The Shapiro delay is the extra time delay light experiences by travelling past a massive object due to general relativistic time dilation. It was first verified by Irwin Shapiro by using radar echoes from Venus when it was near the Sun and later in binary pulsars via pulsar timing.

The Shapiro delay can be described by just two variables, the range r and the shape s=sin i. The range is proportional to the mass and the effect is maximum when s=1.