The equation of time is the difference between which two times?

• A. Local Apparent Time and Local Mean Time
• B. Local Mean Time and Local Apparent Time
• C. Greenwich Apparent Time and Local Mean Time
• D. Local Apparent Time and Greenwich Mean Time

Answer: (A)

The equation of time measures how sundial time (apparent solar time) differs from clock time (mean solar time). At a given location, the Sun’s true position in the sky tells you apparent solar time, while mean solar time uses a uniform rate to average out the Sun’s irregular speed caused by the ellipticity of Earth’s orbit and the tilt of its axis. Because the Sun does not move at a perfectly constant pace, the two times don’t match and the difference can swing from a bit before noon to a bit after, over the course of the year. This difference is what the equation of time expresses.

Using Greenwich times would tie the comparison to a fixed reference and isn’t what the equation of time describes—it’s about the relationship between local apparent solar time and local mean solar time, i.e., the Sun’s actual position vs. the averaged position at the same location.