When the Greenwich observatory (photo, below) was set up in 1675, its primary task was not to keep track of time, but to solve the problem of fixing longitude at sea. The first thing its expert custodians did was to verify a long-held assumption that the Earth’s rate of rotation was constant. To check this assumption, the experts brought two newly invented pendulum clocks. This is how time-keeping at Greenwich began. Using these clocks as benchmark to prepare time-wise star maps night after night to detect any shift in the star positions, the experts established (somewhat erroneously) that the Earth’s rotation is constant. They prepared navigational charts with Greenwich as reference point for all longitudes.
By 1770, all navigators were using them. So when it finally came to selecting a Prime Meridian (0⁰ longitude) for the world, Greenwich was the obvious choice. It became the starting point of both longitude and time. This decision was made official by the Meridian Conference, held in Washington during 1884. Years later in 1940s, it was realized that the Earth has been slowing down and speeding up in irregular manner. Hence, the night sky relative to the Earth also does not remain constant. In 1986, Coordinated Universal Time/UTC, based on atomic measurements, became the world’s preferred time. It is coordinated from Paris and is based upon an average of clock readings from 25 countries around the world.