Editing Ecliptic

{{Short description|Apparent path of the Sun on the celestial sphere}}
{{Good article}}
{{Use dmy dates|date=June 2020}}

[[File:Ecliptic with earth and sun animation.gif|thumb|upright=1.3|As seen from the orbiting [[Earth]], the [[Sun]] [[diurnal motion|appears to move]] with respect to the [[fixed stars]], and the ecliptic is the yearly path the Sun follows on the [[celestial sphere]]. This process repeats itself in a cycle lasting a little over [[tropical year|365 days]].]]

The '''ecliptic''' or '''ecliptic plane''' is the [[orbital plane]] of [[Earth's orbit|Earth around the Sun]].<ref name="AA2010">{{cite book |author1=[[United States Naval Observatory|USNO]] Nautical Almanac Office |author2=UK Hydrographic Office, [[HM Nautical Almanac Office]] |title=The Astronomical Almanac for the Year 2010 |publisher=[[United States Government Publishing Office|GPO]] |date=2008 |page=M5 |isbn=978-0-7077-4082-9}}</ref><ref>{{Cite web|title=LEVEL 5 Lexicon and Glossary of Terms|url=https://ned.ipac.caltech.edu/level5/Glossary/Glossary_E.html}}</ref>{{Efn|Strictly, the plane of the mean orbit, with minor variations averaged out.|name=|group=}} It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making.

From the perspective of an observer on Earth, the Sun's movement around the [[celestial sphere]] over the course of a year traces out a path along the ecliptic against the [[fixed stars|background of stars]] – specifically the [[Zodiac]] constellations.<ref>{{Cite web|title=The Ecliptic: the Sun's Annual Path on the Celestial Sphere|url=http://community.dur.ac.uk/john.lucey/users/solar_year.html}}</ref> The planets of the [[Solar System]] can also be seen along the ecliptic, because their orbital planes are very close to Earth's. The Moon's orbital plane is also similar to Earth's; the ecliptic is so named because the ancients noted that [[eclipse]]s only occur when the Moon is crossing it.<ref name=Ball/>

The ecliptic is an important [[Plane of reference|reference plane]] and is the basis of the [[ecliptic coordinate system]]. Ancient scientists were able to calculate Earth's [[axial tilt]] by comparing the ecliptic plane to that of the [[equator]].

==Sun's apparent motion==
The ecliptic is the apparent path of the Sun throughout the course of a [[year]].<ref>
{{cite book 
 | author = U.S. Naval Observatory Nautical Almanac Office
 | editor = P. Kenneth Seidelmann
 | title = Explanatory Supplement to the Astronomical Almanac
 | publisher = University Science Books, Mill Valley, CA
 | date = 1992
 | isbn = 0-935702-68-7}}, p. 11</ref>

Because Earth takes one year to orbit the Sun, the apparent position of the Sun takes one year to make a complete circuit of the ecliptic. With slightly more than 365 days in one year, the Sun moves a little less than 1° eastward<ref name="celes direc"/> every day. This small difference in the Sun's position against the stars causes any particular spot on Earth's surface to catch up with (and stand directly north or south of) the Sun about four minutes later each day than it would if Earth did not orbit; a day on Earth is therefore 24 hours long rather than the approximately 23-hour 56-minute [[sidereal time|sidereal day]]. Again, this is a simplification, based on a hypothetical Earth that orbits at a uniform angular speed around the Sun. The actual speed with which Earth orbits the Sun varies slightly during the year, so the speed with which the Sun seems to move along the ecliptic also varies. For example, the Sun is north of the celestial equator for about 185 days of each year, and south of it for about 180 days.<ref>''Astronomical Almanac 2010'', sec. C</ref> The variation of orbital speed accounts for part of the [[equation of time]].<ref>''Explanatory Supplement'' (1992), sec. 1.233</ref>
 
Because of the movement of Earth around the Earth–Moon [[center of mass]], the apparent path of the Sun wobbles slightly, with a period of about [[Orbit of the Moon|one month]]. Because of further [[Perturbation (astronomy)|perturbations]] by the other [[planet]]s of the Solar System, the Earth–Moon [[barycenter]] wobbles slightly around a mean position in a complex fashion.

