# Spherical astronomy

Spherical astronomy or positional astronomy is the branch of astronomy that is used to determine the location of objects on the celestial sphere, as seen at a particular date, time, and location on the Earth. It relies on the mathematical methods of spherical geometry and the measurements of astrometry.

This is the oldest branch of astronomy and dates back to antiquity. Observations of celestial objects have been, and continue to be, important for religious and astrological purposes, as well as for timekeeping and navigation. The science of actually measuring positions of celestial objects in the sky is known as astrometry.

The primary elements of spherical astronomy are coordinate systems and time. The coordinates of objects on the sky are listed using the equatorial coordinate system, which is based on the projection of the Earth's equator onto the celestial sphere. The position of an object in this system is given in terms of right ascension (α) and declination (δ). 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.

The coordinates of celestial objects such as stars and galaxies are tabulated in a star catalog, which gives the position for a particular year. However, the combined effects of precession and nutation will cause the coordinates to change slightly over time. The effects of these changes in the movement of the Earth are compensated by the periodic publication of revised catalogs.

To determine the position of the Sun and planets, an astronomical ephemeris (a table of values that gives the positions of astronomical objects in the sky at a given time) is used, which can then be converted into suitable real-world coordinates.

The unaided human eye can detect about 6000 stars, of which about half are below the horizon at any one time. On modern star charts, the celestial sphere is divided into 88 constellations. Every star lies within a constellation. Constellations are useful for navigation. Polaris lies close to due north to an observer in the northern hemisphere. This star is always at a position nearly over the North Pole.