Relativistic aberration

Suppose, in the reference frame of the observer, the source is moving with speed $v\,$ at an angle $\theta_s\,$ relative to the vector from the observer to the source at the time when the light is emitted. Then the following formula, which was derived by Einstein in 1905, describes the aberration of the light source, $\theta_o\,$, measured by the observer:
$\cos \theta_o=\frac{\cos \theta_s-\frac{v}{c}}{1-\frac{v}{c} \cos \theta_s} \,$