Which statement about the Mean Sun's motion regarding right ascension is correct?

• A. It decreases at a uniform rate
• B. It remains constant
• C. It increases at a uniform rate
• D. It increases but not uniformly

Answer: (C)

The important idea is that the mean Sun moves along the sky with a constant angular speed. Because it advances uniformly along the ecliptic over the course of a year, its projection onto the celestial equator—the right ascension—also increases steadily with time. In other words, the mean Sun’s right ascension climbs at a uniform rate (about 24 hours of RA per year, roughly 1 degree per day). The real Sun, influenced by the elliptical shape of Earth's orbit, speeds up and slows down a bit, so its right ascension does not increase perfectly uniformly. But for the mean Sun, the motion is defined to be uniform, making the statement that it increases at a uniform rate the correct one.