Which of the following best describes a property of the Mean Sun?

• 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 fixed relative to the celestial sphere
• D. It moves along the equator and its right ascension decreases uniformly

Answer: (A)

The key idea is that the mean Sun is a fictitious point used to model a uniform daily progression for timekeeping. It is defined to move at a constant rate along the celestial equator, so its longitude increases uniformly. Since right ascension is the angle measured along the celestial equator, a point sliding smoothly eastward on the equator means its right ascension also increases at a constant rate. That’s why the description that it moves along the equator and its right ascension increases uniformly is the best fit.

In contrast, the real Sun travels along the ecliptic, not the equator, so its right ascension does not increase strictly uniformly over time. And the mean Sun isn’t fixed in place. Also, the mean Sun advances eastward, not westward, so its right ascension does not decrease uniformly.