ABSTRACT
This chapter provides several operative definitions of time, together with the transformations among them, in order to complete the notions expounded by taking into account precession, nutation, aberration and the Earth rotation angle (ERA) and its variations. It introduces the notions and algorithms based on the ERA. The chapter details yearly revolution of the Earth provides a new timescale and of a new unit of time, namely, the year, which can be defined in several different ways. International atomic time (TAI) is certainly a very uniform time; nevertheless, according to general relativity, the frequency of any clock varies with the varying gravitational potential in which it is immersed. TAI is certainly a very uniform time; nevertheless, according to general relativity, the frequency of any clock varies with the varying gravitational potential in which it is immersed. The International Terrestrial Reference Frame is the realization of the International Terrestrial Reference System through a number of fiducial stations with well-known coordinates and velocities.
