1. The problem statement, all variables and given/known data A stellar object at some known large distance ejects a ‘jet’ at speed v towards an observer obliquely, making an angle θ with the line of sight. To the observer the jet appears to be ejected sideways at speed V . Prove V = c sin θ (c/v − cos θ )−1 , and show that this can exceed c, for example, when θ = 45◦ . [Indeed, such apparently superluminal jets once had observers worried—briefly.] Question from Rindler 2.21 2. Relevant equations Nothing more is supplied in the question. 3. The attempt at a solution I have tried to use the velocity transformation in the frame of stellar object?(Say frame S). Since I know the speed of the object in observers frame(say S') which is V in the y direction and velocity u'(0,V,0) say in standart configuration.and I know the vcosθ is the speed in x direction vsinθ is the speed in y direction and velocity u(vcosθ,vsinθ,0) in the frame S. However I do not know the observer's relative speed in stellar object's frame. I tried to find by applying velocity transformation in x direction first to find relative speed of observer and then applied in y direction but no use. I have found out to satisfy the given equation speed of the observer should be c relative to the stellar object. What do I miss? I think I'm missing something silly but I cannot find out. I have been thinking for days. Please give me clue.