== Relationship to the celestial equator ==
{{Main|Axial tilt}}
[[File:Earths orbit and ecliptic.svg|thumb|350px|The [[plane (geometry)|plane]] of [[Earth]]'s [[orbit]] projected in all directions forms the reference plane known as the ecliptic. Here, it is shown projected outward (gray) to the [[celestial sphere]], along with Earth's [[equator]] and [[Earth's rotation|polar axis]] (green). The plane of the ecliptic intersects the celestial sphere along a [[great circle]] (black), the same circle on which the Sun seems to move as Earth orbits it. The intersections of the ecliptic and the equator on the celestial sphere are the [[equinox]]es (red), where the Sun seems to cross the celestial equator. ]]

Because [[Earth's rotation|Earth's rotational axis]] is not [[perpendicular]] to its [[Orbital plane (astronomy)|orbital plane]], Earth's [[Equator|equatorial plane]] is not [[Coplanarity|coplanar]] with the ecliptic plane, but is inclined to it by an angle of about 23.4°, which is known as the [[axial tilt|obliquity of the ecliptic]].<ref>''Explanatory Supplement'' (1992), p. 733</ref> If the equator is projected outward to the [[celestial sphere]], forming the [[celestial equator]], it crosses the ecliptic at two points known as the [[equinox]]es. The Sun, in its apparent motion along the ecliptic, crosses the celestial equator at these points, one from south to north, the other from north to south.<ref name="celes direc">The directions ''north'' and ''south'' on the celestial sphere are in the sense ''toward the north [[celestial pole]]'' and ''toward the south celestial pole''. ''East'' is ''the direction toward which Earth rotates'', ''west'' is opposite that.</ref> The crossing from south to north is known as the [[March equinox]], also known as the ''first point of Aries'' and the ''[[Orbital node|ascending node]] of the ecliptic'' on the celestial equator.<ref>''Astronomical Almanac 2010'', p. M2 and M6</ref> The crossing from north to south is the [[September equinox]] or [[Orbital node|descending node]].

{{Main|Axial precession}}

The orientation of [[Earth's rotation|Earth's axis]] and equator are not fixed in space, but rotate about the [[Ecliptic pole|poles of the ecliptic]] with a period of about 26,000 years, a process known as ''lunisolar [[precession]]'', as it is due mostly to the gravitational effect of the [[Moon]] and [[Sun]] on [[Figure of the Earth|Earth's equatorial bulge]]. Likewise, the ecliptic itself is not fixed. The gravitational perturbations of the other bodies of the Solar System cause a much smaller motion of the plane of Earth's orbit, and hence of the ecliptic, known as ''planetary precession''. The combined action of these two motions is called ''general precession'', and changes the position of the equinoxes by about 50 [[Minute of arc|arc seconds]] (about 0.014°) per year.<ref>''Explanatory Supplement'' (1992), sec. 1.322 and 3.21</ref>

{{Main|Astronomical nutation}}

Once again, this is a simplification. Periodic motions of the [[Moon]] and apparent periodic motions of the [[Sun]] (actually of Earth in its orbit) cause short-term small-amplitude periodic oscillations of Earth's axis, and hence the celestial equator, known as [[astronomical nutation|nutation]].<ref>
{{cite book 
 | author = U.S. Naval Observatory Nautical Almanac Office
 |author2=H.M. Nautical Almanac Office
 | title = Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac
 | publisher = H.M. Stationery Office, London
 | date = 1961}}
, sec. 2C</ref>
This adds a periodic component to the position of the equinoxes; the positions of the celestial equator and (March) equinox with fully updated precession and nutation are called the ''true equator and equinox''; the positions without nutation are the ''mean equator and equinox''.<ref>''Explanatory Supplement'' (1992), p. 731 and 737</ref>

