Quote:
Originally Posted by erimir
The shadow will move across the surface of the earth if the moon moves enough to cover the entire diameter of the earth with respect to the sun. It does not need to move a full half orbit around the earth for its shadow to travel across the lighted face of the earth.
The only time it will be casting a shadow on the earth is when it's between the earth and the sun, and given its size and distance from the earth, that is only a fraction of its orbit. Which is to say, something like if you projected the edges of the lighted portion of the earth towards the sun in a roughly cylindrical shape, the moon only needs to travel from the edge of that cylinder to the other edge. Once it is outside that cylinder, it is no longer between the earth and the sun.
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I had to draw myself a little picture, but this does make perfect sense. The fraction of the moon's orbit that takes its shadow across the earth is very small.
