Lets see if this helps
of the tree and the end of its Shadow is similar to the triangle formed
by the top of the stick, the base of the stick and the end of the sticks shadow.
This is because the angles at the base of the tree and stick are both right
angles, and the other angles corresponding angles in the two triangles are
also equal as they are angles formed between either vertical or horizontal lines
and a limiting ray from the Sum. But the Sun is so distant that we assume
that all rays from the sum are parallel.