If you have time of the image accurate to the minute, and you know that the plane, sun and observer were on a line, the observer has to be on the ground track of the shadow of the plane during this minute. If you have that from two planes on different tracks, the observer has to be at the intersection of the ground tracks.

This method is not just from measuring the angle of moon/sun, but using the plane (which has a known position) as a marker. Just by using the angle of sun and moon, you need to be extremely precise. 1 degree error is 1/360 of the circumference of earth distance, that’s about 100km.