International Atomic Time

The astronomical time, called Universal Time, or UT1, has tended to fall a few clicks behind the atomic one, called International Atomic Time, or TAI.

It was devised as a way to align the international atomic time scale, in use since 1967 and derived from the vibration of cesium atoms, with the slightly slower time that Earth keeps as it rotates.

In the summer of 1972, the International Time Bureau for the first time added a leap second to try to resolve a difference between International Atomic Time and the less-precise solar time.

BIPM, a subsidiary of the CGPM, is responsible for generating International Atomic Time, on which utc is based, from the results of 500 clocks distributed around the world.

International atomic time — on which coordinated universal time, or UTC, is based — is currently calculated by averaging measurements from hundreds of atomic clocks.