 the shapiro time delay effect, or gravitational time delay effect, is one of the four classic solar system tests of general relativity. radar signals passing near a massive object take slightly longer to travel to a target and longer to return than they would if the mass of the object were not present. the time delay is caused by the slowing of light as it moves through a change in gravitational potential. in an article entitled fourth test of general relativity, shapiro et al. wrote  the proposed experiment was designed to verify the prediction that the speed of propagation of a light ray decreases as it passes through a region of decreasing gravitational potential ==history== the time delay effect was first noticed in 1964, by irwin shapiro. shapiro proposed an observational test of his prediction: bounce radar beams off the surface of venus and mercury, and measure the round trip travel time. when the earth, sun, and venus are most favorably aligned, shapiro showed that the expected time delay, due to the presence of the sun, of a radar signal traveling from the earth to venus and back, would be about 200 microseconds, well within the limitations of 1960s era technology. the first tests, performed in 1966 and 1967 using the mit haystack radar antenna, were successful, matching the predicted amount of time delay. the experiments have been repeated many times since then, with increasing accuracy. ==calculating time delay== in a near-static gravitational field of moderate strength (say, of stars and planets, but not one of a black hole or close binary system of neutron stars) the effect may be considered as a special case of gravitational time dilation.the measured elapsed time of a light signal in a gravitational field is longer than it would be without the field, and for moderate strength near-static fields the difference is directly proportional to the classical gravitational potential, precisely as given by standard gravitational time dilation formulas. =
Effects_of_gravitation Tests_of_general_relativity