== Obliquity of the ecliptic {{anchor|Obliquity}}==
{{main|Obliquity of the ecliptic}}

''Obliquity of the ecliptic'' is the term used by astronomers for the inclination of Earth's equator with respect to the ecliptic, or of Earth's rotation axis to a perpendicular to the ecliptic. It is about 23.4° and is currently decreasing 0.013 degrees (47 arcseconds) per hundred years because of planetary perturbations.<ref>
{{cite book 
 | last = Chauvenet
 | first = William
 | title = A Manual of Spherical and Practical Astronomy
 | publisher = J.B. Lippincott Co., Philadelphia
 | date = 1906
 | volume = I
 |url=https://books.google.com/books?id=yobvAAAAMAAJ
}}, art. 365–367, p. 694–695, at Google books</ref>

The angular value of the obliquity is found by observation of the motions of Earth and other planets over many years. Astronomers produce new [[fundamental ephemeris|fundamental ephemerides]] as the accuracy of [[Observational astronomy|observation]] improves and as the understanding of the [[Analytical dynamics|dynamics]] increases, and from these ephemerides various astronomical values, including the obliquity, are derived.

[[File:Obliquity of the ecliptic laskar.PNG|thumb|upright=1.4|left|Obliquity of the ecliptic for 20,000 years, from Laskar (1986).<ref name="laskar"/> Note that the obliquity varies only from 24.2° to 22.5° during this time. The red point represents the year 2000.]]

Until 1983 the obliquity for any date was calculated from [[Newcomb's Tables of the Sun|work of Newcomb]], who analyzed positions of the planets until about 1895:

{{math|''ε'' {{=}} 23°27′08.26″ − 46.845″ ''T'' − 0.0059″ ''T''<sup>2</sup> + 0.00181″ ''T''<sup>3</sup>}}

where {{math|''ε''}} is the obliquity and {{math|''T''}} is [[Tropical year|tropical centuries]] from [[Epoch (astronomy)|B1900.0]] to the date in question.<ref>''Explanatory Supplement'' (1961), sec. 2B</ref>

From 1984, the [[Jet Propulsion Laboratory Development Ephemeris|Jet Propulsion Laboratory's DE series]] of computer-generated ephemerides took over as the fundamental ephemeris of the ''Astronomical Almanac''. Obliquity based on DE200, which analyzed observations from 1911 to 1979, was calculated:

{{math|''ε'' {{=}} 23°26′21.45″ − 46.815″ ''T'' − 0.0006″ ''T''<sup>2</sup> + 0.00181″ ''T''<sup>3</sup>}}

where hereafter {{math|''T''}} is [[Julian year (astronomy)|Julian centuries]] from [[Epoch (astronomy)|J2000.0]].<ref>
{{cite book 
 | last = U.S. Naval Observatory
 | first =Nautical Almanac Office
 |author2=H.M. Nautical Almanac Office
 | title = The Astronomical Almanac for the Year 1990
 | publisher = U.S. Govt. Printing Office
 | date = 1989
 | isbn = 0-11-886934-5
}}, p. B18</ref>

JPL's fundamental ephemerides have been continually updated. The ''Astronomical Almanac'' for 2010 specifies:<ref>''Astronomical Almanac 2010'', p. B52</ref>

{{math|''ε'' {{=}} 23°26′21.406″ − 46.836769″ ''T'' − 0.0001831″ ''T''<sup>2</sup> + 0.00200340″ ''T''<sup>3</sup> − 0.576×10<sup>−6</sup>″ ''T''<sup>4</sup> − 4.34×10<sup>−8</sup>″ ''T''<sup>5</sup>}}

