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. This is one of the oldest branches of astronomy. 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 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 are 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 effect 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 is used. This gives the positions of these objects on the celestial sphere at a particular date and time, which can then be converted into suitable 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.

Positional phenomena

* Planets which are in conjunction form a line which passes through the center of the solar system
* The ecliptic is the plane which contains the orbit of a planet, usually in reference to Earth
* Elongation refers to the angle formed by a planet, with respect to the system's center and a viewing point
* Superior planets have a larger orbit than the earth while the inferior planets (Mercury and Venus) orbit the sun inside the earth's orbit
* A transit may occur when an inferior planet passes through a point of conjunction

Ancient structures associated with positional astronomy include

* Arkaim
* Chichen Itza
* The Medicine Wheel
* The Pyramids
* Stonehenge
* The Temple of the Sun

External links

Course Notes and Tutorials
* [http://star-www.st-and.ac.uk/~fv/webnotes/index.html Professor Vincent's course notes at the University of St.Andrews]

* [http://www.astunit.com/tutorials/positional.htm From Stephen Tonkin's Astronomy tutorials]

* [http://www.physics.csbsju.edu/astro/tutorial.html From Professor Kirkman's tutorials at College of Saint Benedict + Saint John's University]

oftware

NOVAS, an integrated package of subroutines for the computation of a wide variety of common astrometric quantities and transformations, in Fortran and C, from the U.S. Naval Observatory.

References

* Robin M. Green, "Spherical Astronomy", 1985, Cambridge University Press, ISBN 0-521-29180-1.
* William M. Smart, edited by Robin M. Green, "Textbook on Spherical Astronomy", 1977, Cambridge University Press, ISBN 0-521-29180-1. (This classic text has been re-issued).

See also

* Astrometry
* Celestial mechanics
* Celestial Navigation
* Equinox
* Solstice
* Astrological aspects
* Diurnal motion
* Eclipse
* Ecliptic
* Elongation
* Epoch
* Equinox
* Halley, Edmund
* History of Astronomy
* Jyotish
* Kepler's laws of planetary motion
* Prograde and retrograde motion
* Occultation
* Parallax
* Sidereal time
* Solstice