I refer you to post #21, where I first used the word penumbra in this thread. You can ascribe the later hyphenation to the four hours of sleep and blistering headache caused by last night's mathsathon, but I would point out that since pen is a non-native prefix the hyphenation is not incorrect, but redundant.
Actually, the definition you used is one of two, and it is the less 'correct' one (insofar as anyone cares how the Catholic church organised its bun fights in days of old.)