Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. It is now sometimes used to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone, although strictly UTC is an atomictime scale which only approximates GMT in the old sense. It is also used to refer to Universal Time (UT), which is the astronomical concept that directly replaced the original GMT in 1928. Observations at the Greenwich observatory ended in 1954.
