A great circle on the sphere where a plane perpendicular through a plumb line at the place of observation and passing through the center of the earth, cuts the celestial sphere

A great circle on the sphere where a plane perpendicular through a plumb line at the place of observation and passing through the center of the earth, cuts the celestial sphere

• A. Celestial equator
• B. Vertical circle
• C. Great circle
• D. Observer's horizon

Answer: (D)

The test hinges on how the horizon appears on the celestial sphere. The horizon is formed by the plane that is perpendicular to the local vertical (the plumb line) and passes through the Earth's center. Since that plane goes through the center of the celestial sphere, its intersection with the sphere is a great circle. That circle is what we call the observer’s horizon on the sky. This horizon circle is a specific great circle tied to the observer’s location, unlike the celestial equator (a fixed great circle tied to Earth's axis), or a vertical circle (a great circle defined by a vertical plane through the plumb line), or a generic great circle. Hence the term that matches is the observer’s horizon.

The test hinges on how the horizon appears on the celestial sphere. The horizon is formed by the plane that is perpendicular to the local vertical (the plumb line) and passes through the Earth's center. Since that plane goes through the center of the celestial sphere, its intersection with the sphere is a great circle. That circle is what we call the observer’s horizon on the sky.

This horizon circle is a specific great circle tied to the observer’s location, unlike the celestial equator (a fixed great circle tied to Earth's axis), or a vertical circle (a great circle defined by a vertical plane through the plumb line), or a generic great circle. Hence the term that matches is the observer’s horizon.