Two Systems: Celestial, Horizon

The horizon coordinate system has the advantage of being orientated towards the sky the observer actually sees. It has the disadvantage of being different for each observer and the location of objects in it change over time. The celestial equatorial system has the advantage of being the same for each observer and the location of stars in it change very little over time. It has the disadvantage of not being naturally oriented towards the observer's sky. Because of these advantages and disadvantages, both systems are frequently used. Depending on the situation it is better to use one or the other.

The animation to the right shows how the horizon system relates to the celestial equatorial system for an observer. Conversion between the two systems obey a few simple principles:

1. Altitude of North Celestial Pole = latitude of observer
2. Altitude of Celestial Equator = (±)90° - latitude of observer
3. Azimuth of North = 0°

where the plus is used for the northern hemisphere and minus for the southern hemisphere.

While these principles are straightforward to employ, converting an actual celestial equatorial coordinate to a horizon coordinate is a bit more tricky. Because the sky is rotating, one needs to know both longitude and time to convert coordinates. As such, it is not covered in this introductory section.