How did sunlight low on the horizon near the terminator aid Galileo's discovery?
Answer
Shadows became elongated, exaggerating the topography
The key advantage of observing near the terminator was the geometry of light when the Sun is at a low angle relative to the Moon's surface, similar to sunrise or sunset on Earth. This low angle causes shadows to stretch out considerably. Galileo utilized this phenomenon because these elongated shadows visually exaggerated any variations in height. Modest hills appeared as towering peaks due to their long shadows, and deep valleys appeared as pronounced dark patches extending from the shadow line. This enhancement through shadow length provided the irrefutable empirical evidence of vertical relief that confirmed the Moon was uneven, relying on early geometric reasoning.
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