Which great circle passes through the zenith and a celestial body and is perpendicular to the horizon?

Which great circle passes through the zenith and a celestial body and is perpendicular to the horizon?

• A. Great circle
• B. Celestial equator
• C. Hour circle
• D. Vertical circle

Answer: (D)

A vertical circle is the great circle that passes through the zenith and nadir and is perpendicular to the horizon. Because its plane is perpendicular to the horizon, it defines the vertical direction in the sky. If the circle also passes through a celestial body, that body lies on the same vertical plane, so the circle through the zenith, the body, and the nadir is the vertical circle. The celestial equator doesn’t generally pass through the zenith and isn’t defined by being perpendicular to the horizon, and an hour circle goes through the celestial pole rather than the zenith. So the vertical circle is the one that best matches all the given conditions.

A vertical circle is the great circle that passes through the zenith and nadir and is perpendicular to the horizon. Because its plane is perpendicular to the horizon, it defines the vertical direction in the sky. If the circle also passes through a celestial body, that body lies on the same vertical plane, so the circle through the zenith, the body, and the nadir is the vertical circle. The celestial equator doesn’t generally pass through the zenith and isn’t defined by being perpendicular to the horizon, and an hour circle goes through the celestial pole rather than the zenith. So the vertical circle is the one that best matches all the given conditions.