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{{Short description|Apparent path of the Sun on the celestial sphere}}
{{Short description|Apparent path of the Sun on the celestial sphere}}
{{Good article}}
{{Use dmy dates|date=June 2020}}
{{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]].]]
[[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.
The '''ecliptic''' or '''ecliptic plane''' is the apparent path of the [[Sun]] on the [[celestial sphere]], resulting from [[Earth's orbit]] 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. The ecliptic is so named because the ancients noted that [[eclipse]]s only occur when the Moon is crossing it.<ref name=Ball/>


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/>
Sun's path can be traced against the [[fixed stars|background of stars]], especially 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 be seen along the ecliptic because their orbital planes are very close to Earth's. The Moon also appears near the plane, with the [[Moon's orbit]]al plane inclined only 5.1°, intersecting the ecliptic at the [[lunar node]]s.


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]].
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 angle of the ecliptic (about 23.4°) to that of the [[equator]]ial plane.


==Sun's apparent motion==
==Sun's apparent motion==
The ecliptic is the apparent path of the Sun throughout the course of a [[year]].<ref>
The ecliptic is [[Sun path|the apparent path]] of the [[Sun]] throughout the course of a [[year]].<ref>
{{cite book  
{{cite book  
  | author = U.S. Naval Observatory Nautical Almanac Office
  | author = U.S. Naval Observatory Nautical Almanac Office
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  | isbn = 0-935702-68-7}}, p. 11</ref>
  | 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 Earth takes one year [[Earth's orbit|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{{efn|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.
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.{{cn|date=May 2026}}


== Relationship to the celestial equator ==
==Inclination to the plane of the Solar System==
{{Main|Axial tilt}}
{{main|Solar System}}
[[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. ]]
[[File:The Mysterious Case of the Disappearing Dust.jpg|thumb|upright|A depiction of the early [[Solar System]]'s [[protoplanetary disk]] from which Earth and other Solar System bodies were formed]]


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]].
Most of the major bodies of the Solar System orbit the Sun in nearly the same plane. This is thought to be due to the planets being formed from a [[protoplanetary disk]], with all the dust and gas orbiting the sun close to a single plane.<ref>{{cite journal|last1=Armitage|first1=Philip J.|title=Dynamics of Protoplanetary Disks|journal=[[Annual Review of Astronomy and Astrophysics]]|date=2011|volume=49|issue=1|pages=195–236|doi=10.1146/annurev-astro-081710-102521|arxiv = 1011.1496 |bibcode = 2011ARA&A..49..195A |s2cid=55900935}}</ref> The ''[[invariable plane]]'' of the Solar System is the plane passing through its [[center of mass]] perpendicular to its [[angular momentum]] vector.<ref>{{Cite journal |last=Souami |first=D. |last2=Souchay |first2=J. |date=2012-07-01 |title=The solar system’s invariable plane |journal=Astronomy & Astrophysics |language=en |volume=543 |pages=A133 |doi=10.1051/0004-6361/201219011 }}</ref> 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°.<ref name=meanplane/> 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/> 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>
 
{| class="wikitable" style="float:center;margin: 0em 0em 0em 0em;"
|-
|width=200px|[[File:Ecliptic plane top view.gif|200px]]
|width=200px|[[File:Ecliptic plane side view.gif|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.
|}


{{Main|Axial precession}}
{{solar system inclinations}}<!-- contains table and definition of reference "meanplane" -->


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>
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{{Main|Astronomical nutation}}
===Inclination to the galactic plane===


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>
The ecliptic is inclined to the [[galactic plane]] by 60°.<ref name="r969">{{cite web | title=Galactic Plane | website=Swinburne | url=https://astronomy.swin.edu.au/cosmos/*/Galactic+Plane | access-date=2026-05-09}}</ref>
 
[[File:Meteor shower in the Chilean Desert (annotated) (potw2227b).jpg|thumb|A [[Dark-sky preserve|dark sky]] view, showing the difference in inclination of the [[galactic plane]] of the [[Milky Way]] to the not much inclined to each other orbital planes of the Solar System planets incl. the ecliptic, which is illuminated by the outlined [[zodiacal light]] and flanked by a [[meteor shower]]]]
 
