Which of the following statements is a property of the Mean Sun?

• A. It moves along the equator with a uniformly increasing right ascension
• B. It moves along the ecliptic with a uniformly increasing right ascension
• C. It moves along the equator with a non-uniform right ascension
• D. It moves along the equator with a decreasing right ascension

Answer: (A)

The Mean Sun is a hypothetical, clock-like reference that moves at a constant rate across the celestial sphere. It stays on the celestial equator, so its declination is always zero, and its right ascension increases at a steady, uniform pace as time passes. That combination—moving along the equator with uniformly increasing right ascension—captures exactly how the Mean Sun is defined as a timekeeping construct.

In contrast, the true Sun travels along the ecliptic, not the equator, and its speed in right ascension varies because of the elliptical orbit and the tilt between the ecliptic and the equator. So its RA does not increase uniformly. Moving along the equator but with non-uniform or decreasing RA would not reflect the standard definition of the Mean Sun.