These expressions for the obliquity are intended for high precision over a relatively short time span, perhaps several centuries.<ref>
{{cite book
 |url = https://archive.org/details/acompendiumsphe00newcgoog
 |title = A Compendium of Spherical Astronomy
 |last = Newcomb
 |first = Simon
 |date = 1906
 |publisher=MacMillan Co., New York
}}, p. 226-227, at Google books</ref> J. Laskar computed an expression to order {{math|''T''<sup>10</sup>}} good to {{math|0.04″}}/1000 years over 10,000 years.<ref name="laskar">
{{Cite journal|title = Secular Terms of Classical Planetary Theories Using the Results of General Relativity
 |last = Laskar
 |first = J.
 |journal = Astronomy and Astrophysics
 |date = 1986|volume = 157
 |issue = 1
 |page = 59
 |bibcode = 1986A&A...157...59L
}}, table 8, at SAO/NASA ADS</ref>

All of these expressions are for the ''mean'' obliquity, that is, without the nutation of the equator included. The ''true'' or instantaneous obliquity includes the nutation.<ref>
{{cite book 
 | last = Meeus
 | first = Jean
 | title = Astronomical Algorithms
 | publisher = Willmann-Bell, Inc., Richmond, VA
 | date = 1991
 |isbn=0-943396-35-2
}}, chap. 21</ref>

{{clear}}

==Plane of the Solar System==
{{main|Solar System}}

{| class="wikitable" style="float:center;margin: 0em 0em .5em 1em;"
|-
|width=200px|[[File:Ecliptic plane top view.gif|200px]]
|width=200px|[[File:Ecliptic plane side view.gif|200px]]
|width=200px|[[File:FourPlanetSunset hao annotated.JPG|200px]]
|-
|colspan=2|Top and side views of the plane of the ecliptic, showing planets [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]]. Most of the planets orbit the [[Sun]] very nearly in the same plane in which Earth orbits, the ecliptic.
|colspan=1|Five planets (Earth included) lined up along the ecliptic in July 2010, illustrating how the planets orbit the Sun in nearly the same plane. Photo taken at sunset, looking west over Surakarta, Java, Indonesia.
|}

Most of the major bodies of the Solar System orbit the Sun in nearly the same plane. This is likely due to the way in which the Solar System formed from a [[protoplanetary disk]]. Probably the closest current representation of the disk is known as the ''[[invariable plane]] of the Solar System''. Earth's orbit, and hence, the ecliptic, is inclined a little more than 1° to the invariable plane, Jupiter's orbit is within a little more than ½° of it, and the other major planets are all within about 6°. Because of this, most Solar System bodies appear very close to the ecliptic in the sky.

The invariable plane is defined by the [[angular momentum]] of the entire Solar System, essentially the vector sum of all of the [[orbit]]al and [[Rotation#Astronomy|rotational]] angular momenta of all the bodies of the system; more than 60% of the total comes from the orbit of Jupiter.<ref name=meanplane>{{cite web |date=3 April 2009 |title=The Mean Plane (Invariable Plane) of the Solar System passing through the barycenter |url=http://home.surewest.net/kpheider/astro/MeanPlane.gif |access-date=10 April 2009 |url-status=dead |archive-url=https://web.archive.org/web/20130603143144/http://home.surewest.net/kheider/astro/MeanPlane.gif |archive-date=3 June 2013 }} produced with {{cite web |url=http://chemistry.unina.it/~alvitagl/solex/ |title=Solex 10 |first=Aldo |last=Vitagliano |format=computer program |access-date=10 April 2009 |archive-url=https://www.webcitation.org/5gOzK38bc?url=http://chemistry.unina.it/~alvitagl/solex/ |archive-date=29 April 2009 |url-status=dead }}</ref> That sum requires precise knowledge of every object in the system, making it a somewhat uncertain value. Because of the uncertainty regarding the exact location of the invariable plane, and because the ecliptic is well defined by the apparent motion of the Sun, the ecliptic is used as the reference plane of the Solar System both for precision and convenience. The only drawback of using the ecliptic instead of the invariable plane is that over geologic time scales, it will move against fixed reference points in the sky's distant background.<ref>{{cite book |last=Danby |first=J.M.A. |title=Fundamentals of Celestial Mechanics |publisher=Willmann-Bell, Inc., Richmond, VA |year=1988 |isbn=0-943396-20-4 |at=section&nbsp;9.1}}</ref><ref>{{cite book |last=Roy |first=A.E. |title=Orbital Motion |publisher=Institute of Physics Publishing |year=1988 |isbn=0-85274-229-0 |edition=third |at=section&nbsp;5.3}}</ref>

