I guarantee anyone that has taken a sunset shot has seen a penumbra, but likely never knew the term for what they were seeing. After the sun has set, when it is dusk, and the sky is darkening there is a transition on the eastern horizon between the last bit of sky lit by the sun and that were darkness is falling. At this point of transition is the penumbra: a partial shadow cast by the earth onto the sky. Or as someone much wiser than I defined it in relation to the world of astronomy…
The shadow cast, in an eclipse, where the light
is partly, but not wholly, cut off by the intervening
body; the space of partial illumination between the umbra,
or perfect shadow, on all sides, and the full light.
–Sir I. Newton.
An example displaying the very early presence of a â€œpenumbraâ€ can be seen in the image found with in the following blog post “Yosemite Sunset“.