“Mean solar time,” is what is shown on most watches and clocks. It is the solar time that would be measured by observation if the Sun traveled at a uniform apparent speed throughout the year.

The real Sun and the imaginary “mean Sun,” from which mean solar time is measured, may be as much as 16 minutes apart because during the course of the year.

The difference between mean and apparent solar time is known as the “equation of time.” This is usually expressed as a correction, never exceeding 16 minutes, that is added to or subtracted from apparent solar time to determine mean solar time.