"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.](/files/startswithabang/files/2017/02/Total-solar-eclipse-illustration-600x450.jpg)
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.
- Log in to post comments
"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 :)
.. 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.
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.
@ 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.
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...