Which term designates the great circle that passes through a celestial body and is perpendicular to the celestial equator?

Which term designates the great circle that passes through a celestial body and is perpendicular to the celestial equator?

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

Answer: (B)

In the celestial sphere, circles of constant right ascension are called hour circles, and they are great circles that pass through the celestial poles. A given hour circle goes through the celestial body and the poles, and because any circle through the poles intersects the celestial equator at a right angle, this hour circle is perpendicular to the celestial equator. That combination—passing through the body and through the poles, and crossing the equator at right angles—defines the hour circle. A vertical circle, by contrast, passes through the zenith and nadir and isn’t defined by a specific body in relation to the equator. The celestial equator is a reference circle itself, not the circle through the body. And while a great circle is a broad category, the hour circle is the specific great circle that lines up with the body’s constant right ascension and is perpendicular to the celestial equator.

In the celestial sphere, circles of constant right ascension are called hour circles, and they are great circles that pass through the celestial poles. A given hour circle goes through the celestial body and the poles, and because any circle through the poles intersects the celestial equator at a right angle, this hour circle is perpendicular to the celestial equator. That combination—passing through the body and through the poles, and crossing the equator at right angles—defines the hour circle.

A vertical circle, by contrast, passes through the zenith and nadir and isn’t defined by a specific body in relation to the equator. The celestial equator is a reference circle itself, not the circle through the body. And while a great circle is a broad category, the hour circle is the specific great circle that lines up with the body’s constant right ascension and is perpendicular to the celestial equator.