What feature did Galileo focus on to infer lunar topography using light and shadow?
Answer
The terminator
Galileo's methodology for discovering surface elevation relied heavily on observing the dynamic interplay of light and shadow, focusing specifically on the terminator. The terminator represents the moving border separating the sunlit portion of the Moon from the shadowed region. When the Sun is low on the lunar horizon, analogous to dawn or dusk on Earth, shadows cast by elevated objects are dramatically elongated. Galileo noticed bright spots—mountain peaks—jutting out from the shadow line. By interpreting these elongated shadows, he was able to deduce the presence of significant topographical variations, such as towering mountains and deep valleys, which would be invisible or minimally apparent under full illumination.
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