Greenwich Mean Time

(G-M-T)

GMT was the original official world time measured at the former site of the Royal Greenwich Observatory. Greenwich sits on meridian zero. In 1925 a new reference was specified and the term "Universal Time" introduced. Universal Time is tied to the rotation of the Earth with respect to a "mean Sun" along the prime meridian through the Greenwich Observatory. The difference between Universal Time and GMT is quite small but scientifically significant. In computers, you will often have to specify time differentials from GMT to your local time for mail programs and news reading programs.

More Information

• U.S. Naval Observatory Discussion
• Official U.S. Time
• Time FAQ Index