GREENWICH MEAN TIME

(GMT), the former name for mean solar time of the longitude (0) of the former Royal Observatory at Greenwich, Eng. This longitude is called the Greenwich meridian (q.v.). Greenwich Mean Time was used for clearly designating epoch by avoiding confusing references to local time systems (zones). In accord with tradition, the epoch 0000 GMT (denoting the start of a solar day) occurred at noon. In 1925 the numbering system for GMT was changed so that the day began at midnight, as did the civil day. Some confusion in terminology resulted, though, and in 1928 the International Astronomical Union changed the designation of the standard time of the zero meridian to Universal Time (q.v.), which remains in general use. The term GMT is still used for some purposes, e.g., navigation, in English-speaking countries. See also solar time.