{{clear}}

==Celestial reference plane==
{{main|Celestial equator|Ecliptic coordinate system}}

{{Gallery
| title        = 
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| File:Ecliptic vs equator small.gif
 | The apparent motion of the [[Sun]] along the ecliptic (red) as seen on the inside of the [[celestial sphere]]. [[Ecliptic coordinate system|Ecliptic coordinates]] appear in (red). The [[celestial equator]] (blue) and the [[Equatorial coordinate system|equatorial coordinates]] (blue), being inclined to the ecliptic, appear to wobble as the Sun advances.
 | class1=
 | alt1=
| File:Ecliptic inclination dziobek.PNG
 | Inclination of the ecliptic over 200,000 years, from Dziobek (1892).<ref>
 {{cite book 
 | last = Dziobek
 | first = Otto
 | title = Mathematical Theories of Planetary Motions
 | publisher = Register Publishing Co., Ann Arbor, Michigan
 | date = 1892
 |url = https://books.google.com/books?id=WTEaAAAAYAAJ&q=dziobek+mathematical}}, p. 294, at Google books</ref> This is the inclination to the ecliptic of 101,800 CE. Note that the ecliptic rotates by only about 7° during this time, whereas the [[celestial equator]] makes several complete cycles around the ecliptic. The ecliptic is a relatively stable reference compared to the celestial equator.
 | class2=
 | alt2=
}}

The ecliptic forms one of the two fundamental [[Plane (geometry)|planes]] used as reference for positions on the celestial sphere, the other being the [[celestial equator]]. Perpendicular to the ecliptic are the [[ecliptic pole]]s, the north ecliptic pole being the pole north of the equator. Of the two fundamental planes, the ecliptic is closer to unmoving against the background stars, its motion due to planetary [[precession]] being roughly 1/100 that of the celestial equator.<ref name="montenbruck">
{{cite book 
 | last = Montenbruck
 | first = Oliver
 | title = Practical Ephemeris Calculations
 | publisher = Springer-Verlag
 | date = 1989
 | isbn = 0-387-50704-3
}}, sec 1.4</ref>

[[Spherical coordinate system|Spherical coordinates]], known as ecliptic longitude and latitude or celestial longitude and latitude, are used to specify positions of bodies on the celestial sphere with respect to the ecliptic. Longitude is measured positively eastward<ref name="celes direc"/> 0° to 360° along the ecliptic from the March equinox, the same direction in which the Sun appears to move. Latitude is measured perpendicular to the ecliptic, to +90° northward or −90° southward to the poles of the ecliptic, the ecliptic itself being 0° latitude. For a complete spherical position, a distance parameter is also necessary. Different distance units are used for different objects. Within the Solar System, [[astronomical unit]]s are used, and for objects near [[Earth]], [[Earth radius|Earth radii]] or [[kilometre|kilometers]] are used. A corresponding right-handed [[Cartesian coordinate system|rectangular coordinate system]] is also used occasionally; the ''x''-axis is directed toward the March equinox, the ''y''-axis 90° to the east, and the ''z''-axis toward the north ecliptic pole; the astronomical unit is the unit of measure. Symbols for ecliptic coordinates are somewhat standardized; see the table.<ref>''Explanatory Supplement'' (1961), sec. 2A</ref>

