Can somebody give me some insight as to how I solve the following questions: 1) A rocket travels from the earth to the moon(distance measured from earth 380,000km) at a speed of v = 0.8c. (i) How long does the trip take according to an observer on earth? (ii) How long does the trip take according to the astronaut (iii) What is the earth-moon distance measured by an astronaut on the rocket? So for part (i), I'm thinking along these lines: The earth is reference frame S, the rocket S'. Therefore, the relative velocitys between the reference frames is 0.8c? The distance to the moon(the distance the rocket travels) according to the man on earth is 384000x10^3km which is x. This is where I get stuck. t = (t' + (vx/c^2))x (1-u^2/c^2)^-0.5 I seem to have too many unknowns?