In astronomy, astrology, and geodesy, the ______ or first vertical is the vertical circle passing east and west through the zenith and intersecting the horizon in its east and west points.

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

Answer: (B)

Vertical circles on the celestial sphere are great circles formed by planes that pass through the observer’s zenith. The one that goes east–west through the zenith is the prime vertical, sometimes called the first vertical. Its defining feature is that the plane containing the zenith also contains the east–west line, so it intersects the horizon precisely at the points due east and due west while it passes through the zenith. This distinguishes it from the meridian, which passes through the zenith and the north and south points on the horizon; the celestial equator is a tilted great circle representing Earth’s equator projected into the sky and is not aligned specifically with east–west, and the horizon circle just marks the boundary between sky and ground. So the described circle is the prime vertical.