Editing Shapiro time delay (section)

== History ==
The time delay effect was first predicted 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,<ref name=Shapiro1964/> well within the limitations of 1960s-era technology.

The first tests, performed in 1966 and 1967 using the [[Massachusetts Institute of Technology|MIT]] [[Haystack Observatory|Haystack radar antenna]], were successful, matching the predicted amount of time delay.<ref name=Shapiro1968>{{cite journal | display-authors=4| author=Irwin I. Shapiro| author2=Gordon H. Pettengill| author3=Michael E. Ash| author4=Melvin L. Stone| author5=William B. Smith| author6=Richard P. Ingalls| author7=Richard A. Brockelman| name-list-style=amp | title=Fourth Test of General Relativity: Preliminary Results | journal=[[Physical Review Letters]] | date=1968 | volume=20 | pages=1265–1269 | doi=10.1103/PhysRevLett.20.1265 | issue=22 | bibcode=1968PhRvL..20.1265S}}</ref> The experiments have been repeated many times since then, with increasing accuracy.