The length of a shadow, the height of the Sun above the lunar horizon and a little trigonometry give the height of the peak throwing it, and that is how the first lunar mountains were measured in 1610.

Galileo had trained as an artist and understood shadows; he worked out that some peaks were several kilometres high, which was a scandal in a cosmology where the Moon was a perfect sphere. His numbers were too big, mostly because he underestimated the Moon's size, but the method is exactly the one still used on every rocky world we photograph.