This reminds me of an Italo Calvino short story about a guy doing backyard telescope observatory space viewing (whew) and he spots someone holding a sign on a distant start stating “I know what you did”. The viewer now starts calculating exactly where he would have been at the time including light speed and distance light travels to reach him and all sorts of other calcs until he determines the time in the past and finally realizes “Whoa! He saw that!?”.