This document was written by Kawasaki Naoyuki in Sapporo on November 15, 2002.
If you have any question, send me e-mail to kawasaki@azores.cc.hokudai.ac.jp .
UTC is a world standard time widely adopted. JST can be calculated by adding 9 hours to UTC. UTC is given by TAI + accumulated leap seconds. TAI has no leap second. GMT has already lost its role as a world standard time being replaced by UTC. In addition, the role of GMT as an earth rotation time was taken over by UT (UT0, UT1 and UT2). GMT is no longer an academic term. But the word "GMT" is often used in the place, which should be replaced by UTC because of its public recognizability.
TAI, Temps Atomique International, is given by
BIPM : Bureau International des Poids et Mesures
.
It is generated by the inputs of over 200 atomic clocks around the world.
BIPM is the organization in Paris founded by Convention du Mètre.
The International System of Units (SI) is also maintained by BIPM.
Now, an atomic clock keeps ticking a precise second defined physically in SI,
while the rotation speed of the earth is gradually slowing down
at the rate of around a second per year.
This is a problem because the error can be accumulated to upset day and night in the long term.
In UTC, a leap second is added or subtracted from time to time to keep the gap of
the standard time and the earth rotation time within 0.9 second.
The rotation of the earth is measured by
International Earth Rotation Service (IERS).
As a result of the compensation, TAI and UTC have a gap of 32 seconds as of January, 2002.
JST is provided by Communications Research Laboratory (CRL). The time ticked by 10 atomic clocks in CRL is sent to BIPM to be a source of TAI. CRL also provides standard frequency broadcast to set radio-wave clocks.
We are living in the standard time of the UTC series.
But why is it U-T-C ?
In English order, it should be "CUT" as the acronym of "Coordinated Universal Time."
In French order, it should be "TUC" as the acronym of "Temps Universel Coordonné."
Actually, it is said that the order of UTC is a result of the battle
between French supporters and English supporters.