The Strangest Eclipse Fact Of All: The Moon's Shadow Isn't A Circle (Synopsis)

"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 Moon is spherical, and so its shadow should be a circle by simple geometry, right? Only, if we view it when it strikes Earth, it’s not even close to a circle. It’s stretched into an ellipse, and further complicated by irregular, sharp edges and corners. Why would it appear that way? As it turns out, three factors combine to get us there.

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. 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.

The first is the fact that Earth is a sphere, not a disk, so any shadow falling on it gets stretched. The second is that the Moon’s sharp peaks, valleys and craters mean that its shadow gets irregularly distorted in a way that changes as its orbit continues. And the third is that Earth isn’t smooth, but exhibits significant changes in elevation and terrain.

The shadow of the moon falls on Earth as seen from the International Space Station during the March 29, 2005 solar eclipse. Image credit: NASA / ISS / Expedition 12. The shadow of the moon falls on Earth as seen from the International Space Station during the March 29, 2005 solar eclipse. Image credit: NASA / ISS / Expedition 12.

Add them all up, and you’ve got the incredibly bizarre and ever-changing shape of the Moon’s shadow. Come see what the eclipse of August 21, 2017 will hold!

More like this

"...and the Sun has perished out of heaven, and an evil mist hovers over all." -Homer's Odyssey The great american eclipse is coming, and you're in luck! This astronomical sight is unlike anything else that occurs on Earth (or any rocky planet in the Solar System), and there are three incredible…
“We have finally established the contours that define the supercluster of galaxies we can call home … This is not unlike finding out for the first time that your hometown is actually part of much larger country that borders other nations.” -Brent Tully It's been another remarkable week here at …
“It will shine still brighter when night is about you. May it be a light to you in dark places, when all other lights go out.” ―Galadriel, LOTR, J.R.R. Tolkien Well, we've been anticipating it for months (or years), but this is our very first time meeting up since the total solar eclipse here at …
"The moon shuts off the beams of the sun as it passes across it, and darkens so much of the earth as the breadth of the blue-eyed moon amounts to." -Empedocles, ~450 B.C. Less than two weeks ago, I saw my first annular eclipse, with some spectacular results at the moment of maximum eclipse. From…

"The Moon is spherical, and so its shadow should be a circle by simple geometry, right?"

Only if you're a citizen of flat-land :) In 3d spheres cast elipses on other spheres even in simple geometry :)

By Sinisa Lazarek (not verified) on 08 Feb 2017 #permalink

.. in other words, IMO don't think it's strange that the shadow is ellipse since everyday experience shows you that.. one ball casting shadow on another ball.. That it's a jagged ellipse is an interesting thing indeed.

By Sinisa Lazarek (not verified) on 08 Feb 2017 #permalink

Sinisa:
A circular shadow would project as an ellipse on any planer surface that is inclined to the direction of the light. Once you make it a sphere, then its no longer even an ellipse. But, the size of the penumbra versus the earths radius is pretty small, so the ellipse isn't a bad first approximation.

Of course the shadow isn't a cylinder, its actually a cone, but a plane intersecting a cone is a conic section still.

By Omega Centauri (not verified) on 08 Feb 2017 #permalink

@ Omega
"A circular shadow would project as an ellipse on any planer surface that is inclined to the direction of the light."

true.. I didn't mention that part because if they are all aligned perfectly, you would get a circle. But you are correct, and goes in line with what I wrote #2 .. that IMO in most of our everyday experience... we see way more ellipses then we see perfect circle shadows.

By Sinisa Lazarek (not verified) on 08 Feb 2017 #permalink

Of course it's a circular shadow!

... if you look at it from the center of the Earth-facing Moon.

A considerable amount of uphill walking and holding your breath will be required, though...