Time standard

The term time scale has both a strictly quantitative and a qualitative rather important. Both views can be the basis of appropriate time systems.

Quantitatively accurate time scale

In the area of most science and technology " time scale " means a sharply defined, regular course of the timeline, which can be related to measured or calculated time points. Such scales provide an accurate measure of time measurement and have

The most important time scales are those that are based on Greenwich Mean Time, and thus on the Earth's rotation:

  • The World Time or Universal Time (UT1 ). It is related to the true rotation of the earth together with the star time and corresponds to the mean solar time at Greenwich.
  • The Coordinated Universal Time ( UTC) is the mean Earth's rotation, but is based on atomic time with the atom- physically defined SI second. Due to leap seconds will ensure that the UTC is never more than 0.9 seconds of UT1 is different.

An intermediate function has

  • The International Atomic Time (TAI ), which is represented by a global network of atomic clocks. Its basis, the SI second was derived from the earth's rotation from 1900 to 1905. It is synchronous to UTC, apart from its leap seconds.
  • GPS time is also in synchronism with the TAI.

On the other hand need astronomy, celestial mechanics and space with the strict relation to the orbital motion of the earth around the sun, which is why in 1960

  • Ephemeris Time (ET ) has been introduced. Based on the atomic second, the movements are calculated in the solar system in this completely uniform time scale.
  • The Dynamic Time (TD ), which has replaced the Ephemeris in 1984 and is based on the international atomic time. Their difference Delta T to the world Time is now bigger and is currently around 65 seconds.
  • Because of the theory of relativity must be strictly between terrestrial ( TDT ) and barycentric dynamic time (TDB ) are different. The latter refers to the current focus of the solar system.

In contrast to this strict sense time scales are relative time measurements - eg with a stopwatch - where the zero point because of the mere measurement of a time difference can be arbitrary. In everyday life, this meaning of the word "time" prevails and therefore can be adapted to the purpose well.

Qualitative time scales

Qualitative time scales, however, are to be preferred if the exact " scale " is less important than the succession of phenomena observed. Such time scales list events or sections that have taken place at a certain time, on their order. But the exact amount of time between them is less important than the interrelation of various series of events.

"Time" is understood here as a kind of distance between two or more events, the time scale is a systematic consequence of these events and ensures its context. Depending on the purpose of a time scale, it uses an appropriate time unit ( seconds, day, year, etc.). For example, would a graph of geological eras for purposes of illustration not serve in the second unit, usually select the geologists here even as a unit the Year Million.

In the history of science can be worked sometimes without strict measure of time if, instead of years the succession of dynasties determined, for example the time frame.

Natural science GPS time#Timekeeping ΔT Myr Dynasty Millennium
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