User's Manual Part 2

B-2 Universal Coordinated Time

Another way of keeping time is similar to solar time, called Universal

Time. Within this category are variations. UT1 is a measure of the rotation

angle of the Earth as observed astronomically. Solar time varies slightly.

Because of the Earth’s tides, the earth slows down, wobbles, and

introduces slight variations in measurements. UT1 accounts for these

variations, making it useful for astronomy.

Universal Coordinated Time is the basis for worldwide civil time-keeping.

Timing laboratories around the world contribute to provide the

international standard Universal Coordinated Time (UTC).

NOTE: “UTC” technically does not represent a series of words.

During international discussions, the three letters were agreed upon

as a “symbol” rather than an abbreviation or acronym.

The UTC second is based on the atomic transition of the element cesium

under specific conditions. It is independent of astronomical variations, and

because of its stability is accurate to a nanosecond (1,000,000,000

of a

second) per day.