The problem statement, all variables and given/known data The star Alpha goes supernova. Ten years later and 100 ly away, as measured by astronomers in the galaxy, star Beta explodes. An alien spacecraft passing through the galaxy finds that the distance between the two explosions is 120 ly. According to the aliens, what is the time between the explosions? Relevant equations c^2*t^2 - x^2 = c^2(t')^2 - (x')^2 The attempt at a solution I tried using the spacetime interval equation but the distances are too small to see a difference and gives me a wrong answer. Then i did not include c^2 with the equation and i got the correct answer. Which is 67.1 years. I also tried to multiply x^2 and (x')^2 with c^2 and got the same answer. Is it under specific units, like light years, where we can neglect the c^2 or did I just used the wrong equation in the first place?