Slowing and bending of light in the vicinity of massive bodies
     Around 1970, Irwin Shapiro and colleagues at MIT's Lincoln Laboratory measured the times for radar signals to travel to Venus and back to Earth. The round-trip travel time for the signals was about 1000 seconds when Venus was on the far side of the Sun from Earth. Shapiro determined that the closer a signal's path was to the Sun, the greater the slowing of the signals. When a round-trip signal passed within about 10 LS of the Sun's center, the signal was delayed by about 170 microseconds. When a round-trip signal passed within about 120 LS of the Sun, the signal was delayed by about 80 microseconds. The time delays specified by Eqs. (33), (37) and (38) are 182 microseconds and 84 microseconds, in general agreement with experimental results.
     The bending of the paths of photons due to the Sun's mass has also been determined by observing stars and quasars which are seen to pass behind the Sun as Earth orbits the Sun. The first of these observations was made in 1919 by Arthur Eddington. The observed bending when a photon's path just grazes the Sun's surface is 1.75 arcseconds, and the observed bending when the path comes within 78 LS of the Sun's center is .05 arcseconds. The bendings specified by Eqs. (33), (37) and (38) are 1.746 arcseconds and .051 arcseconds, in agreement with the experimental results.