![]() Historically this was measured either by reference to the position of the Sun, or by reference to the positions of the stars. Our day is based on the length of time it takes for the Earth to spin around once on its axis. ![]() When the Royal Observatory was founded at Greenwich in 1675, it was generally believed that the Earth was spinning at a steady rate (or in technical speak, isochronous). After that date, all time determinations were made from observations of the stars. Today’s atomic clocks are capable of keeping time to better than one second in 1,400,000 years.Īt Greenwich, time determinations were made from observations of the Sun until Edmond Halley obtained the Observatory’s first transit telescope in 1721. ![]() By the mid twentieth century, this had been improved by a factor of 10,000, with the best clocks being able to keep time to around a few seconds a year. By contrast, the first pendulum clocks were able to keep time to about 10 seconds a day. Until the introduction of the pendulum clock in the latter half of the seventeenth century, timekeepers were unable to keep time to better than about 15 minutes a day. They need to be adjusted so that they go at the correct rate and reset periodically to show the correct time. Timekeepers are instruments that measures the passage of time. It also refers to the specific instruments that were used at the Royal Observatory, together with some of the principles of their operation.īesides the sundial, methods for determing the local time from observations of the sun or stars have been known since ancient times. It covers basic astronomical principles, together with some of the many definitions of time that scientists use. The purpose of this page is to provide background information about the different ways in which time has been determined since the Observatory’s founding in 1675.
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