A coordinate system is necessary in order to accurately calculate positions both on the Earth's surface and on the celestial sphere. Although there are several different coordinate systems, they all have a horizontal and vertical or polar axis. An exact location is defined by the intersection of two sets of coordinates.
The Earth's equator represents the reference plane. Parallel to this are 90 circles of latitude running in northerly and southerly directions (= Northern and Southern latitudes). 360 meridians (= Western and Eastern circles of longitude) run to the North and South poles. These lines are then superimposed onto the celestial sphere where the celestial equator represents the base level while the celestial North and South poles are equivalent to their terrestrial counterparts. The degree of latitude equates to the declination while the degree of longitude equates to the right ascension.
The plane of the ecliptic, i.e. the Sun's apparent orbit around the Earth, represents the reference plane. The given coordinates are the ecliptic longitude - which is the number of degrees that can be found in an ephemeris, and the ecliptic latitude. The latter describes the distance to the ecliptic along a circle of longitude that is vertical to the ecliptic. The ecliptic system is used to locate the position of celestial bodies in the solar system.
It is also called the Azimuth system.
The actual horizon represents the reference plane which, in contrast to the apparent horizon, runs through the centre of the Earth. The poles are the zenith and the nadir, the points on the celestial sphere located directly above and below the observer. The coordinates of this system are: The great circle that runs from the Zenith to the Nadir, and, extending from a Northerly point in an clockwise direction, the longitude along the horizontal circle which is measured in degrees and is called the Azimuth.
The horizontal system is used to define the position of a celestial body in relation to the horizon.
In navigation the common definition is from North = 0° clockwise, East = 90°, South = 180°, West = 270°, as shown in the diagram on the side.
Astrodienst uses an older system, counting from the South = 0° clockwise, West = 90°, North = 180°, East = 270°.