Eclipse: It’s All about the Umbra

Aug 21, 2017

By Caleb Scharf

In recent days I’ve been reading a few eye-witness accounts of total solar eclipses across the ages. What’s common to almost all of these descriptions is the genuine sense of awe that a blocked-out sun evokes. That awe come from different pieces of the experience. It’s in the alien vista of the Sun’s corona, or in the spectacle of Bailey’s beads, or the sudden chill that descends on an otherwise warm sunny day and the wind that whips up.

But what’s bothered me is why some of these phenomena should have such a visceral impact. After all, a stray cloud can do a pretty good job at blotting the Sun out, as can a well-placed umbrella or hand.

And that got me interested in the actual dimensions of the shadow cast by the Moon. This shadow is really divided into three zones, depending on the eclipse configuration. The inner umbra of total eclipse, the outer penumbra of partial eclipse, and the antumbra between these two if the moon’s distance allows an annular eclipse – the Sun peeking around the lunar disk.

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