Equation of time

The equation of time describes the discrepancy between two kinds of solar time. The word equation is used in the medieval sense of reconcile a difference. The two times that differ are the apparent solar time, which directly tracks the motion of the sun, and mean solar time, which tracks a theoretical "mean" sun with noons 24 hours apart. Apparent (or true) solar time can be obtained by measurement of the current position (hour angle) of the Sun, as indicated (with limited accuracy) by a sundial. 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 resolve to zero.