Equation of time - Wikipedia
https://en.wikipedia.org/wiki/Equation_of_timeThe 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 diurnal motion of the Sun, and mean solar time, which tracks a theoretical mean Sun with uniform motion along the celestial equator.