Editing Equatorial coordinate system (section)

===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>