How do you find similar triangles and shadows?

Answer 1

If you mean the use of similar triangles and the similarity of the linear dimensions of the shadows to calculate the linear dimensions of any object which can not be measured directly you need one indispensable condition to solve this problem.

This condition is the need for a parallel lines (such as directions to practically infinitely distant star) or lines which we can assume that in parallel with the permissible error of proportion (such as rays of the sun), Otherwise, if this condition is not feasible then due to parallax effects, the results of the calculations will be not reliable.

If this condition (the presence of parallel lines) is satisfied then we can measure with reasonable accuracy the length of the elements of imaginary triangles and applying one of the Thales theorems, to make a proportion and calculate the required interval (distance or height).