What general-relativity effect delays radar signals passing near the Sun?

The general-relativity effect that delays radar signals passing near the Sun is the Shapiro time delay.

A light or radar signal takes longer to travel through a gravitational field than it would along an equivalent path in flat spacetime. In the usual weak-field description, the effect is associated with gravitational time dilation and the curved spacetime around the intervening mass. The signal is also deflected, but for the classic Solar System test, the extra travel time is the key measurement.

Irwin Shapiro proposed the test in 1964 in his paper “Fourth Test of General Relativity.” He suggested bouncing radar pulses off Venus or Mercury when the signal path passed close to the Sun. At a favorable Earth–Sun–Venus alignment, the predicted round-trip delay was about 200 microseconds, equivalent to roughly 60 kilometers of extra light-travel distance.

MIT’s Haystack radar antenna produced successful tests in 1966 and 1967. The effect is one of the four classic tests of general relativity. It is often confused with gravitational lensing: lensing changes a signal’s path and apparent direction, while the Shapiro effect specifically concerns the additional travel time.