Universal Time

[a] Using telescopes, GMT was calibrated to the mean solar time at the prime meridian through the Royal Observatory, Greenwich.

At the end of this conference, on 22 October 1884,[b] the recommended base reference for world time, the "universal day", was announced to be the local mean solar time at the Royal Observatory in Greenwich, counted from 0 hours at Greenwich mean midnight.

[6] During the period between 1848 and 1972, all of the major countries adopted time zones based on the Greenwich meridian.

[7] In 1928, the term Universal Time (UT) was introduced by the International Astronomical Union to refer to GMT, with the day starting at midnight.

[9] As the general public had always begun the day at midnight, the timescale continued to be presented to them as Greenwich Mean Time.

[citation needed] When introduced, broadcast time signals were based on UT, and hence on the rotation of the Earth.

Starting in 1956, WWV broadcast an atomic clock signal stepped by 20 ms increments to bring it into agreement with UT1.

[citation needed] Historically, Universal Time was computed from observing the position of the Sun in the sky.

But astronomers found that it was more accurate to measure the rotation of the Earth by observing stars as they crossed the meridian each day.

Nowadays, UT in relation to International Atomic Time (TAI) is determined by Very Long Baseline Interferometry (VLBI) observations of the positions of distant celestial objects (stars and quasars), a method which can determine UT1 to within 15 microseconds or better.

Next, the time scales based on Earth's rotation are not uniform and therefore, are not suitable for predicting the motion of bodies in our solar system.