A vertical line at the location of the observer which coincides with the plumb line and is normal to the observer's horizon

• A. Observer's horizon
• B. Zenith
• C. Celestial poles
• D. Observer's vertical

Answer: (D)

This is about the local vertical—the line through the observer that is perpendicular to the horizon plane and follows the direction of gravity, known as the plumb line. The horizon is defined as a plane tangent to the Earth's surface at the observer, so the vertical line is exactly normal to that horizon plane. Extending this line upward would pass through the zenith, the point on the celestial sphere directly above the observer, but the thing being described is the line itself. The celestial poles are fixed directions tied to Earth's axis, not the observer’s vertical, and the observer’s horizon is the plane perpendicular to this vertical. So the described line is the observer’s vertical.