As we said above, we must correct any Doppler shift for the inclination of the orbit, i. If we take a tilted ellipse and project it onto the sky, we still have an ellipse. However, that ellipse will have a different eccentricity than the true ellipse. When we look at an elliptical orbit, how can we tell if it is tilted or not? For a tilted orbit the foci will not appear in the right place for the projected ellipse. Therefore, if we see two stars orbiting a point different from the center of mass, we will know that the orbit is inclined. We can determine the inclination from the degree to which the foci appear to be displaced. In this technique, we don't actually know where the center of mass is, so a process must be used in which we try one position for the center of mass and try to match the projected orbit, repeating the process until a good fit is achieved.
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