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 “penumbra” can be seen in the image found with in the following blog post “Yosemite Sunset“.
JMG-Galleries - Yosemite Sunset
on March 26 2007
[...] There’s nothing like seeing Yosemite in the winter. This was taken in 2002, back when I was shooting film. The valley floor was cold enough for the snow to stick around and everything looked amazingly beautiful. In this photo just as the days last light filled the sky and as the penumbra became visible everything turned magically pink. [...]