   # Vertical Circle Explained

Astronomy uses a number of terms which may not be understandable to an average reader. In this article, we will explain the term ‘vertical circle’ as well as several closely related terms.

## Vertical Circle Definition

Vertical circle is defined as a great circle on an imaginary sphere enclosing the Earth or celestial sphere at a 90 degrees angle to the horizon, which means that it passes through both poles of the horizon - nadir and zenith. Since vertical circle is aligned with the horizon at an angle of 90 degrees, there is a vertical circle for every azimuth on the north-east direction of the horizon. However, vertical circle can also be on the east-west direction. But in this case, it is known as the prime vertical. When it is on the north-south direction, it is referred to as the principal vertical or the Local Celestial Meridian (LCM).

• Azimuth. In astronomy (and some other disciplines), azimuth refers to one of two coordinates (the other is altitude) of an object’s position. While the altitude determines the position of the object above the horizon at an angle of 90 degrees, azimuth refers to the angle of the object’s position in the clockwise direction north of the vertical circle.

• Great circle. Also called orthodrome and Riemannian circle, the great circle is defined as the largest circle of the celestial sphere, passing through its very centre. Its diameter is thus the same as the diameter of the sphere. Therefore, the great circle and the sphere also have the same circumference and centre.

• Nadir. This term refers to a point of the horizon that is opposite to the zenith or the highest point of the celestial sphere. This means that nadir is pointing in the vertical direction at a given location, that is in the direction of gravitational force.

• Zenith. Point on of the horizon opposite to the nadir.

• Horizontal coordinate system. Vertical circle plays an important role in the horizontal coordinate system which determines the position of an object using altitude and azimuth as the coordinates, and the observer’s horizon to divide the sphere into upper and lower hemispheres. The horizontal coordinate system is one of several celestial coordinate systems, all of which are used to determine the position of celestial bodies.