Let me quote the english language:
"The umbra (Latin for "shadow") is the darkest part of the shadow. From within the umbra, the light source is completely occluded. "
"The penumbra (from the Latin paenes "almost, nearly" and umbra "shadow") is the region in which only a portion of the light source is obscured by the occulting body."

Let me quote the Elnofex language:
"Tamriel means "Starry Heart""

Let me quote the Monomyth:


It is also common knowledge that Umbra is a person, who with powers stolen from Calvicus Vile created a finite Daedric Realm within Nirn called Umbriel.
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I am going to guess that Tam- means Starry and -Riel means Heart. If Umbra means shadow, then Umbriel means Shadowy Heart.

On a different note, Penumbra is Auribis. What is producing the light that is obscured, and what is obscuring it to create a Penumbra? Also, id there is a Penumbra, then there is an Umbra.
I believe that the light-source is AE, the Mad Godhead. The full umbra is Padomay, while the full force of light is Anu. I have yet to work out what is obscuring the light. Further, the Penubra is divided into segments itself. Aetherius is farthest from the Shadow, Mundas is in the middle, and Oblivion is closest to the Shadow.

Does any of this make sense?

If Tamriel is the Starry Heart, then its producing the light. The penumbra being outer layers of the aurbis, then keep going until you hit the umbra. The proper ehlofex would be Tamri-El, el being star.

So yeah sounds accurate. At least that was MK's interpretation of Tamriel.

but what is causing the shadow? light would, with no obstruction whatsoever, continue into space indefinitely.

Real world physics probably won't carry on into Tes metaphysics unless it was intentionally made that way. And in that case only certain specific points would carry over, rendering any connection to physics pointless anyway. So even if light itself would go out into space, the starry heart does not have to continue into Oblivion. Though I suppose it could, it seems like a bit of a stretch to me. I do admit it would be awesome if they went that in depth with the wordplay and whatnot (which I suppose they have done before). So all in all it's an interesting theory, but I'm not sure how accurate it may be.

I believe this to be what's called "over-extending the metaphor."

I believe so, but only as far including Umbra and looking for what's casting the shadow.

thanks for all the responses