The easiest way I know to explain it is this:
-- Take a flashlight and a ball into a dark room.
-- Turn on the flashlight and point it at the ball.
-- Half of the ball is lighted up by the flashlight, and the other half is dark.
-- There is no way you can turn or twist the ball to make more or less
than 50% of it lighted up and more or less than 50% of it dark.
Everything in the solar system ... as long as it's shaped like a ball ... is
half illuminated by the sun and half dark.
