Halloween may be months away, but the night sky is about to get pretty spooky.
A total lunar eclipse will be visible across North America late Monday night, and the entire continental United States will get a clear view of the occurrence, weather permitting. The Earth, currently positioned between the sun and the moon, will cast a shadow over the moon’s entire surface. The moon will then glow red, varying in color from bright copper to crimson — hence the nickname “blood moon.”
Ominous as they might seem, red moons are completely natural products of total lunar eclipses, which occur in stages and last for a few hours.
First, the moon’s edge begins to enter the outer portion of the Earth’s shadow, known as its penumbra. Some shading along the moon’s surface begins to appear, chasing the light that still reaches the celestial body from the sun.
Then the moon enters the umbra, the darkest part of Earth’s shadow. “Few sights in astronomy are more eerie and impressive than watching this red-black shadow creeping, minute by minute, across the bright lunar landscape, slowly engulfing one crater after another,” writes Alan MacRobert at Sky & Telescope. An hour later, only a bright sliver of the moon remains outside the shadow.
That’s when the reddish hue begins to appear, and the source of the color is nothing short of mind-boggling. Because the Earth is between the sun and the moon, sunlight must first pass through our planet’s atmosphere before reaching the moon. As sunlight skims the rim of the globe, our atmosphere bends and scatters it. When the refracted light from all the sunrises and sunsets from around the Earth hits the moon, it casts a red glow on the surface.
Unlike a solar eclipse, a lunar eclipse can be seen with the naked eye if the sky is clear. It also won’t blind you when you stare directly at it. Here’s what a total lunar eclipse looked like in 2011, the last time North America witnessed one:
This time around, the eclipse will last from about 1:20 a.m. until 5:33 a.m on the East Coast, with the moon at its bloodiest at 3:07 a.m. West Coast dwellers don’t have to stay up too late to catch the show, which begins at 10:20 p.m. and ends at 2:33 a.m. For other time zones, check out this handy chart from Sky & Telescope. Or plug in your city at this U.S. Naval Observatory data generator to determine peak viewing times. Or, if you want to watch from the comfort of your own bed, tune into NASA TV.
Monday night’s eclipse is the first of series of four total eclipses visible in North America in the next year. The phenomenon is called a tetrad, and it’s quite rare. For example, between 1600 and 1900, no complete lunar eclipses were seen at all. This century, however, is more promising, NASA’s Fred Espenak, who specializes in eclipse predictions, told ABC‘s Elizabeth Gonzalez on Monday. “Frequency sort of goes through 585-year cycles,” he said. “So you go through centuries where you don’t have any, and centuries where you have a number of them.”
If we’re talking in centuries, the next tetrad will be here before we know it — in 2032.