A great circle which is perpendicular to the polar axis of the celestial sphere. It is an extension on the plane of the earth's equator outward until it intersects the celestial sphere.

A great circle which is perpendicular to the polar axis of the celestial sphere. It is an extension on the plane of the earth's equator outward until it intersects the celestial sphere.

• A. Celestial equator
• B. Nadir
• C. Zenith
• D. Hour circle

Answer: (A)

The celestial equator is the projection of Earth's equator onto the celestial sphere. It forms a great circle because it lies in the plane of Earth's equator, which passes through the center of the Earth, and extending that plane outward cuts the celestial sphere in a circle. This circle sits perpendicular to the polar axis (the line through the celestial poles) and serves as the reference circle for declination and the baseline for right ascension. The other terms aren’t this circle: the nadir is just the point on the celestial sphere directly below you, the zenith is the point directly above you, and an hour circle is the circle through a given celestial object and the celestial poles, not the projection of Earth's equator.

The celestial equator is the projection of Earth's equator onto the celestial sphere. It forms a great circle because it lies in the plane of Earth's equator, which passes through the center of the Earth, and extending that plane outward cuts the celestial sphere in a circle. This circle sits perpendicular to the polar axis (the line through the celestial poles) and serves as the reference circle for declination and the baseline for right ascension. The other terms aren’t this circle: the nadir is just the point on the celestial sphere directly below you, the zenith is the point directly above you, and an hour circle is the circle through a given celestial object and the celestial poles, not the projection of Earth's equator.