Spherical astronomy

Observations of celestial objects have been, and continue to be, important for religious and astrological purposes, as well as for timekeeping and navigation.

The latitude and local time can then be used to derive the position of the object in the horizontal coordinate system, consisting of the altitude and azimuth.

However, the combined effects of axial precession and nutation will cause the coordinates to change slightly over time.

The effects of these changes in Earth's motion are compensated by the periodic publication of revised catalogs.

The unaided human eye can perceive about 6,000 stars, of which about half are below the horizon at any one time.