{| class="wikitable" style="float:right; margin:0em 1em .5em 0em;"
|+ Summary of notation for ecliptic coordinates<ref>''Explanatory Supplement'' (1961), sec. 1G</ref>
| rowspan="2" bgcolor="#89CFF0" |
| colspan="3" align="center" bgcolor="#89CFF0" | '''Spherical'''
| rowspan="2" align="center" bgcolor="#89CFF0" | '''Rectangular''' 
|- bgcolor="#89CFF0" align="center"
| Longitude
| Latitude
| Distance
|- align="center"
| bgcolor="#89CFF0" | '''Geocentric'''
| ''λ''
| ''β''
| ''Δ''
|
|- align="center"
| bgcolor="#89CFF0" | '''Heliocentric'''
| ''l''
| ''b''
| ''r''
| ''x'', ''y'', ''z''<ref group="note">Occasional use; ''x'', ''y'', ''z'' are usually reserved for [[Equatorial coordinate system|equatorial coordinates]].</ref>
|-
| colspan="5" | {{Reflist|group="note"}}
|}
Ecliptic coordinates are convenient for specifying positions of Solar System objects, as most of the planets' orbits have small [[Orbital inclination|inclinations]] to the ecliptic, and therefore always appear relatively close to it on the sky. Because Earth's orbit, and hence the ecliptic, moves very little, it is a relatively fixed reference with respect to the stars.

Because of the [[precession|precessional motion of the equinox]], the ecliptic coordinates of objects on the celestial sphere are continuously changing. Specifying a position in ecliptic coordinates requires specifying a particular equinox, that is, the equinox of a particular date, known as an [[Epoch (astronomy)|epoch]]; the coordinates are referred to the direction of the equinox at that date. For instance, the ''Astronomical Almanac''<ref>''Astronomical Almanac 2010'', p. E14</ref> lists the [[Heliocentric#Modern use of geocentric and heliocentric|heliocentric]] position of [[Mars]] at 0h [[Terrestrial Time]], 4 January 2010 as: longitude 118°09′15.8″, latitude +1°43′16.7″, true heliocentric distance 1.6302454 AU, mean equinox and ecliptic of date. This specifies the [[mean equinox]] of 4 January 2010 0h TT [[ecliptic#Relationship to the equator|as above]], without the addition of nutation.

==Eclipses==
{{main|Eclipse}}
[[File:Eclipse vs new or full moons, annotated.svg|thumb|As the Earth revolves around the Sun, approximate [[axial parallelism]] of the Moon's orbital plane ([[Orbital inclination|tilted]] five degrees to the ecliptic) results in the revolution of the [[lunar nodes]] relative to the Earth. This causes an [[eclipse season]] approximately every six months, in which a [[solar eclipse]] can occur at the [[new moon]] phase and a [[lunar eclipse]] can occur at the [[full moon]] phase.]]

Because the [[orbit of the Moon]] is inclined only about 5.145° to the ecliptic and the Sun is always very near the ecliptic, [[eclipse]]s always occur on or near it. Because of the inclination of the Moon's orbit, eclipses do not occur at every [[Conjunction (astronomy and astrology)|conjunction]] and [[Opposition (planets)|opposition]] of the Sun and Moon, but only when the Moon is near an [[Orbital node|ascending or descending node]] at the same time it is at conjunction ([[new moon|new]]) or opposition ([[full moon|full]]). The ecliptic is so named because the ancients noted that eclipses only occur when the Moon is crossing it.<ref name=Ball>
{{cite book
|url=https://archive.org/details/atreatiseonsphe00ballgoog
|title=A Treatise on Spherical Astronomy
|first=Robert S.
|last=Ball
|date=1908
|publisher=Cambridge University Press
|page=[https://archive.org/details/atreatiseonsphe00ballgoog/page/n98 83]}}
</ref>

==Equinoxes and solstices==
[[File:Optical effect march sunset - NOAA.jpg|thumb|upright=1.3|At [[Earth's poles]] the Sun appears at the horizon only and all day around [[equinox]], marking the change between the half year long [[polar night]] and [[polar day]]. The picture shows the [[South Pole]] right before March equinox, with the Sun appearing through [[refraction]] despite being still below the horizon.]]

