the study of the psr b1913+16 binary pulsar also led to the first accurate determination of neutron star masses, using relativistic timing effects. when the two bodies are in close proximity, the gravitational field is stronger, the passage of time is slowed – and the time between pulses (or ticks) is lengthened. then as the pulsar clock travels more slowly through the weakest part of the field it regains time. a special relativistic effect, time dilation, acts around the orbit in a similar fashion. this relativistic time delay is the difference between what one would expect to see if the pulsar were moving at a constant distance and speed around its companion in a circular orbit, and what is actually observed.