Early astronomers studied the motion of the Sun and other celestial bodies to predict the changes of the seasons and determine when seeds needed to be sown. Subsequently, the equinoxes and solstices that mark seasonal transitions acquired religious connotations, and their accurate prediction became essential. Inspired by that need, the Babylonians discovered that the solar day—the time it takes Earth to complete a 360° rotation with respect to the Sun—varies throughout the year relative to the average, or mean, solar day, which can be determined by observations of the fixed stars. The equation of time is defined as the cumulative difference between time based on the solar day, for which noon corresponds to the Sun’s highest point in the sky, and time based on the mean solar day. In other words, it is the difference between time as measured by a sundial and time as measured by a standard clock...
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1 November 2008
November 01 2008
On ships, trains, and the equation of time
Because of Earth’s elliptical orbit, tilted axis, and standardized time zones, high noon as measured by a clock does not coincide with the Sun’s apex in the sky.
Physics Today 61 (11), 76–77 (2008);