How did Galileo interpret the dark areas extending into the light near the lunar terminator?
Answer
Deep valleys or plains shadowed by peaks
When observing the Moon near the terminator, Galileo noted a stark contrast: bright spots representing illuminated mountain peaks, and dark areas extending into the light. He correctly deduced that these dark extensions were not features intrinsic to the surface being dark, but rather the shadows cast by those very elevated peaks he observed. Therefore, the large dark patches were interpreted as deep valleys and broad plains that were momentarily obscured from direct sunlight by the surrounding high ground, providing further evidence of extreme vertical relief, or 'depressions,' on the Moon.
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