The shape of the Moon’s shadow is not only not circular on Earth’s surface, but resembles a bizarrely elongated, irregular polygon. Science now understands exactly why. Image credit: NASA / SVS / Ernie Wright, via https://svs.gsfc.nasa.gov/4515.

The strangest eclipse fact of all: the Moon’s shadow isn’t a circle

Sometimes science is stranger than fiction.

“To morrow, I believe, is to be an eclipse of the sun, and I think it perfectly meet and proper that the sun in the heavens, and the glory of the Republic should both go into obscurity and darkness together.” -Benjamin F. Wade

The Earth is nearly a perfect circle, and when the Sun strikes it, it casts a shadow. That circle, during a perfect alignment between the Earth, Sun and Moon, is visible on the lunar surface, and what we see lines up exactly with what we expect: a near-perfect circle. The Moon, too, is nearly a perfect circle. Whether you see it lit up during a full Moon, the faint outline from earthshine during a crescent phase or are viewing it silhouetted against the Sun during a solar eclipse, it, too, appears circular. But if you were to view the shadow the Moon makes on Earth during this time, not only wouldn’t it be circular, but three combining factors mean that it’s not even close.

An illustration of the Sun-Moon-Earth configuration setting up a total solar eclipse. The Earth’s non-flatness means that the Moon’s shadow gets elongated when it’s close to the edge of the Earth. Image credit: Starry Night education software.

There’s a grand illusion you see if you look at Earth from space: it looks like you’re watching a circular planet. But that’s a trick of…