{| class="wikitable" style="float: right;margin:0em 0 .5em 1em;"
|+'''Positions of [[equinox]]es and [[solstice]]s'''
| rowspan="2" bgcolor="#F4C2C2"|&nbsp;
| align="center" bgcolor="#F4C2C2" | '''[[ecliptic coordinate system|ecliptic]]'''
| align="center" bgcolor="#F4C2C2" | '''[[equatorial coordinate system|equatorial]]'''
|- align="center" bgcolor="#F4C2C2" 
| longitude
| [[right ascension]]
|- align="center"
| bgcolor="#F4C2C2" | '''[[March equinox]]'''
| 0°
| 0h
|- align="center"
| bgcolor="#F4C2C2" | '''[[June solstice]]'''
| 90°
| 6h
|- align="center"
| bgcolor="#F4C2C2" | '''[[September equinox]]'''
| 180°
| 12h
|- align="center"
| bgcolor="#F4C2C2" | '''[[December solstice]]'''
| 270°
| 18h
|}
{{main|Equinox (celestial coordinates)}}

The exact instants of [[equinox]]es and [[solstice]]s are the times when the apparent [[ecliptic coordinate system|ecliptic longitude]] (including the effects of [[aberration of light|aberration]] and [[nutation]]) of the [[Sun]] is 0°, 90°, 180°, and 270°. Because of [[perturbation (astronomy)|perturbations]] of [[Earth's orbit]] and anomalies of [[Gregorian calendar|the calendar]], the dates of these are not fixed.<ref>Meeus (1991), chap. 26</ref>

==In the constellations==
[[File:Constellations, equirectangular plot, Menzel families.svg|thumb|300px|Equirectangular plot of declination vs right ascension of the modern constellations with a dotted line denoting the ecliptic. Constellations are colour-coded by family and year established.]]
The ecliptic currently passes through the following thirteen [[constellation]]s:
{{columns-list|colwidth=18em|
*[[Pisces (constellation)|Pisces]]
*[[Aries (constellation)|Aries]]
*[[Taurus (constellation)|Taurus]]
*[[Gemini (constellation)|Gemini]]
*[[Cancer (constellation)|Cancer]]
*[[Leo (constellation)|Leo]]
*[[Virgo (constellation)|Virgo]]
*[[Libra (constellation)|Libra]]
*[[Scorpius]]
*[[Ophiuchus]]<ref>
{{cite book
|url=https://archive.org/details/astronomywithna02servgoog
|title=Astronomy With the Naked Eye
|first=Garrett P.
|last=Serviss
|publisher=Harper & Brothers, New York and London
|date=1908
|pages=[https://archive.org/details/astronomywithna02servgoog/page/n151 105], 106}}</ref>
*[[Sagittarius (constellation)|Sagittarius]]
*[[Capricornus]]
*[[Aquarius (constellation)|Aquarius]]
}}

There are twelve constellations that are not on the ecliptic, but are close enough that the Moon and planets can occasionally appear in them.<ref>{{cite book |last=Kidger |first=Mark |date=2005 |title=Astronomical Enigmas: Life on Mars, the Star of Bethlehem, and Other Milky Way Mysteries |publisher=The Johns Hopkins University Press |pages=38–39 |isbn=9780801880261}}</ref><ref name=mosley>{{cite web |url=http://www.ips-planetarium.org/?page=a_mosley1999b |publisher=International Planetarium Society |date=2011 |first=John |last=Mosley |title=The Real, Real Constellations of the Zodiac |access-date=21 March 2017 |archive-date=1 July 2017 |archive-url=https://web.archive.org/web/20170701021831/http://www.ips-planetarium.org/?page=a_mosley1999b |url-status=live }}</ref>

*[[Cetus (constellation)|Cetus]]
*[[Pegasus (constellation)|Pegasus]]
*[[Aquila (constellation)|Aquila]]
*[[Scutum (constellation)|Scutum]]
*[[Serpens (constellation)|Serpens]]
*[[Hydra (constellation)|Hydra]]
*[[Corvus (constellation)|Corvus]]
*[[Crater (constellation)|Crater]]
*[[Sextans (constellation)|Sextans]]
*[[Canis Minor (constellation)|Canis Minor]]
*[[Auriga (constellation)|Auriga]]
*[[Orion (constellation)|Orion]]

