A vertical circle that is at the right angles to the meridian and that therefore passes through the east and west points of the horizon is called:

Concept:

The visible horizon:

• The circle of contact of the earth and the cone of visual rays pass through the point of observation.

Co-latitude: 

• The angular distance from the observer's zenith (Z) to the celestial pole (P) along the observer's meridian is known as co-latitude and is given by (90°- f), where f is the latitude of the observer.

Prime vertical:

• A vertical circle that is at the right angles to the meridian and that therefore passes through the east and west points of the horizon.

Right ascension: 

• The right ascension of a celestial body is the angular distance along the arc of the celestial equator measured from the First point of Aeries (g) to the foot of the hour circle.
• It is measured from East to West direction i.e., anti-clockwise in the Northern hemisphere.