Altazimuth Coordinate System

The altazimuth coordinates

The altazimuth or horizontal coordinate system is on of the celestial coordinate systems used in astronomy.

Other celestial coordinate systems are:

The equatorial system that can be fixed (if the hour angle is used) or mobile (based on the right ascension), both with the origin at the center of the Earth (geocentric), the celestial equatorial plane as fundamental plane and the celestial North and South as poles;
The ecliptic system, having its origin at the center of the Sun (heliocentric), the ecliptic plane as fundamental plane and poles those of the ecliptic;
The galactic system, with its origin at the center of the Sun, fundamental plane the galactic one and the galactic poles;
The supergalactic system.

The horizontal system is a spherical coordinate system having its origin at the observer's location and as reference planes on the celestial sphere:

a) the celestial (or rational) horizon, which is the great circle that from the observer's standpoint separates the celestial visible hemisphere from the non-visible one;

b) the celestial (or local) meridian, which is the great circle that passes through the zenith and the celestial poles.

The celestial horizon is therefore the great circle of the celestial sphere laying on the plane normal to the local gravity vector (the vertical axis). The celestial horizon is, of course, an imaginary circle.

The zenith is defined as the intersection of the celestial spherical surface with the vertical axis through the observer's location. It therefore represents the pole of the observer's visible hemisphere. The opposite pole (on the non-visible hemisphere) is named nadir.

The celestial meridian is an imaginary circle passing through the Zenith (Z), and the North (NCP) and South (SCP) celestial poles. It always lies on a plane perpendicular to the horizon.

The intersection of the celestial meridina passing through the celestial body and the horizon is used to measure the angular distance along the horizon.

Using these two reference planes, the coordinates of a celestial body are measured as follows:

a) The abscissa, measured on the horizontal plane, is given by the azimuth (A) that is the angle of the object around the horizon, usually measured from the North; in other words, it is the angular distance between the celestial (or local) meridian and the meridian through the object;

b) The ordinate, measured along the meridian passing through the object, is given by the altitude (h), sometimes referred to as elevation, that is the angle between the object and the horizon.

The azimuth, A, is usually measured in clockwise direction from North toward East and is represented by a numerical value varying from 0 degree to 360 degree.

The altitude, h, is expressed with a numerical value from 0 degree (at the horizon) to +90 degree (distance of the Zenith to the horizon).

Alternatively to the altitude, the Zenith distance, z, may be used to provide the angular distance of the object to the Zenith; it is then the complement of the altitude h: z = 90° - h.