Editing Ecliptic coordinate system (section)

== Spherical coordinates ==

{| class="wikitable" style="float:right; text-align:center;"
|+ Summary of notation for ecliptic coordinates<ref>''Explanatory Supplement'' (1961), sec. 1G</ref>
! rowspan="2" style="background-color:#89CFF0;" |
! colspan="3" style="background-color:#89CFF0;" | Spherical
! rowspan="2" style="background-color:#89CFF0;" | Rectangular
|-
! style="background-color:#89CFF0;" | Longitude
! style="background-color:#89CFF0;" | Latitude
! style="background-color:#89CFF0;" | Distance
|-
! style="background-color:#89CFF0;" | Geocentric
| {{math|''λ''}}
| {{math|''β''}}
| {{math|''Δ''}}
|
|-
! style="background-color:#89CFF0;" | Heliocentric
| {{math|''l''}}
| {{math|''b''}}
| {{math|''r''}}
| {{math|''x''}}, {{math|''y''}}, {{math|''z''}}<ref group="note">Occasional use; {{math|''x''}}, {{math|''y''}}, {{math|''z''}} are usually reserved for [[Equatorial coordinate system|equatorial coordinates]].</ref>
|-
| colspan="5" | {{Reflist|group="note"}}
|}

;Ecliptic longitude
:''Ecliptic longitude'' or ''celestial longitude'' (symbols: heliocentric {{mvar|l}}, geocentric {{mvar|λ}}) measures the angular distance of an object along the [[ecliptic]] from the primary direction. Like [[right ascension]] in the [[equatorial coordinate system]], the primary direction (0° ecliptic longitude) points from the Earth towards the Sun at the [[March equinox]]. Because it is a right-handed system, ecliptic longitude is measured positive eastwards in the fundamental plane (the ecliptic) from 0° to 360°. Because of [[axial precession]], the ecliptic longitude of most "fixed stars" (referred to the equinox of date) increases by about 50.3 [[arcsecond]]s per year, or 83.8 [[arcminute]]s per century, the speed of general precession.<ref>{{cite journal|author1=N. Capitaine|author2=P.T. Wallace|author3=J. Chapront|title=Expressions for IAU 2000 precession quantities|journal=Astronomy & Astrophysics|date=2003|volume=412|issue=2|page=581|doi=10.1051/0004-6361:20031539|url=http://syrte.obspm.fr/iau2006/aa03_412_P03.pdf |archive-url=https://web.archive.org/web/20120325210757/http://syrte.obspm.fr/iau2006/aa03_412_P03.pdf |archive-date=2012-03-25 |url-status=live|bibcode=2003A&A...412..567C|doi-access=free}}</ref><ref>J.H. Lieske ''et al.'' (1977), "[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1977A%26A....58....1L&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=46303c7cf308007 Expressions for the Precession Quantities Based upon the IAU (1976) System of Astronomical Constants]".  ''Astronomy & Astrophysics'' '''58''', pp. 1-16</ref> However, for stars near the ecliptic poles, the rate of change of ecliptic longitude is dominated by the slight movement of the ecliptic (that is, of the plane of the Earth's orbit), so the rate of change may be anything from minus infinity to plus infinity depending on the exact position of the star.
;Ecliptic latitude
:''Ecliptic latitude'' or ''celestial latitude'' (symbols: heliocentric {{mvar|b}}, geocentric {{mvar|β}}), measures the angular distance of an object from the [[ecliptic]] towards the north (positive) or south (negative) [[ecliptic pole]]. For example, the [[Ecliptic pole|north ecliptic pole]] has a celestial latitude of +90°. Ecliptic latitude for "fixed stars" is not affected by precession.
;Distance
:''Distance'' is also necessary for a complete spherical position (symbols: heliocentric {{mvar|r}}, geocentric {{mvar|Δ}}). Different distance units are used for different objects. Within the [[Solar System]], [[astronomical unit]]s are used, and for objects near the [[Earth]], [[Earth radius|Earth radii]] or [[kilometer]]s are used.

=== Historical use ===
From antiquity through the 18th&nbsp;century, ecliptic longitude was commonly measured using twelve [[Astrological sign|zodiacal signs]], each of 30°&nbsp;longitude, a practice that continues in modern [[astrology]]. The signs approximately corresponded to the [[constellation]]s crossed by the ecliptic. Longitudes were specified in signs, degrees, minutes, and seconds. For example, a longitude of {{nowrap|♌ 19°&thinsp;55′&thinsp;58″}} is 19.933°&nbsp;east of the start of the sign [[Leo (astrology)|Leo]]. Since Leo begins 120°&nbsp;from the March equinox, the longitude in modern form is {{nowrap|139°&thinsp;55′&thinsp;58″}}.<ref>{{cite book |url=https://archive.org/details/acompleatsystem01leadgoog |title=A Compleat System of Astronomy |first=Charles |last=Leadbetter |year=1742 |publisher=J. Wilcox, London |page=[https://archive.org/details/acompleatsystem01leadgoog/page/n102 94]}}; numerous examples of this notation appear throughout the book.</ref>

{{See also|solar term}}

In China, ecliptic longitude is measured using 24&nbsp;[[Solar term]]s, each of 15°&nbsp;longitude, and are used by [[Chinese calendar|Chinese lunisolar calendars]] to stay synchronized with the seasons, which is crucial for agrarian societies.