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gradually weakening. You have heard above
how half of it receives its light from the whole sun.
But when the time comes for it to eclipse, no brightness
comes from it anywhere. And this does not happen
except when it has turned half its body toward
the sun so that it is completely illuminated.
But the moon, which does not always go as straight
as the sun does, sometimes goes along a path such
that the earth entirely shadows it. For the earth is
large than it is, and for this reason can put it in shadow this way.
For between the moon and the earth there is a line that
bends toward the moon when it is on its usual course,
and when the sun remakes it with its rays and gives it
brightness. (see note) From this it regains its light. But the less
it passes through the shadow, the less it loses of
the brightness that it receives from the sun, wherever it
might be, except when it must be taken into shadow.
You can understand it like this: If a line passes through the earth
via its midpoint, extending outside both parts and reaching
straight as far as the body of the sun, and the other end