A system of time measurement based on the rotation of the Earth, which is assumed to be constant. The Earth’s rotation rate is not in fact precisely constant when checked against atomic clocks. Mean solar time has therefore been superseded by International Atomic Time TAI. Leap seconds are occasionally introduced to allow TAI to keep pace with the Earth’s rotation. Since the Earth’s rotation axis is inclined to the plane of the ecliptic and its orbit around the Sun is elliptical rather than circular, the Sun’s apparent motion through the sky is not uniform over the course of a year. Apparent solar time, as measured directly by a sundial, differs from mean solar time by an amount that varies through the year and is known as the equation of time. To define mean solar time, the abstract concept of the mean Sun was introduced. This hypothetical object follows a circular orbit around the celestial equator at a constant speed, completing one circuit in a tropical year.