Which term describes the intersection of the celestial sphere with a plane that passes through the sphere's center?

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

Answer: (C)

A great circle is the intersection of the celestial sphere with a plane that passes through the sphere’s center. Because the plane goes through the center, the resulting circle is the largest possible on the sphere, with the same radius as the sphere itself. This concept applies to the sky as well: the celestial equator is a specific example of a great circle, and other familiar lines on the sky such as hour circles (lines of constant right ascension) and vertical circles are also great circles. If the cutting plane did not pass through the center, the intersection would be a smaller circle called a small circle. So the general term for this intersection is great circle.