Full Idea
The height of the flagpole explains the length of the shadow because the interaction between the sunlight and the flagpole occurs before the interaction between the sunlight and the ground.
Gist of Idea
Flagpoles explain shadows, and not vice versa, because of temporal ordering
Source
Wesley Salmon (Four Decades of Scientific Explanation [1989], 3.6)
Book Reference
Salmon,Wesley C.: 'Four Decades of Scientific Explanation', ed/tr. Humphreys,Paul [Pittsburgh 2006], p.103
A Reaction
[Bromberger produced the flagpole example] This seems to be correct, and would apply to all physical cases, but there may still be cases of explanation which are not causal (in mathematics, for example).
Related Idea
Idea 4805 If laws explain the length of a flagpole's shadow, then the shadow also explains the length of the pole [Psillos]