NIST's primary mission is to promote U.S. economic growth by working with industry to develop and apply technology, measurements, and standards. It was established by Congress to assist industry in the development of technology. How Time is Measured A better and more detailed description of time scales is available from the U.S. Naval Observatory. Much of the information posted here originates from that source. Sidereal Time Sidereal time has units equal to the period of the earth's rotation with respect to a point nearly fixed with respect to the stars.
Sidereal time varies with the rotation of the earth.
It is defined to be as uniform as possible despite minor variations in the rotation of the earth. UT0 is the rotational timescale of a particular place of observation. It is observed as the motion of stars or extraterrestrial radio sources. UT1 is computed by correcting UT0 for the effect of polar motion on the longitude of the observing site. It varies from uniformity because of the irregularities in the Earth's rotation.