The concept Clock with auxiliary dial displaying the equation of time. The equation of time values for each day of the year, compiled by astronomical observatories, were widely listed in almanacs and ephemerides. The equation of time is the east or west component of the analemma, a curve representing the angular offset of the Sun from its mean position on the celestial sphere as viewed from Earth. Mean solar time, for the same place, would be the time indicated by a steady clock set so that over the year its differences from apparent solar time would have a mean of zero. Apparent solar time can be obtained by measurement of the current position ( hour angle) of the Sun, as indicated (with limited accuracy) by a sundial. The two times that differ are the apparent solar time, which directly tracks the diurnal motion of the Sun, and mean solar time, which tracks a theoretical mean Sun with uniform motion along the celestial equator. The word equation is used in the medieval sense of "reconciliation of a difference". The equation of time describes the discrepancy between two kinds of solar time.
See the section " Sign of the equation of time" below. This graph shows how many minutes the clock is ahead (+) or behind (−) the apparent sun. The equation of time - above the axis a sundial will appear fast relative to a clock showing local mean time, and below the axis a sundial will appear slow. Apparent solar time minus mean solar time
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