... Star Atlas . ... Gyrochronology transforms a rotating star into a clock which is set using the Sun and keeps time well," says Sydney Barnes, Lowell Observatory astronomer. ... By showing that the rotation period of a star is a steadily changing and tight function of its age and color, gyrochronology allows the age to be determined by measuring the two other properties ? ... Barnes derives ages for sample stars where rotation and color are known, but the stellar ages using other methods are not known...