Spin-down is the gradual lengthening of the period of a pulsar as it loses energy with age. Observed spin-down rates range from 10-12 sec per sec for the youngest pulsars to 10-19 sec per sec for recycled pulsar. The Crab pulsar is slowing at a rate of 3.5 × 10-13 sec per sec. Occasionally a glitch occurs in the spin-down in which there is a sudden increase in rotation rate. This is thought to be the equivalent of an earthquake where the crust of the neutron star settles a little bit, decreasing the radius and moment of inertia.