[[File:Motion of Earth & Sun Around the Milky Way 07Jul2022 credits & captions - moon 5 deg incl to ecliptic 9.jpg|thumb|The general motion and orientation of the Sun, with Earth and the Moon as its Solar System satellites]]
 
== Referencing Earth's equator ==
{{Further|Axial tilt}}
[[File:Earths orbit and ecliptic.svg|thumb|upright=1.3|The ecliptic plane (gray) projected outward to the [[celestial sphere]]. The 23.44° tilt of Earth's axis defines its [[celestial equator]] (red). The [[equinox]]es (red dots) occur biannually, when the Sun appears [[Zenith|directly above]] the equator.]]
 
Because [[Earth's rotation|Earth's rotational axis]] is not perpendicular to its [[Orbital plane (astronomy)|orbital plane]], Earth's [[equator]]ial plane is not [[coplanar]] with the ecliptic plane, but is inclined to it by an angle of about 23.4°, which is known as the ''[[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.{{efn|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.}} 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. The crossing from north to south is the [[September equinox]] or [[Orbital node|descending node]].<ref>''Astronomical Almanac 2010'', p. M2 and M6</ref>
 
===Celestial reference plane===
{{main|Celestial equator|Ecliptic coordinate system}}
 
[[File:Ecliptic vs equator small.gif|thumb|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.]]
 
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  
{{cite book  
  | author = U.S. Naval Observatory Nautical Almanac Office
  | last = Montenbruck
  |author2=H.M. Nautical Almanac Office
  | first = Oliver
  | title = Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac
  | title = Practical Ephemeris Calculations
  | publisher = H.M. Stationery Office, London
  | publisher = Springer-Verlag
  | date = 1961}}
  | date = 1989
, sec. 2C</ref>
| isbn = 0-387-50704-3
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>
}}, 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{{efn|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.{{cn|date=May 2026}}
 
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'' 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.<ref>''Astronomical Almanac 2010'', p. E14</ref>


== Obliquity of the ecliptic {{anchor|Obliquity}}==
== Change of inclination {{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>
The obliquity of the ecliptic of 23.4° is currently decreasing 0.013 degrees (47 arcseconds) per hundred years because of planetary perturbations.<ref>
{{cite book  
{{cite book  
  | last = Chauvenet
  | last = Chauvenet
Line 61: Line 116:
}}, art. 365–367, p. 694–695, at Google books</ref>
}}, 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.
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.<ref name="laskar"/>
 
[[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:
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:
Line 121: Line 174:
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==Plane of the Solar System==
===Precession===
{{main|Solar System}}
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''.
[[File:Motion of Sun, Earth and Moon around the Milky Way.jpg|thumb|The general motion and orientation of the Sun, with Earth and the Moon as its Solar System satellites]]
{| 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>
Likewise, the orientation of [[Earth's rotation|Earth's axis]] and equator are not fixed in space, but rotate about the [[poles of the ecliptic]] with a period of about 26,000 years, a process known as ''lunisolar [[precession]]'' or [[axial precession]], as it is due mostly to the gravitational effect of the [[Moon]] and Sun on [[Figure of the Earth|Earth's equatorial bulge]].


{{clear}}
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>


==Celestial reference plane==
===Nutation===
{{main|Celestial equator|Ecliptic coordinate system}}
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>
 
{{Gallery
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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  
{{cite book  
  | last = Montenbruck
  | author = U.S. Naval Observatory Nautical Almanac Office
  | first = Oliver
  |author2=H.M. Nautical Almanac Office
  | title = Practical Ephemeris Calculations
  | title = Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac
  | publisher = Springer-Verlag
  | publisher = H.M. Stationery Office, London
  | date = 1989
  | date = 1961}}
| isbn = 0-387-50704-3
, 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>
}}, 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==
==Equinoxes and solstices==
Line 259: Line 222:


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>
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>
==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>


==In the constellations==
==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.]]
[[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:
 
