Glossary/Equation of Time
Equation of Time
The Equation of Time is the difference between solar time (i.e. sundial time) and mean solar time (i.e. time shown by clocks), the sundial sometimes appearing fast and sometimes slow according to the clock. Before time signals, accurate clocks and watches had to be set using a sundial, so the Equation of Time had to be known. This was often a printed table stuck inside the clock or watch case. Expensive clocks, such as the Bath Clock, had an equation dial showing how much the sundial should be fast or slow compared with the clock. The year's equation figures are published in the Astronomical Ephemeris. The difference between solar time and mean solar time varies between about 16 1/2 minutes and -14 1/4 minutes throughout the year.