How did Galileo deduce the existence of mountains on the Moon using observations near the terminator?

Answer

By observing rugged, elongated shadows cast by low-angle light.

When the sun struck the surface at a low angle near the terminator, rugged, elongated shadows were projected by features rising above the surface, allowing him to deduce peaks and valleys.

How did Galileo deduce the existence of mountains on the Moon using observations near the terminator?
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