What observation confirmed the presence of height variations like mountains near the line dividing light and dark on the Moon?
Answer
Distinct shadows cast by features near the terminator
Galileo's observation was critically dependent on analyzing the light transition zone on the Moon, known as the terminator, which is the line separating the illuminated side from the dark side. If the surface were perfectly smooth, the transition between light and dark would be continuous and gradual. However, Galileo observed sharp shadows being cast by features located near this line. The existence of such distinct shadows provides definitive proof of vertical relief, indicating that structures like mountains or valleys exist, as only elevated objects cast shadows proportionate to a light source angle.
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