Greenwich Mean Time


Yet another system of time-keeping has been in effect since 1884. By international agreement, the transit telescope at the Old Royal Observatory in Greenwich, England, defines the Prime Meridian for the planet Earth. At the same time, the Old Royal Observatory was also given the distinction of being the standard reference for time.

Most announcements of international events, such as astronomical happenings or radio broadcasts, are given in terms of Greenwich Mean Time, now called coordinated universal time, or UCT. To determine when an event will take place in your local time zone all you need to do is add or subtract the appropriate number of hours from the UCT listing.

Time zones west of Greenwich lag UCT, while those east lead GMT. Los Angles, for example, is eight zones behind Greenwich, so when it is starting time (9:00 am) in LA, it is already quitting time (5:00 pm) in Greenwich. The Universal Clock contains one memory address that stores the difference between UCT and your local time.

Revised 2013 by Larry Gentleman