The Mean Sun property: What path does it move along?

• A. It moves along the ecliptic, its right ascension is constantly increasing at a uniform rate
• B. It moves along the ecliptic, its right ascension is not constantly increasing at a uniform rate
• C. It moves along the equator, its right ascension is not constantly increasing at a uniform rate
• D. It moves along the equator, its right ascension is constantly increasing at a uniform rate

Answer: (D)

The mean Sun follows the ecliptic, the plane of Earth’s orbit projected onto the sky. Its longitude along that path increases at a constant mean rate (one full turn per year), which is why we model it as moving uniformly along the ecliptic. However, right ascension is measured along the celestial equator, which is tilted relative to the ecliptic by the obliquity of the Earth. When you translate a position on the ecliptic into equatorial coordinates, that tilt makes the right ascension change non-uniformly with time. So the mean Sun moves along the ecliptic with steady angular speed in longitude, but its right ascension does not increase at a constant rate.