Unit of time

The natural units for timekeeping used by most historical societies are the day, the solar year and the lunation. Such calendars include the Sumerian, Egyptian, Chinese, Babylonian, ancient Athenian, Buddhist, Hindu, Islamic, Icelandic, Mayan, and French Republican calendars. The modern calendar has its origins in the Roman calendar, which evolved into the Julian calendar, and then the Gregorian calendar. == Scientific ==

• The Planck time is the time that light takes to travel one Planck length. (5.391 247 × 10⁻⁴⁴ sec., Full Form: .000 000 000 000 000 000 000 000 000 000 000 000 000 000 053 912 47 sec.) • The Jiffy is the amount of time light takes to travel one femtometre (about the diameter of a nucleon). • The atomic time relates to the orbital period of a ground state electron around a hydrogen atom and is about 24.2 attoseconds. • The svedberg is a time unit used for sedimentation rates (usually of proteins). It is defined as 10−13 seconds (100 fs). • The TU (for time unit) is a unit of time defined as 1024 μs for use in engineering. • The galactic year, based on the rotation of the galaxy and usually measured in million years. • The geological time scale relates stratigraphy to time. The deep time of Earth's past is divided into units according to events that took place in each period. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event. The largest unit is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and ages. It is not a true mathematical unit, as all ages, epochs, periods, eras, or eons don't have the same length; instead, their length is determined by the geological and historical events that define them individually. A table with approximate time lengths can be seen at Divisions of geologic time Note: The light-year is not a unit of time, but a unit of length of about 9.5 petametres (). Note: The parsec is not a unit of time, but a unit of length of about 30.9 trillion kilometres, despite movie references otherwise. == List ==

All of the formal units of time are scaled multiples of each other. The most common units are the second, defined in terms of an atomic process; the day, an integral multiple of seconds; and the year, usually 365 days. The other units used are multiples or divisions of these three. == See also ==