Spherical astronomy, orpositional astronomy, is a branch ofobservational astronomy used to locateastronomical objects on thecelestial sphere, as seen at a particular date, time, and location onEarth. It relies on the mathematical methods ofspherical trigonometry and the measurements ofastrometry.
This is the oldest branch of astronomy and dates back toantiquity. Observations of celestial objects have been, and continue to be, important for religious andastrological purposes, as well as fortimekeeping andnavigation. The science of actually measuring positions of celestial objects in the sky is known as astrometry.
The primary elements of spherical astronomy arecelestial coordinate systems and time. The coordinates of objects on the sky are listed using theequatorial coordinate system, which is based on the projection of Earth'sequator onto the celestial sphere. The position of an object in this system is given in terms ofright ascension (α) anddeclination (δ). The latitude and local time can then be used to derive the position of the object in thehorizontal coordinate system, consisting of thealtitude andazimuth.
The coordinates of celestial objects such asstars andgalaxies are tabulated in astar catalog, which gives the position for a particular year. However, the combined effects ofaxial precession andnutation 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.
To determine the position of theSun andplanets, an astronomicalephemeris (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.
Theunaided human eye can perceive about 6,000 stars, of which about half are below thehorizon at any one time. On modernstar charts, thecelestial sphere is divided into88 constellations. Every star lies within aconstellation. Constellations are useful for navigation.Polaris lies nearly due north to an observer in theNorthern Hemisphere. Thispole star is always ata position nearlydirectly above theNorth Pole.
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