I am to measure the rotational velocity of Io relative to Jupiter, and will do so using a spectrograph. I will measure the frequency change of a specific mineral emmision band in the spectra, and then check how much it has shifted from its original wavelenght with a table. Then I use the formula: [tex]v = (\Delta \lambda/\lambda) c[/tex] where [tex]\Delta \lambda[/tex] is the change of frequency and [tex]\lambda[/tex] is the rest wavelenght. C is speed of light. to calculate the velocity for each object. The problem is how do I calculate the velocities relative to each other? Io rotates around jupiter, using my method i believe i can only calculate its speed moving relative to earth?