Which term is also known as the first vertical?

• A. Prime Vertical
• B. Observer's Meridian
• C. Celestial Equator
• D. Horizon Circle

Answer: (A)

In sky geometry, vertical circles are great circles that pass through the zenith and nadir. The prime vertical runs in the east–west direction, passing through the zenith and the points where the horizon meets due east and due west. Because it lies in the east–west vertical plane, it is traditionally called the first vertical. The observer's meridian is the north–south vertical circle that contains the zenith and nadir (and the celestial poles). The celestial equator is a projection of Earth’s equator onto the celestial sphere, and the horizon circle is simply the boundary between the visible and invisible sky. So the term for the first vertical is prime vertical.