Relevant equations t' = t * sqrt(1 - v^2/c^2), where t' is the proper time interval, and t is the measured time interval in another reference frame. The problem statement, all variables and given/known data/ Q: A spaceship is moving past us at a velocity of v = 3c/5. If, in our frame of reference, we measure a time interval of 1 second, what will be the time measured by clocks within the moving ship? Actually, I'm questioning the solution that I'm given. I can get the same answer in both ways, but I want to confirm if I am correct in theory. So my question would be: should I assign t = 1 or t' = 1? The solution I was given suggests t = 1 such that t' = 0.8s. The attempt at a solution In my opinion, it's t' = 1 because each event (successive positions of the second hand; let's suppose it's analog) of the clock with us is observed to occur at the same position, while that of the clock(s) within the moving ship are observed to occur at different positions. Hence 1 second is the proper time interval. This gives me t = 1.25s, which is the measured period of the second hand's motion on the ship. Then taking 1/1.25 = 0.8, we have 0.8 of a cycle of the second hand's motion on the moving ship - the time showed on the clock on the ship is 0.8s.