The term time scale has both a strictly quantitative and a more qualitative meaning. Both perspectives can be the basis of corresponding time systems .
Quantitatively exact time scale
In the field of most natural sciences and technology , “time scale” means a sharply defined, regular course of the time axis to which measured or calculated points in time can be related. Such scales form and have an accurate measure of time measurement
- as a basis principally the earth's rotation , but also because of its small irregularities
- a physical model that must be implemented with sufficient accuracy for technical application (e.g. by time signals or atomic clocks ), and a precisely defined zero point .
- If the timescale is to be more precise than 1 second per year, it must also take into account small irregularities in the earth's rotation and relativistic effects.
The most important time scales are those 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 earth rotation over sidereal time and corresponds to the mean solar time in Greenwich .
- The Coordinated Universal Time (UTC) corresponds to the average Earth's rotation, but is based on the atomic time with the atomic second . By leap seconds will ensure that the UTC never deviates more than 0.9 seconds of UT1.
Has an intermediate function
- the International Atomic Time (TAI), which is represented by a worldwide network of atomic clocks. Its basis, the SI second, was derived from the Earth's rotation from 1900 to 1905. It runs synchronously with UTC, apart from its leap seconds.
- the GPS time , also synchronized with the TAI.
On the other hand, astronomy, celestial mechanics and space travel need a strict reference to the orbital movement of the earth around the sun.
- The Ephemeris Time (ET) was introduced around 1960. Based on the atomic second, the movements in the solar system were calculated on this completely even time scale .
- The terrestrial time TT replaced the ephemeris time around 1984. Except for a fixed difference, it runs synchronously with TAI, but its Delta T difference to world time is slowly increasing and is currently around 69 seconds.
- The Barycentric Dynamic Time TDB was introduced as a further successor to the Ephemeris Time . On earth, it differs from the TT due to relativistic effects, but only by a maximum of 2 ms.
In contrast to time scales of this strict sense, there are relative time measurements - e.g. B. with a stopwatch - where the zero point can be any difference because of the mere measurement of a time . In everyday life , this meaning of the word "time" prevails and can therefore be well adapted to the respective purpose.
Qualitative time scales
Qualitative time scales, on the other hand, are preferable when the exact “yardstick” is less important than the sequence of the phenomena under consideration. Such time scales list events or sections that have occurred at a certain point in time in their order. But the exact length of time between them is less important than the mutual assignment of different series of events.
“ Time ” is understood here as a kind of distance between two or more events, the time scale is a systematic sequence of these events and ensures their connection. Depending on the purpose of a timescale , it uses a suitable time unit ( second , day, year, etc.). For example, a representation of geological ages in the unit of seconds would not serve the purpose of illustration, mostly geologists even choose the million years as the unit .