==Astrology==
{{main|Astrology}}

The ecliptic forms the center of the [[zodiac]], a celestial belt about 20° wide in latitude through which the Sun, Moon, and planets always appear to move.<ref>
{{cite book |url=https://books.google.com/books?id=BqLifAK2zJkC&q=history+of+astronomy |title=A History of Astronomy |last=Bryant |first=Walter W. |date=1907 |page=3 |publisher=Forgotten Books |isbn=9781440057922}}</ref>
Traditionally, this region is divided into 12 [[astrological sign|signs]] of 30° longitude, each of which approximates the Sun's motion in one month.<ref>Bryant (1907), p. 4.</ref> In ancient times, the signs corresponded roughly to 12 of the constellations that straddle the ecliptic.<ref>See, for instance, {{cite book |url=https://archive.org/details/bub_gb_8CwSAAAAYAAJ |quote=astrology. |title=Astrology for All |publisher=L.N. Fowler & Company |last=Leo |first=Alan |date=1899 |page=[https://archive.org/details/bub_gb_8CwSAAAAYAAJ/page/n13 8]}}</ref>
These signs are sometimes still used in modern terminology. The "[[First Point of Aries]]" was named when the [[March equinox]] Sun was actually in the constellation [[Aries (constellation)|Aries]]; it has since moved into [[Pisces (constellation)|Pisces]] because of [[axial precession|precession of the equinoxes]].<ref>{{cite book |last=Vallado |first=David A. |title=Fundamentals of Astrodynamics and Applications |publisher=Microcosm Press |location=El Segundo, CA |date=2001 |isbn=1-881883-12-4 |edition=2nd |page=153}}
</ref>

==See also==
* [[Formation and evolution of the Solar System]]
* [[Invariable plane]]
* [[Protoplanetary disk]]
* [[Celestial coordinate system]]

==Notes and references==
{{notelist}}
{{Reflist}}

== External links ==
{{Wiktionary|ecliptic}}
{{Wikiversity|Ecliptic|at=Quizzes|at-link=Ecliptic/Quizzes}}
*[http://www.dur.ac.uk/john.lucey/users/solar_year.html The Ecliptic: the Sun's Annual Path on the Celestial Sphere] Durham University Department of Physics
*[http://astro.unl.edu/naap/motion1/animations/seasons_ecliptic.html Seasons and Ecliptic Simulator] University of Nebraska-Lincoln
*[http://stars.astro.illinois.edu/celsph.html MEASURING THE SKY A Quick Guide to the Celestial Sphere] James B. Kaler, University of Illinois
*[http://aa.usno.navy.mil/data/docs/EarthSeasons.php Earth's Seasons] {{Webarchive|url=https://web.archive.org/web/20071013000301/http://aa.usno.navy.mil/data/docs/EarthSeasons.php |date=13 October 2007 }} U.S. Naval Observatory
*[http://www.astrologyclub.org/articles/ecliptic/ecliptic.htm The Basics - the Ecliptic, the Equator, and Coordinate Systems] AstrologyClub.Org
*{{cite journal
|title=The definition of the ecliptic
|last1=Kinoshita
|first1=H.
|last2=Aoki
|first2=S.
|date=1983
|journal=Celestial Mechanics
|volume=31
|issue=4
|pages=329–338|bibcode = 1983CeMec..31..329K |doi = 10.1007/BF01230290 |s2cid=122913096
}}; comparison of the definitions of LeVerrier, Newcomb, and Standish.
{{Zodiac}}
{{Astronomy in medieval Islam}}
{{Indian astronomy}}
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{{Authority control}}

[[Category:Astronomical coordinate systems]]
[[Category:Dynamics of the Solar System]]
[[Category:Technical factors of astrology]]
[[Category:Planes (geometry)]]