The complement of the declination is the angular distance measured from the celestial pole to the star along the hour circle. What is it called?

• A. Polar Distance
• B. Zenith Distance
• C. Right Ascension
• D. Altitude

Answer: (A)

The key idea is how angles on the celestial sphere are measured relative to the poles and the equator. Declination tells you how far a star is north or south of the celestial equator. The remaining angle to reach the pole sits along the hour circle, which is the great circle through the star and the celestial poles. That angle is called the polar distance. It is the complement of the star’s declination, so p = 90° − δ. For example, if a star has a declination of +25°, its polar distance is 65°. This measure specifically references the distance to the pole along the hour circle, not to the zenith, nor along the equator, and it’s different from right ascension or altitude.