for nearly four hours. The peak eclipse will take place just before 4:22 p.m. EDT.

Unfortunately, Buckley said, skywatchers in the United States will not be able to catch the eclipse, as it will take place during daylight hours, when the moon is behind the Earth. Europe, Asia, Africa, Australia and parts of South America will be able to witness some or all of the show.

Luckily, there are options to view the eclipse online. One of the best may be Slooh, which airs live views of space and tracks celestial events. The company will be streaming a telescopic view of the entire eclipse starting at 1 p.m.

Those who would prefer to take in an eclipse with their own eyes are in luck, Buckley said, though they might have to bundle up.

“The next total lunar eclipse will be on the night of Jan. 20-21, 2019,” he said. “Maximum eclipse will be just a little after midnight. Make sure you have a good parka!”