Gravitomagnetic time delay
In according to general relativity, a massive spinning body endowed with angular momentum S will alter the space-time fabric around it in such a way that several effects on moving test particles and propagating electromagnetic waves occur.
In particular, the direction of motion with respect to the sense of rotation of the central body is relevant because co-and counter-propagating waves carry a "gravitomagnetic" time delay ΔtGM which could be, in principle, be measured if S is known.
On the contrary, if the validity of general relativity is assumed, it is possible to use ΔtGM to measure S. Such effect must not be confused with the much larger Shapiro time delay ΔtGE induced by the "gravitoelectric" Schwarzschild-like component of the gravitational field of a planet of mass M considered non-rotating. Unlike the small ΔtGM, the Shapiro time delay has been accurately measured in several radar-ranging experiments with solar system interplanetary spacecraft.
