ΓΝΩΜΩΝ. A page in French on the web says Thales measured the height of the Great Pyramid at Giza by comparing shadows at the same time of day.

Consider a pyramid, height h unknown. However the length of its shadow, l, is known for a certain time of day, as is the shadow l’ and height, h’ of a reference such as a 6’ man.

h   h'

- = --

l   l'



    h'l

h = ---

     l'

Example: l = 962’, h’ = 6’ and l’ = 12’. Solving for h = 481’ = 6 x 962 / 12.