Doppler extrema for a circularly moving source
= Doppler extrema for a circularly moving source
A source of fixed proper <frequency> and <speed> $v$ on a circle has instantaneous <frequency> extrema $\nu_0\sqrt{(1\pm\beta)/(1\mp\beta)}$ for a stationary coplanar receiver outside the circle. The extrema occur when the emission ray is tangent to the source circle, so line-of-sight <speed> reaches $\pm v$.