1. The problem statement, all variables and given/known data Atomic hydrogen emits a characteristic spectral line with a frequency of 1421 MHz. A radio telescope observes this line in a certain galaxy at a frequency of 1432 MHz. Is the galaxy approaching or receding, and at what speed (in km/s)? 2. Relevant equations 3. The attempt at a solution I assume the galaxy is approaching as the waves tend to bunch up and thus the perceived frequency is actually higher than the actual frequency. To find the speed I tried calculating the change in wavelength as true-observed and got 0.0016m. Using this fact I used the equation: recession velocity/wave speed = change in wavelength/actual wavelength And got a value of 0.007579, which my text says should be the fraction of the speed of light which the galaxy is approaching at. However, this value is positive and I'm told it should be negative if approaching! Where am I going wrong? Thanks in advance.