Which statement is true about the Mean Sun according to the material?

• A. It moves along the equator and its right ascension increases uniformly
• B. It moves along the ecliptic and its right ascension increases uniformly
• C. It remains stationary relative to the stars
• D. It moves along the celestial pole

Answer: (A)

The main idea here is how the Mean Sun is defined for timekeeping: it is a theoretical point that moves at a constant rate along the celestial equator, so its right ascension changes uniformly with time. Right ascension is measured along the celestial equator from the vernal equinox, so when the Mean Sun travels on the equator, its RA increases at a steady pace, roughly 24 hours of RA per year. In contrast, the actual Sun follows the tilted ecliptic and its RA does not increase uniformly because the Sun’s true path and speed vary with orbital geometry. The Mean Sun is not fixed relative to the stars, nor does it move along the celestial pole. So the statement that fits best is that the Mean Sun moves along the equator and its right ascension increases uniformly.