the binary pulsar psr b1913+16 (or the "hulse-taylor binary pulsar") was first discovered in 1974 at arecibo by joseph hooton taylor, jr. and russell hulse, for which they won the 1993 nobel prize in physics. while hulse was observing the newly discovered pulsar psr b1913+16, he noticed that the rate at which it pulsed varied regularly. it was concluded that the pulsar was orbiting another star very closely at a high velocity, and that the pulse period was varying due to the doppler effect: as the pulsar was moving towards earth, the pulses would be more frequent; and conversely, as it moved away from earth fewer would be detected in a given time period. one can think of the pulses like the ticks of a clock; changes in the ticking are indications of changes in the pulsars speed toward and away from earth. hulse and taylor also determined that the stars were approximately equally massive by observing these pulse fluctuations, which led them to believe the other object was also a neutron star. pulses from this system are now tracked to within 15 μs.