# Chapter 22: The Earth's Shadow

**At Grade**Created by: CK-12

In a lunar eclipse, if the width of the shadow of the Earth is twice the width of the Moon, then the width of the Earth itself is (very nearly) three times that of the Moon --- not twice, as one might perhaps think. Here is why:

The Sun is not a point of light but an extended source, with a disk covering a circular patch in the sky, about 0.5 degrees across. This makes the shadow of the Earth not a cylinder, stretching to infinity without narrowing down, but a cone, with an angle of 0.5 degrees across its apex C (drawing). AB is here the diameter of the Earth, and the directions AC and BC represents rays from opposite edges of the Sun's disk, rays whose directions differ by 0.5 degrees.

Illustration of a Lunar eclipse.

If \begin{align*} x\end{align*}

The width of the Moon as seen from point H is KD = \begin{align*} x\end{align*}

It follows then that AC = \begin{align*} 3R\end{align*}