Isaac Physics "An Umbrella"

Hi, I would really appreciate if anyone could offer some help to this question: https://isaacphysics.org/questions/maths_ch7_2_q7?stage=all
Specifically part d, as I don't really understand what the hint is saying. Thanks in advance.

Reply 1

mqb2766

Original post by rt4e

Not worked the question through but a hemi-disc is a half disc. So the illuminated part of the umbrella is bounded by a hemi-disc cutting through the curved surface of the umbrella and another one which represents the base. So its a bit like figure 11 part c1) (illumination over tropic of cancer) in
https://www.open.edu/openlearn/mod/oucontent/view.php?id=13345&extra=thumbnailfigure_idm403
where the earth is sliced across the equator. The day/night boundary is one hemi-disc and the other is where youve sliced it.