International Atomic Time (TAI) is calculated by the BIPM from the readings of more than 200 atomic clocks located in metrology institutes and observatories in more than 30 countries around the world. TAI is made available every month in the BIPM Circular T (ftp://184.108.40.206/pub/tai/publication/). BIPM estimates that TAI does not lose or gain with respect to an imaginary perfect clock by more than about one tenth of a microsecond per year, or less than 1 second over a million year.
Coordinated Universal Time (UTC)
is the basis for legal time worldwide and follows TAI exactly except
for an integral number of seconds, presently -37
These leap seconds are inserted on the advice of the
International Earth Rotation Service (IERS)
, to ensure that, on average over the years, the Sun is overhead within 0.9 seconds
of 12:00:00 UTC on the meridian of Greenwich.
UTC is than the modern successor of Greenwich Mean Time, GMT ,
(see below)which was used when the unit of time was the mean solar day.
UTC in the display means UTC(KIM). Indonesian Standard Time (WIB, Waktu Indonesia Barat: West Indonesian Time, WITA, Waktu Indenesia Tengah: Central Indonesian Time, and WIT, Waktu Indonesia Timur: East Indonesian Time) are then realized based on UTC(KIM)
Greenwich Mean Time (GMT) is equivalent to universal time (UT). GMT is the "civil" name for the standard; UT is the "scientific" name for the same standard. The distinction between UTC and UT is that UTC is based on an atomic clock (i.e TAI) and UT is based on astronomical observations, which for all practical purposes is an invisibly fine hair to split. Because the earth's rotation is not uniform (it slows down and speeds up in complicated ways), UT does not always flow uniformly. Leap seconds are introduced as needed into UTC so as to keep UTC within 0.9 seconds of UT1, which is a version of UT with certain corrections applied.
last edited 2016-12-27