Definition radiometric dating
Series of events can be referred to a time scale, which is an ordered set of times derived from observations of some phenomenon.
Two independent, fundamental time scales are those called dynamical—based on the regularity of the motions of celestial bodies fixed in their orbits by gravitation—and atomic—based on the characteristic frequency of electromagnetic radiation used to induce quantum transitions between internal energy states of atoms.
The equations of motion that define TDB include relativistic terms.
The atomic clocks that form TAI, however, are corrected only for height above sea level, not for periodic relativistic variations, because all fixed terrestrial clocks are affected identically.
When the increasing accuracy of clocks led to the adoption of the mean solar day, which contained 86,400 seconds, this mean solar second became the basic unit of time.
The adoption of the SI second, defined on the basis of atomic phenomena, as the fundamental time unit has necessitated some changes in the definitions of other terms.
Accuracy in specifying time is needed for civil, industrial, and scientific purposes.
The abbreviations given here are derived from English or French terms.
It has made possible new, highly accurate techniques for measuring time and distance.
These techniques, involving radar, lasers, spacecraft, radio telescopes, and pulsars, have been applied to the study of problems in celestial mechanics, astrophysics, relativity, and cosmogony.
Universal time (UT), once corrected for polar variation (UT1) and also seasonal variation (UT2), is needed for civil purposes, celestial navigation, and tracking of space vehicles.
The decay of radioactive elements is a random, rather than a repetitive, process, but the statistical reliability of the time required for the disappearance of any given fraction of a particular element can be used for measuring long time intervals.