Issues of Time for Reference Systems

At the International Astronomical Union General Assembly in 1991 the terminologies Geocentric Coordinate Time (TCG) and Barycentric Coordinate Time (TCB) were introduced as the time coordinates of four dimensional geocentric and barycentric coordinate systems, respectively. The reasons for these time-like arguments and their applications will be discussed. The relationships between the different time-like arguments will be discussed along with their applications and relationships to the astronomical constants.