Equation of Time The Equation of Time is the difference on each day of the year between the time shown on a clock and the time indicated by the shadow of the sun on a sundial. Clock time and sun time differ from each other by a few minutes throughout the year. Also, because of the elliptical orbit of the earth round the sun, there has never been a day that is exactly 24 hours long. In fact, the earth makes one perfect rotation on its axis in relation to the stars in 23 hours, 56 minutes and 4.09 seconds. Therefore, each day that the earth moves in space around its orbit, it requires a bit of extra turn to come back and face the sun on the next day. Additionally, the time needed to execute that extra bit of turn differs slightly from day to day, because the earth travels at different speeds in its elliptical orbit. Since the invention of the pendulum, clocks have been set for an average day of 24 hours, hence the 'mean' in Greenwich Mean Time. Within a short time of the establishment of the Greenwich Observatory, the first Astronomer Royal, John Flamsteed, published tables for the correction of sundial time to mean clock time: this was called the Equation of Time.
