It was found from the observations of the radiopulsars that the intervals between their pulses are changing periodically, besides, for all pulsars this period is the same and equal to one stellar (sidereal) year. Explain this phenomenon
The variation comes from the Earth's movement around the Sun. When the Earth is moving away from the pulsar in question, each subsequent pulse has slightly further to travel (up to 30km for a 1 second pulsar on the ecliptic down to zero for a pulsar located due North or South) and so is slightly delayed compared to the one before. Six months later, each subsequent pulse has slightly less far to travel, and so arrives a little earlier compared to the one before. The variation ranges from zero at the North or South poles to 1 part in 10000 on the ecliptic (this just comes from the ration between the Earth's orbital velocity and the speed of light).