Which pair of time scales are involved in the equation of time?

• A. Greenwich Apparent Time and Greenwich Mean Time
• B. Universal Time and Ephemeris Time
• C. Sidereal Time and Solar Time
• D. Local Apparent Time and Local Mean Time

Answer: (D)

The equation of time relates the time shown by a sundial to the time kept by a clock. A sundial shows apparent solar time, based on the Sun’s actual position, while clocks run on mean solar time, which is a smooth, averaged version of the Sun’s motion. The two scales in play are local apparent time and local mean time—the difference between them is what the equation of time expresses.

This pair makes sense because the Sun’s real motion across the sky isn’t perfectly uniform due to the Earth’s elliptical orbit and axial tilt, so we average it to get a steady clock time. The other options mix time standards that aren’t tied to this Sun-based comparison: global time standards, time scales tied to stars, or purely Greenwich-based references, rather than the local apparent vs mean solar time that the equation of time directly compares.