This document discusses various systems of time measurement. It describes: 1. Solar time, which is based on the sun's apparent daily motion, and mean solar time, which averages small variations. 2. Sidereal time, which is based on fixed stars' daily motions rather than the sun. 3. Universal Time (UT) and its variants, which are based on mean solar time at the Greenwich meridian. 4. International Atomic Time (TAI) and Coordinated Universal Time (UTC), which are atomic clock-based time standards. 5. Time scales used in global navigation satellite systems like GPS, GLONASS, Galileo, and BeiDou.