Well well well. The time is now again. During the early morning hours of Tuesday December 21, there will be a total eclipse of the moon. It will be visible in North America, but I’ll get to that in a moment.
You often see all kinds of words like ecliptic, node, umbra, penumbra, etc. when reading or learning about lunar eclipses. The only two words we really need to concern ourselves with are the penumbra and the umbra. Both of these refer to two distinct shadows cast by Earth into outer space by light emanating from the Sun. Let’s, for the sake of ease, keep these descriptions simple.
The penumbra is a cylindrical shadow. It’s diffuse and lighter in shade. The reason for the light diffusion is because the Sun, despite looking small from 93 million miles away, is very large compared to Earth. So there is some scattering of light from all the points of light all over the exterior of the Sun.
The umbra, on the other hand, is a cone-shaped shadow inside the penumbra cylinder. It’s base is on Earth and it’s tip is a point of convergence in space. The umbra is a more concentrated shadow; . . . → Read More: Total Lunar Eclipse – December 21, 2010