The Century’s Longest Lunar Eclipse Will Shroud the Moon This Week

For any stargazers out there feeling eclipse withdrawals, the universe has good news: the longest lunar eclipse of the 21st century will occur on the evening of July 27. The bad news? If you’re reading this within U.S. borders, you won’t be able to watch from home.

This eclipse will be completely visible in Eastern Africa and Central Asia from 3:30 p.m. to 5:13 p.m. EDT, while viewers in Western Africa, Eastern Asia, South America, Europe, and Australia will catch a partial glimpse, according to Temi Adebowale at Popular Mechanics. Including phases where the moon is partially masked, the event will last nearly four hours total.

Unlike solar eclipses like last year’s much-hyped summer spectacular, in which the moon passes between the Earth and the sun, blocking the sun from view, a lunar eclipse occurs when the Earth passes between the moon and the sun, obscuring the moon from the sun’s illumination. So, rather than darkened skies, viewers will be treated to a ruddy red visage that’s often dubbed a “blood moon” (though the most finicky reserve this term for a series of four consecutive lunar eclipses).

Eerie as it may be, the moon’s face appears rouged for the same reason the sky looks blue to our eyes. Though concealed by the sun, the moon is still flecked with a few stray particles of light filtered through Earth’s atmosphere. Air on Earth scatters shorter-wavelength light, including most of the blues and greens (which trickle down to us on the planet’s surface), leaving the longer reds to spotlight the moon.

The reason this lunar eclipse will be a particularly long one is because the moon will be passing almost directly through the central part of Earth’s shadow, while the moon is at a particularly distant point from Earth in its monthly orbit. During the summer months, Earth also swings out to its furthest distance from the sun, allowing it to cast a particularly long shadow. These three factors are what have lengthened the duration of July's eclipse to 1 hour and 43 minutes, which NASA estimates will be the longest total lunar eclipse of the 21st century. To put this in perspective, it falls just four minutes shy of the longest lunar eclipse possible for Earth’s most faithful satellite.

As an added celestial bonus, around the same time, stargazers may spot Mars at its closest position to Earth in 15 years, making the red planet appear about 10 times brighter than usual. Peak proximity will occur at 3:50 a.m. EDT on July 31. Yankee night owls, rejoice—this phenomenon will be visible worldwide. And the lucky viewers abroad may have the chance to see both celestial bodies enrobed in crimson at once.

For those outside the lunar eclipse’s line of sight, the Virtual Telescope Project will be livestreaming the moon’s descent into darkness beginning at 2:30 p.m. EDT.

Because the line of view is directed at the moon instead of the sun, lunar eclipses, unlike solar eclipses, are completely safe for full frontal viewing. So for any presidents that might be reading this, don’t worry: this time, it’s OK to stare.

Update, July 23, 2018: This piece's headline has been changed to reflect that the eclipse is occuring this week.

Katherine J. Wu | | READ MORE

Katherine J. Wu is a Boston-based science journalist and Story Collider senior producer whose work has appeared in National Geographic, Undark magazine, Popular Science and more. She holds a Ph.D. in Microbiology and Immunobiology from Harvard University, and was Smithsonian magazine's 2018 AAAS Mass Media Fellow.