Systems of Time
There are four types of systems in common use:
- Atomic time: the unit of duration corresponds to a defined number of wavelengths of radiation of a specified atomic transition of a chosen isotope – the SI second
- Dynamical time: the unit of duration is based on the orbital motion of the Earth, Moon, and planets
- Sidereal time: the unit of duration is the period of the Earth’s rotation with respect to a point nearly fixed with respect to the stars
- Universal time: the unit of duration represents the solar day, defined to be as uniform as possible, despite variations in the rotation of the Earth