Editing Equatorial coordinate system (section)

== Spherical coordinates ==

=== Use in astronomy ===
[[File:Equatorial_and_horizontal_celestial_coordinates_E.svg|350px|thumb|Equatorial (red) and horizontal (blue) celestial coordinates]]
A [[star]]'s spherical coordinates are often expressed as a pair, [[right ascension]] and [[declination]], without a [[distance]] coordinate. The direction of sufficiently distant objects is the same for all observers, and it is convenient to specify this direction with the same coordinates for all. In contrast, in the [[horizontal coordinate system]], a star's position differs from observer to observer based on their positions on the Earth's surface, and is continuously changing with the Earth's rotation.

[[Telescope]]s equipped with [[equatorial mount]]s and [[setting circles]] employ the equatorial coordinate system to find objects. Setting circles in conjunction with a [[star chart]] or [[ephemeris]] allow the telescope to be easily pointed at known objects on the celestial sphere.

===Declination===
{{Main|Declination}}

The declination symbol {{math|''δ''}}, (lower case "delta", abbreviated DEC) measures the angular distance of an object perpendicular to the celestial equator, positive to the north, negative to the south. For example, the north celestial pole has a declination of +90°. The origin for declination is the celestial equator, which is the projection of the Earth's equator onto the celestial sphere.  Declination is analogous to terrestrial [[latitude]].<ref name="calculator28">
{{cite book
 | title=Practical Astronomy with Your Calculator, third edition
 | author=Peter Duffett-Smith
 | year=1988
 | publisher=[[Cambridge University Press]]
 | isbn=0-521-35699-7
 | pages=[https://archive.org/details/practicalastrono0000duff/page/28 28–29]
 | url=https://archive.org/details/practicalastrono0000duff/page/28
 }}</ref><ref name="simple">
{{cite book
 | title=Astronomy Made Simple
 | url=https://archive.org/details/astronomymadesim00hamb
 | url-access=registration
 | author=Meir H. Degani
 | isbn=0-385-08854-X
 | date=1976
 | publisher=Doubleday & Company, Inc
 | page=[https://archive.org/details/astronomymadesim00hamb/page/216 216]
}}</ref><ref>
''Astronomical Almanac 2010'', p. M4
</ref>

===Right ascension===
[[File:Hour angle still1.png|thumb|right|300px|As seen from above the [[Earth]]'s [[geographic pole|north pole]], a star's {{colorbox|cyan}}{{nbsp}}[[hour angle|local hour angle]] (LHA) for an {{colorbox|red}}{{nbsp}}observer near New York. Also depicted are the star's {{colorbox|green}}{{nbsp}}[[right ascension]] and {{colorbox|blue}}{{nbsp}}Greenwich hour angle (GHA), the {{colorbox|magenta}}{{nbsp}}[[sidereal time|local mean sidereal time]] (LMST) and {{colorbox|purple}}{{nbsp}}[[sidereal time|Greenwich mean sidereal time]] (GMST). The symbol ♈︎ identifies the [[equinox|March equinox]] direction.]]

{{Main|Right ascension}}
The right ascension symbol {{math|''α''}}, (lower case "alpha", abbreviated RA) measures the angular distance of an object eastward along the [[celestial equator]] from the March [[equinox]] to the [[hour circle]] passing through the object. The March equinox point is one of the two points where the [[ecliptic]] intersects the celestial equator. Right ascension is usually measured in [[sidereal time|sidereal]] hours, minutes and seconds instead of degrees, a result of the method of measuring right ascensions by [[Meridian circle|timing the passage of objects across the meridian]] as the [[Earth's rotation|Earth rotates]]. There are {{sfrac|360°|24<sup>h</sup>}} = 15° in one hour of right ascension, and 24<sup>h</sup> of right ascension around the entire [[celestial equator]].<ref name="calculator28"/><ref>
{{cite book
 | url=https://books.google.com/books?id=PJoUAQAAMAAJ
 | title = An Introduction to Astronomy
 | last1 = Moulton
 | first1 = Forest Ray
 | page=127
 | date = 1918
}}</ref><ref>
''Astronomical Almanac 2010'', p. M14
</ref>

When used together, right ascension and declination are usually abbreviated RA/Dec.

===Hour angle===
{{Main|Hour angle}}

Alternatively to [[right ascension]], [[hour angle]] (abbreviated HA or LHA, ''local hour angle''), a left-handed system, measures the angular distance of an object westward along the [[celestial equator]] from the observer's [[meridian (astronomy)|meridian]] to the [[hour circle]] passing through the object. Unlike right ascension, hour angle is always increasing with the [[rotation of Earth]]. Hour angle may be considered a means of measuring the time since upper [[culmination]], the moment when an object contacts the meridian overhead.

A culminating star on the observer's meridian is said to have a zero hour angle (0<sup>h</sup>). One [[sidereal time|sidereal hour]] (approximately 0.9973 [[solar time|solar hours]]) later, Earth's rotation will carry the star to the west of the meridian, and its hour angle will be 1<sup>h</sup>. When calculating [[horizontal coordinate system|topocentric]] phenomena, right ascension may be converted into hour angle as an intermediate step.<ref>
{{cite book
 | title=Practical Astronomy with Your Calculator, third edition
 | author=Peter Duffett-Smith
 | year=1988
 | publisher=Cambridge University Press
 | isbn=0-521-35699-7
 | pages=[https://archive.org/details/practicalastrono0000duff/page/34 34–36]
 | url=https://archive.org/details/practicalastrono0000duff/page/34
 }}</ref><ref>''Astronomical Almanac 2010'', p. M8</ref><ref>Vallado (2001), p. 154</ref>