A "super blood wolf moon?" Nothing is howling, bleeding, or appearing in a Marvel film, but the actual event is just as extraordinary.
When I was eleven, I remember watching the November 1993 lunar eclipse with my parents and the neighborhood kids from our Alabama driveway, as the moon slowly vanished and turned red, then came back. It was beautiful. But it was also inspiring. Why does this happen? Why do we always get a lunar eclipse during a full moon? What makes it red? My search for answers to questions like these fueled my interest in science in school. The answers—the workings of nature—turned out to be beautiful as that ghostly red moon itself, and that realization led me to become a physics professor to share that beauty with the next generation of students.
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This Sunday night, the sky will serve up a rare treat: A total lunar eclipse. Observers in the U.S. will see a normal full moon at first, but at 10:33pm ET, the Earth's shadow will begin to pass in front of the Moon, blocking almost all of the sun's light from reaching it. Observers will see the moon appear to be progressively "swallowed up" starting from the lower left. This process will end at 11:41pm, when the Earth's shadow covers the whole of the moon's surface. This is the beginning of "totality." This will last until around 12:43am ET, when the motion of the Earth's shadow will carry it past the moon, and the moon will gradually again be lit by the sun. At 1:50am ET, the Moon will be fully visible again.
But the moon won't be completely invisible during the period of totality, even when the Earth's shadow completely covers it. A little bit of sunlight, mostly the red part, is refracted by the Earth's atmosphere and reaches the moon, bending around the edges of the Earth. This small amount of light still illuminates the Moon enough for us to see it. Instead of being bright and white, the moon will be very dim and red, ten thousand or so times dimmer than usual; people call this a "blood moon." The moon will be dim enough that it won't overwhelm starlight, so observers will get the rare treat of seeing the moon and the stars together.
Total lunar eclipses like this are rare. Usually, when the moon passes behind the Earth, we get an ordinary full moon, since the moon is slightly above or below the Earth's shadow and is still lit by the sun. This happens because the moon's orbit is tilted about five degrees compared to the Earth's orbit. Rarely though, the moon passes directly behind the Earth—not above or below—and the Earth's shadow blocks some or all of the sun's light, giving us the rare treat of an eclipse.
According to calculations by Fred Espenak at NASA's Goddard Space Flight Center (whose math is still valid even when NASA is shut down,) we will have 85 total lunar eclipses in the 21st century. There will also be 57 partial lunar eclipses, where the alignment is close enough for the Earth's shadow to cover some, but not all, of the moon.
Lunar eclipses are more common than solar eclipses, like the magnificent treat we got in August of 2017. A solar eclipse happens when the moon's shadow blocks sunlight from reaching the Earth, instead of the other way around. The moon is only about a quarter of the diameter of Earth, and its shadow is only barely big enough to block out the Sun, so it requires a much more precise alignment for a solar eclipse than a lunar eclipse.
There is nothing terribly special about a "supermoon." The moon's orbit, like the orbits of all celestial bodies, is an ellipse. Its distance from Earth varies by about ten percent as it travels around its orbit; a "supermoon" is just a full moon where the moon is near its closest point to us. This lunar eclipse is one of those times, so the Moon will look about 10 percent bigger than it usually does. Compared to the drama of the eclipse, this is no big deal. Finally, a "wolf moon" is just an old name given to the first full moon of a new year.
The next total lunar eclipse visible from the U.S. will happen on May 26, 2021. It will only be visible from the West Coast and Hawai'i, but observers on the West Coast will be able to see the eclipsed moon just before dawn as the Moon is about to set over the western horizon. This event will likely be a treat for landscape photographers, who can capture the blood-red moon, the stars, and the majestic landscapes of California, Oregon, and Washington in a single frame.
Walter Freeman is assistant teaching professor of physics at Syracuse University, New York.
The views expressed in this article are the author's own.
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