Alternatively known as ‘Alt/Az coordinates’, this system of celestial coordinates is dependent on the observer’s latitude and longitude. Using the observer’s local horizon as a reference plane, the position of an object on the celestial sphere at a particular time is given by its:
- Altitude- the angular distance above the horizon
- Azimuth- the angular distance measured east from north and parallel to the horizon
The horizonal coordinate system depends on the location of the observer and the time of the observation. It measures the altitude and azimuth of an object in degrees to position it on the sky.
The altitude (alt) of an object can lie between 0
o (indicating it is on the horizon) and 90
o at the
zenith (or -90
o if it lies below the horizon). The azimuth (az) changes from 0
o for an object due North to 90
o (due East) to 180
o (due South) to 270
o (due West).
The
horizontal coordinate system is fixed to the Earth and not the
stars and therefore, unlike in the
equatorial coordinate system, the position of an object changes with time.