Shapiro time delay

Several years ago it became evident that a new test of general relativity was technically feasible.The proposed experiment was designed toverify the prediction that the speed of propagation of a light ray decreases as it passes through a region of increasing gravitational potential.For a radar pulse transmitted from the earthand reflected by another planet, the increase in the round-trip time delay, attributable to thepredicted gravitational influence of the sun onthe propagation, would be ~=200 sec if the pathof the pulse were to graze the solar limb.

— Fourth Test of General Relativity: Preliminary Results, by Irwin I. Shapiro, Gordon H. Pettengill, Michael E. Ash, Melvin L. Stone,William B. Smith, Richard P. Ingalls, and Richard A. Brockelman.

Source of image and caption