ABSTRACT
Time systems based on astronomical events are ultimately flawed. Atomic clocks use the ultra stable property of Cesium or Rubidium electronic transitions. Local sidereal time is a system designed for use by astronomers. It is based on the Earth's rotation and does not account for its orbit around the Sun. Julian dates are a day-number system that allows users to calculate the elapsed time between two dates. The formula converts dates into an integer that allows one to quickly work out the interval. An epoch is a moment in time used as a reference point for a time-changing attribute, for instance, the coordinate of a star. Astrometric data often references the epoch of the measurement or coordinate system. The astronomical unit or AU for short is also used. It is most useful when used in the context of the measurement of stellar distances in parsecs.
