A rocket travels from earth to the moon (distance measured from the earth 384000km) at a speed v = 0.8c. (i) How long does it the trip take according to an observer on earth? Just used straightforward Newtonian mechanics, t = 384000*10^3/(0.8*3*10^8) = 1.6s. (ii) How long does the trip take according to the astronaut? Used the lorentz transformation, t' = \gamma (t - u/c^2 x) = (0.36)^(-1/2) * (1.6 - 0.8/c^2 * 384000*10^3) = 0.96s I believe this part is correct? Now the part I am stuck on is: (iii) What is the earth-moon distance measured by an astronaut on the rocket? Could someone please explain to me the principles behind the calculation? I am finding it difficult to get my head around SR! Many thanks, James.