Gyrochronology is a method for deriving the age of a low-mass star like the Sun from its rotation period.
The term is derived from the Greek words gyros, chronos and logos, roughly translated as rotation, age and study respectively.
It was coined in 2003 by Sydney Barnes

to describe the associated procedure for deriving stellar ages, and developed extensively in empirical form in 2007
.

The technique builds on an insight of Andrew Skumanich,
who realized that another measure of stellar rotation (v sin i) declined steadily with stellar age.
Gyrochronology uses the rotation period P of the star instead of the doubly ambiguous v sin i and in particular, accounts for the substantial mass dependence
of stellar rotation, as exemplified by early rotation period work on the Hyades open cluster
.
These two improvements are largely responsible for the precision in the ages provided by gyrochronology.

The basic idea underlying gyrochronology is that the rotation period P, of a main sequence cool star is a deterministic function of its age t, and its mass M
(or a suitable proxy such as color).
The detailed dependencies of rotation are such that the periods converge rapidly to a certain function of age and mass, mathematically denoted by P = P (t, M),
even though stars have a range of allowed initial periods.
Consequently, cool stars do not occupy the entire 3-dimensional space of (mass, age, period), but instead define a 2-dimensional surface in this space.
Therefore, measuring two of these variables yields the third. Of these quantities, the mass (or a proxy such as color) and the rotation period are the easier variables to measure,
providing access to the star's age, otherwise difficult to obtain.