The ecliptic currently passes through the following thirteen [[constellation]]s (the [[zodiac]], except [[Ophiuchus]]):<ref name=Shapiro>{{cite web |url=https://www.ips-planetarium.org/page/a_shapiro1977 |publisher=International Planetarium Society |date=1977 |first=Lee |last=Shapiro |title=The Real Constellations of the Zodiac |access-date=2 May 2026 |archive-date=20 September 2018 |archive-url=https://web.archive.org/web/20180920162613/https://www.ips-planetarium.org/page/a_shapiro1977 |url-status=live}}</ref>
{{columns-list|colwidth=18em|
{{columns-list|colwidth=18em|
*[[Pisces (constellation)|Pisces]]
*[[Pisces (constellation)|Pisces]]
Line 287: Line 267:
}}
}}


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>
There are an additional twelve constellations that are not on the ecliptic, but are close enough that the Moon and planets can occasionally appear in some of 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=1999 |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]]
*[[Cetus (constellation)|Cetus]]
Line 301: Line 281:
*[[Auriga (constellation)|Auriga]]
*[[Auriga (constellation)|Auriga]]
*[[Orion (constellation)|Orion]]
*[[Orion (constellation)|Orion]]
Venus is the only planet that occasionally passes through all 25 of these constellations.<ref name=mosley/>


==Astrology==
==Astrology==
Line 316: Line 298:
* [[Protoplanetary disk]]
* [[Protoplanetary disk]]
* [[Celestial coordinate system]]
* [[Celestial coordinate system]]
* [[Analemma]]


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


Latest revision as of 05:51, 14 May 2026

File:Ecliptic with earth and sun animation.gif
As seen from the orbiting Earth, the Sun 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 365 days.

The ecliptic or ecliptic plane is the apparent path of the Sun on the celestial sphere, resulting from Earth's orbit around the Sun.[1][2][lower-alpha 1] It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. The ecliptic is so named because the ancients noted that eclipses only occur when the Moon is crossing it.[3]

Sun's path can be traced against the background of stars, especially the Zodiac constellations.[4] The planets of the Solar System can be seen along the ecliptic because their orbital planes are very close to Earth's. The Moon also appears near the plane, with the Moon's orbital plane inclined only 5.1°, intersecting the ecliptic at the lunar nodes.

The ecliptic is an important reference plane and is the basis of the ecliptic coordinate system. Ancient scientists were able to calculate Earth's axial tilt by comparing the angle of the ecliptic (about 23.4°) to that of the equatorial plane.

Sun's apparent motion

The ecliptic is the apparent path of the Sun throughout the course of a year.[5]

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[lower-alpha 2] 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 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.[6] The variation of orbital speed accounts for part of the equation of time.[7]

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 one month. Because of further perturbations by the other planets of the Solar System, the Earth–Moon barycenter wobbles slightly around a mean position in a complex fashion.[citation needed]

Inclination to the plane of the Solar System

Main article: Solar System
File:The Mysterious Case of the Disappearing Dust.jpg
A depiction of the early Solar System's protoplanetary disk from which Earth and other Solar System bodies were formed

Most of the major bodies of the Solar System orbit the Sun in nearly the same plane. This is thought to be due to the planets being formed from a protoplanetary disk, with all the dust and gas orbiting the sun close to a single plane.[8] The invariable plane of the Solar System is the plane passing through its center of mass perpendicular to its angular momentum vector.[9] 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°.[10] 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 orbital and rotational angular momenta of all the bodies of the system; more than 60% of the total comes from the orbit of Jupiter.[10] 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.[11][12]

File:Ecliptic plane top view.gif File:Ecliptic plane side view.gif
Top and side views of the plane of the ecliptic, showing planets Mercury, Venus, Earth, and Mars. Most of the planets orbit the Sun very nearly in the same plane in which Earth orbits, the ecliptic.

