The equation of time is the difference between apparent solar time, as shown by a sundial, and mean time, as kept by a regular clock. Because the Earth's orbit is elliptical and its axis is tilted, the sun appears to run fast or slow through the year, so a sundial can differ from a clock by up to about sixteen minutes, with the two agreeing on only four days a year.
The equation was of great practical importance before the adoption of standardised mean time, since clocks were regularly set from sundials. Many seventeenth and eighteenth-century clockmakers pasted printed equation tables inside longcase clock doors, and sundials were sometimes engraved with correction figures. The finest makers, including Thomas Tompion, Daniel Quare and Joseph Williamson, made complex equation clocks that showed both solar and mean time on the dial, using a kidney-shaped cam rotating once a year to drive an additional hand or adjust the indication. Equation work is also found on astronomical clocks and some later regulators and watches. Clocks with original equation work are rare and highly valued, and collectors examine the mechanism carefully to confirm it has not been added or altered.
