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Spherical astronomy, or positional astronomy, is a branch of
observational astronomy Observational astronomy is a division of astronomy that is concerned with recording data about the observable universe, in contrast with theoretical astronomy, which is mainly concerned with calculating the measurable implications of physical ...
used to locate
astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often u ...
s on the
celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...
, as seen at a particular date, time, and location on Earth. It relies on the mathematical methods of spherical geometry and the measurements of
astrometry Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way. History ...
. This is the oldest branch of astronomy and dates back to
antiquity Antiquity or Antiquities may refer to: Historical objects or periods Artifacts *Antiquities, objects or artifacts surviving from ancient cultures Eras Any period before the European Middle Ages (5th to 15th centuries) but still within the histo ...
. Observations of celestial objects have been, and continue to be, important for religious and
astrological Astrology is a range of divinatory practices, recognized as pseudoscientific since the 18th century, that claim to discern information about human affairs and terrestrial events by studying the apparent positions of celestial objects. Dif ...
purposes, as well as for
timekeeping Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, to co ...
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
celestial coordinate system Astronomical coordinate systems are organized arrangements for specifying positions of satellites, planets, stars, galaxies, and other celestial objects relative to physical reference points available to a situated observer (e.g. the true horizo ...
s and time. The coordinates of objects on the sky are listed using the
equatorial coordinate system The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fu ...
, which is based on the projection of 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 In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the ...
(δ). The latitude and local time can then be used to derive the position of the object in the
horizontal coordinate system The horizontal coordinate system is a celestial coordinate system that uses the observer's local horizon as the fundamental plane to define two angles: altitude and azimuth. Therefore, the horizontal coordinate system is sometimes called as the ...
, consisting of the
altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
and
azimuth An azimuth (; from ar, اَلسُّمُوت, as-sumūt, the directions) is an angular measurement in a spherical coordinate system. More specifically, it is the horizontal angle from a cardinal direction, most commonly north. Mathematicall ...
. The coordinates of celestial objects such as
star A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth ma ...
s and galaxies are tabulated in a star catalog, which gives the position for a particular year. However, the combined effects of
axial precession In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In the absence of precession, the astronomical body's orbit would show axial parallelism. In particu ...
and
nutation Nutation () is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behaviour of a mechanism. In an appropriate reference frame ...
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 the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
and planets, an astronomical
ephemeris In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly ve ...
(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 perceive about 6,000 stars, of which about half are below the horizon at any one time. On modern star charts, the
celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...
is divided into 88 constellations. Every star lies within a constellation. Constellations are useful for navigation. Polaris lies nearly due north to an observer in the Northern Hemisphere. This pole star is always at a position nearly directly above the
North Pole The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's axis of rotation meets its surface. It is called the True North Pole to distinguish from the Magn ...
.


Positional phenomena

* Planets which are in
conjunction Conjunction may refer to: * Conjunction (grammar), a part of speech * Logical conjunction, a mathematical operator ** Conjunction introduction, a rule of inference of propositional logic * Conjunction (astronomy), in which two astronomical bodies ...
form a line which passes through the center of the Solar System. * The ecliptic is the
plane Plane(s) most often refers to: * Aero- or airplane, a powered, fixed-wing aircraft * Plane (geometry), a flat, 2-dimensional surface Plane or planes may also refer to: Biology * Plane (tree) or ''Platanus'', wetland native plant * ''Planes' ...
which contains the orbit of a planet, usually in reference to Earth. * Elongation refers to the
angle In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the ''vertex'' of the angle. Angles formed by two rays lie in the plane that contains the rays. Angles a ...
formed by a planet, with respect to the system's center and a viewing point. ** A quadrature occurs when the position of a body (moon or planet) is such that its elongation is 90° or 270°; i.e. the body-earth-sun angle is 90° * Superior planets have a larger orbit than Earth's, while the inferior planets (Mercury and Venus) orbit the Sun inside 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 Arkaim (russian: Аркаим) is an archaeological site, dated to 2050-1900 BCE, of an ancient fortified settlement, belonging to Sintashta culture, situated in the steppe of the Southern Urals, north-northwest of the village of Amursky and e ...
*
Chichen Itza Chichen Itza , es, Chichén Itzá , often with the emphasis reversed in English to ; from yua, Chiʼchʼèen Ìitshaʼ () "at the mouth of the well of the Itza people" was a large pre-Columbian city built by the Maya people of the Termin ...
* The Medicine Wheel * The Pyramids *
Stonehenge Stonehenge is a prehistoric monument on Salisbury Plain in Wiltshire, England, west of Amesbury. It consists of an outer ring of vertical sarsen standing stones, each around high, wide, and weighing around 25 tons, topped by connecting h ...
* The Temple of the Sun


See also


References

* Robin M. Green, ''Spherical Astronomy'',
1985 The year 1985 was designated as the International Youth Year by the United Nations. Events January * January 1 ** The Internet's Domain Name System is created. ** Greenland withdraws from the European Economic Community as a result of a ...
, Cambridge University Press, * William M. Smart, edited by Robin M. Green, ''Textbook on Spherical Astronomy'',
1977 Events January * January 8 – Three bombs explode in Moscow within 37 minutes, killing seven. The bombings are attributed to an Armenian separatist group. * January 10 – Mount Nyiragongo erupts in eastern Zaire (now the Democratic Re ...
, Cambridge University Press, . (This classic text has been re-issued)


External links

;Software * NOVAS is 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.
jNOVAS
is a java wrapper for library developed and distributed by The United States Naval Meteorology and Oceanography Command (NMOC) with included JPL planetary and lunar ephemeris DE421 binary file published by the Jet Propulsion Laboratory. ;Course notes and tutorials

* ttp://www.astunit.com/tutorials/positional.htm From Stephen Tonkin's Astronomy tutorials
From Professor Kirkman's tutorials at College of Saint Benedict + Saint John's University
{{Astronomy navbox Astrometry Astrological aspects