Which statement describes the Mean Sun's right ascension behavior?

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

Answer: (C)

Right ascension is measured along the celestial equator, while the mean Sun moves along the ecliptic, which is tilted relative to the equator. If the mean Sun advances at a constant speed along the ecliptic, its longitude increases uniformly, but the right ascension you see on the sky does not increase at a constant rate because of the tilt between the two great circles. The projection of a uniform motion on the ecliptic into the equatorial system makes d(RA)/dt vary with time, even though RA itself keeps increasing eastward. The result is a monotonic, but nonuniform, increase in the mean Sun’s right ascension over the year.