SizesProp02
Illustration of Proposition 2 of the book On Sizes.
If a sphere is illuminated by a sphere greater than itself, the illuminated portion of the former sphere will be greater than a hemisphere (Heath, 1913:358).
Prop. 2 shows that, because the sun is much larger than the moon, the lit side of the moon is greater than a hemisphere.
Blue points can be dragged around with the mouse.