Greenwich mean
time (GMT), or Universal Time (UT), is the time on the meridian through Greenwich, England. It is used as
the zero for longitudinal measurement, according to the Mean SunÑan imaginary body moving around the celestial equator with constant angular speed, making a complete circuit with respect to the vernal equinox in one tropical year. GMT, established as the
world standard in 1884, was given the name UT in 1928 (see
time). The International Time Bureau in Paris now coordinates astronomical measurements and atomic clock readings from around the world to arrive at a coordinated UT.