Im trying to understand frames of reference, im very new to relativity so sorry if im being silly! Suppose you have a 2 spacecraft one travelling relative to the sun, and the other traveling relative to the first spacecraft in a perpendicualar direction. i know the velocitys of space craft A relative to the sun and the velocity of B relative to A. Am i right in saying theres three frames of reference? What im trying to consider is relating spacecraft B to the sun...so am i right in saying i relate A to the sun with lorentz transformations and B to A and relate the two? Im just a little confused since theres the aditional moving object! Any explaination would help!