The period derivative is the rate at which an object's orbital or rotation period

is changing - ie the instaneous change in period divided by the change in time. In calculus terms this is simply**P**and is often expressed as a dimensionless quantity.**dP/dt**Sometimes physicists use the simple dot above a quantity to represent the derivative with respect to time

, so the period derivative**t**is often called**dP/dt**, ie:**P-dot**here the subscripts 0 and f refer to the initial and final conditions respectively.

**Applications**- In pulsar astronomy the period derivative can be used to estimate a pulsar's characteristic age and magnetic field strength.

- The ratio of a binary's orbital period to orbital period derivative
gives a characteristic timescale upon which it is evolving.**P/(dP/dt)**