Radii from
Eclipse Timing

From point 1 to point 2 the smaller star moves a distance on its orbit equal to its diameter D. But if the magnitude of the velocity v for the smaller star can be determined from Doppler methods and we carefully measure the time t from point 1 to point 2, the diameter follows from multiplying the velocity by this time: D = v t.

By similar reasoning, in the time from when the eclipse begins until it ends, the smaller star covers a distance equal to the diameter of the larger star. Thus, by carefully measuring this interval and multiplying by the velocity we can determine the diameter of the larger star.

The following animation illustrates the relationship of eclipse timing and stellar diameters.