A great circle passing through a celestial body and whose plane is perpendicular to the plane of the celestial equator.

A great circle passing through a celestial body and whose plane is perpendicular to the plane of the celestial equator.

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

Answer: (D)

On the celestial sphere, great circles are the intersections with planes through the center. If a circle’s plane is perpendicular to the plane of the celestial equator, that plane must contain the two celestial poles. Planes like this define meridians, or hour circles, which are the great circles of constant right ascension that pass through the poles. Because the circle also passes through the given celestial body, it is specifically the hour circle corresponding to that body’s right ascension. The celestial equator itself lies in the equatorial plane, so its plane is not perpendicular to itself. A vertical circle comes from a vertical plane and isn’t fixed as perpendicular to the equator in general, and a general great circle doesn’t specify the required orientation. Thus the described circle is best identified as an hour circle.

On the celestial sphere, great circles are the intersections with planes through the center. If a circle’s plane is perpendicular to the plane of the celestial equator, that plane must contain the two celestial poles. Planes like this define meridians, or hour circles, which are the great circles of constant right ascension that pass through the poles. Because the circle also passes through the given celestial body, it is specifically the hour circle corresponding to that body’s right ascension.

The celestial equator itself lies in the equatorial plane, so its plane is not perpendicular to itself. A vertical circle comes from a vertical plane and isn’t fixed as perpendicular to the equator in general, and a general great circle doesn’t specify the required orientation. Thus the described circle is best identified as an hour circle.