Eclipse, Umbra, and Penumbra - the Dummies Approach

Eclipse diagram with umbra and penumbra

The Antisocial Network staffers find that their sources – our DotD candidates – demonstrate their topical ignorance in several different ways. The most common evidence, though, is not knowing some very basic information about their topic. That's the reason behind today's nomination, eHowian Chris Deziel, and his post, "What Is the Darkest Portion of the Moon's Shadow During a Solar Eclipse?" at Sciencing.com.

Deziel performed his usual workmanlike job on the question, assuming that by "workmanlike" you mean he informed the OQ that the darkest part of the moon's shadow is called the umbra. He even went so far as to mention that the lighter part of the shadow is known as the penumbra. That's where the discussion ended – except for a tangential discussion about annular eclipses in which Chris failed to mention what "annular" means (it means "ring-shaped"). We suspect he needed those 114 words to meet the minimum word count.

Where Chris blew it was in forgetting his audience: eHow (Sciencing) posts are written at a seventh- or eighth-grade level, which (to us, anyway) suggests that the OQ might have wanted to know the origin of the word "umbra" (Latin for "shade") and perhaps the meaning of the prefix "pen-" (paene, Latin for "almost"). Nope, not there...

More damaging to both Deziel's credibility (and Sciencing's credibility, such as it is) is a complete lack of explanation for why there are different darkness zones in the moon's shadow during an eclipse. Did Chris not know? Did he not think it important? Did he not realize that he almost got there in this paraphrase,

"Because the sun is so much larger than the moon, however, the moon's shadow appears much smaller on Earth that [sic] the moon itself. That's because sunlight is impinging at an angle from the much larger disk of the sun. The umbra forms a cone that narrows to a width of 100 miles by the time it reaches Earth."

Darn, he almost got there!

If you actually want to know, it's pretty simple: the sun is not a point source of light. Only a point source casts a shadow without a penumbra; but we usually can't differentiate umbra from penumbra in shadows because the width of the penumbra depends on the relative sizes of the light source and the object plus the distance source and object compared to the distance between object and shadow.

That's basic knowledge in both physics and astronomy, something Deziel should have known and mentioned if only because he claims to have a physics degree. He didn't mention this, and he failed to explain the origins of words he used. We consider both to be critical errors, errors of the sort that qualify Chris as our Dumbass of the Day.

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