Tomorrow’s Total Lunar Eclipse, and a Mystery

Tomorrow night (January 20-21) will present the whole U.S. with a total lunar eclipse, the best one until May 15, 2022.

Totality here in Alabama will occur approximately from 10:40 to 11:40 p.m. CST. Clear weather will be restricted mostly to the southeastern U.S., and portions of the Northern Plains and Great Lakes:

A Mystery (to me, anyway)

There’s one aspect of the eclipse I cannot figure out. I’m sure the explanation will be simple, and when someone explains it to me, my response will be, “DOH!”.

The illumination of the moon during totality is due to light scattered through Earth’s atmosphere. Just as we see red sunsets, that red light will be shining on the moon from an annulus of red sunset light circling the Earth.

What I don’t understand, though, is the role of sunlight refraction (bending of sunlight) as it passes through the atmosphere at an oblique angle. The refraction occurs whether it is the moon or the sun being viewed through the limb, and I will use the example moonlight shining through the limb.

My understanding is that light (from either the moon or sum) bends as I crudely show in the following cartoon. The “mystery” arises from the fact that we know that the appearance of the moon is that it is flattened due to refraction:

The moon composite photo is from the ISS, so it is exactly analogous to the situation shown in the drawing.

So, the mystery: Why is the moon flattened rather elongated? I simply don’t know. But I’m sure the explanation is simple.

via Roy Spencer, PhD.

January 19, 2019 at 08:29AM

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