1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution First off this isn't really homework, just a problem I can't seem to wrap my head around. I worked through the questions in the frame of the Earth and got the right answers: a) 12:50 b) 7.2 x 10^11 m c) 1:30 d) doesn't matter Solving (c) in the earth frame is easy. The rocket is moving away from the earth with v = 0.8c, emits a pulse at v=c at a distance 7.2 x 10^11 m away, so time is just d/v. My question is in solving part (c) in the frame of the rocketship. In this frame, the earth is moving away from the rocketship with v=-0.8c, so now there is a relative speed (-0.2c) between the pulse emitted from the rocket (stationary in this frame) and the earth moving away with v=-0.8c.