Okay, I will explain like you're 5. If you have seen any videos from LEO satellites, including the ISS, you would notice that any particular point on the groun

Okay, I will explain like you're 5. If you have seen any videos from LEO satellites, including the ISS, you would notice that any particular point on the ground moves very fast from the camera's point of view. In the photo presented on the post, you can see the tracks of man made light pollution - the bright light emanating from cities being smeared across the image plane. But you can see the start and the end of those tracks, and they are small. Which means you can gauge the relative position of a single point on the ground at the beginning and at the end of the exposure. And it hasn't moved much. Ergo, you can estimate that the exposure time is quite small. Let me know if that wasn't clear enough, I am not used to explaining things to this level.