1, 4713 (MJD), defined by the equation MJD = JD - 2,400,000.5, begins at midnight rather than noon and, for the 20th and 21st centuries, is expressed by a number with fewer digits. 14, 1981 (Gregorian calendar date), corresponds to JD 2,444,923.0; the preceding midnight occurred at JD 2,444,922.5 and MJD 44,922.0.
Historical details of the week, month, year, and various calendars are treated in the article calendar.
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.
Two time scales that have no relative secular acceleration are called equivalent.
That is, a clock displaying the time according to one of these scales would not—over an extended interval—show a change in its rate relative to that of a clock displaying time according to the other scale.
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.
Accuracy in specifying time is needed for civil, industrial, and scientific purposes.
Although defining time presents difficulties, measuring it does not; it is the most accurately measured physical quantity.
Universal Time (UT; mean solar time or the prime meridian of Greenwich, England), Coordinated Universal Time (UTC; the basis of legal, civil time), and leap seconds are treated under the heading Rotational time.
Ephemeris Time (ET; the first correct dynamical time scale) is treated in the section Dynamical time, as are Barycentric Dynamical Time (TDB) and Terrestrial Dynamical Time (TDT), which are more accurate than Ephemeris Time because they take relativity into account.
Atomic clocks serve as the basis of scientific and legal clock times.