A great circle passing through the observer's zenith and any celestial body. Such a circle is perpendicular to the horizon and represents the line of intersection of a vertical plane with the celestial sphere.

A great circle passing through the observer's zenith and any celestial body. Such a circle is perpendicular to the horizon and represents the line of intersection of a vertical plane with the celestial sphere.

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

Answer: (C)

A vertical circle is produced by a vertical plane that passes through the observer’s zenith. The intersection of that vertical plane with the celestial sphere forms a circle that passes through the zenith and any celestial body lying in that azimuth, and because the plane goes through the sphere’s center, this circle is a great circle. This circle sits perpendicular to the horizon circle, which is the intersection of the horizon plane with the celestial sphere. So the described line—passing through the zenith and a body, perpendicular to the horizon, and defined by a vertical plane—fits the concept of a vertical circle.

A vertical circle is produced by a vertical plane that passes through the observer’s zenith. The intersection of that vertical plane with the celestial sphere forms a circle that passes through the zenith and any celestial body lying in that azimuth, and because the plane goes through the sphere’s center, this circle is a great circle. This circle sits perpendicular to the horizon circle, which is the intersection of the horizon plane with the celestial sphere. So the described line—passing through the zenith and a body, perpendicular to the horizon, and defined by a vertical plane—fits the concept of a vertical circle.