This is a picture of the Royal Observatory in Greenwich.Greenwich Meidian Time:
Greenwich, England has been the home of Greenwich Mean Time (GMT) since 1884. GMT is sometimes called Greenwich Meridian Time because it is measured from the Greenwich Meridian Line at the Royal Observatory in Greenwich. Greenwich is the place from where all time zones are measured.
GMT is World Time and the basis of every world time zone which sets the time of day and is at the centre of the time zone map. GMT sets current time or official time around the globe. Most time changes are measured by GMT. Although GMT has been replaced by atomic time (UTC) it is still widely regarded as the correct time for every international time zone.
Greenwich Mean Time is international time, the basis of the world time clock. GMT marks precision time and military time (sometimes called Zulu Time). Defines date and time and the exact time. The atomic time clock is adjusted by leap seconds to maintain synchronicity with GMT.
