Comparison and coordination of time scales

Historically, time scales have been based on the earth's rotation. Even today, the atomic-based time scale (UTC) is adjusted annually to keep it in time with the rotating earth. However, continual research for greater accuracy in timekeeping has led to the adoption of time scales based on other phenomena, so that four classes of time scales are currently in use. These rational, dynamical, atomic, and hybrid time scales are the subject of the present discussion.