Template:Solar system inclinations

Inclination to the galactic plane

The ecliptic is inclined to the galactic plane by 60°.[13]

File:Meteor shower in the Chilean Desert (annotated) (potw2227b).jpg
A dark sky view, showing the difference in inclination of the galactic plane of the Milky Way to the not much inclined to each other orbital planes of the Solar System planets incl. the ecliptic, which is illuminated by the outlined zodiacal light and flanked by a meteor shower
File:Motion of Earth & Sun Around the Milky Way 07Jul2022 credits & captions - moon 5 deg incl to ecliptic 9.jpg
The general motion and orientation of the Sun, with Earth and the Moon as its Solar System satellites

Referencing Earth's equator

Further information: Axial tilt
File:Earths orbit and ecliptic.svg
The ecliptic plane (gray) projected outward to the celestial sphere. The 23.44° tilt of Earth's axis defines its celestial equator (red). The equinoxes (red dots) occur biannually, when the Sun appears directly above the equator.

Because Earth's rotational axis is not perpendicular to its orbital plane, Earth's equatorial plane is not coplanar with the ecliptic plane, but is inclined to it by an angle of about 23.4°, which is known as the obliquity of the ecliptic.[14] If the equator is projected outward to the celestial sphere, forming the celestial equator, it crosses the ecliptic at two points known as the equinoxes. 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.[lower-alpha 2] The crossing from south to north is known as the March equinox, also known as the first point of Aries and the ascending node of the ecliptic on the celestial equator. The crossing from north to south is the September equinox or descending node.[15]

Celestial reference plane

Main articles: Celestial equator and Ecliptic coordinate system
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 coordinates appear in (red). The celestial equator (blue) and the equatorial coordinates (blue), being inclined to the ecliptic, appear to wobble as the Sun advances.

The ecliptic forms one of the two fundamental planes used as reference for positions on the celestial sphere, the other being the celestial equator. Perpendicular to the ecliptic are the ecliptic poles, 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.[16]

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[lower-alpha 2] 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 units are used, and for objects near Earth, Earth radii or kilometers are used. A corresponding right-handed 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.[17]

Summary of notation for ecliptic coordinates[18]
Spherical Rectangular
Longitude Latitude Distance
Geocentric λ β Δ
Heliocentric l b r x, y, z[note 1]
  1. ↑ Occasional use; x, y, z are usually reserved for equatorial coordinates.

Ecliptic coordinates are convenient for specifying positions of Solar System objects, as most of the planets' orbits have small 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.[citation needed]

Because of the 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; the coordinates are referred to the direction of the equinox at that date. For instance, the Astronomical Almanac lists the 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 as above, without the addition of nutation.[19]

Change of inclination

The obliquity of the ecliptic of 23.4° is currently decreasing 0.013 degrees (47 arcseconds) per hundred years because of planetary perturbations.[20]

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 ephemerides as the accuracy of observation improves and as the understanding of the dynamics increases, and from these ephemerides various astronomical values, including the obliquity, are derived.[21]

Until 1983 the obliquity for any date was calculated from work of Newcomb, who analyzed positions of the planets until about 1895:

ε = 23°27′08.26″ − 46.845″ T − 0.0059″ T2 + 0.00181″ T3

where ε is the obliquity and T is tropical centuries from B1900.0 to the date in question.[22]

From 1984, the 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:

ε = 23°26′21.45″ − 46.815″ T − 0.0006″ T2 + 0.00181″ T3

where hereafter T is Julian centuries from J2000.0.[23]

JPL's fundamental ephemerides have been continually updated. The Astronomical Almanac for 2010 specifies:[24]

ε = 23°26′21.406″ − 46.836769″ T − 0.0001831″ T2 + 0.00200340″ T3 − 0.576×10−6″ T4 − 4.34×10−8″ T5

These expressions for the obliquity are intended for high precision over a relatively short time span, perhaps several centuries.[25] J. Laskar computed an expression to order T10 good to 0.04″/1000 years over 10,000 years.[21]

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.[26]

Precession

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.

Likewise, the orientation of Earth's axis and equator are not fixed in space, but rotate about the poles of the ecliptic with a period of about 26,000 years, a process known as lunisolar precession or axial precession, as it is due mostly to the gravitational effect of the Moon and Sun on Earth's equatorial bulge.

The combined action of these two motions is called general precession, and changes the position of the equinoxes by about 50 arc seconds (about 0.014°) per year.[27]

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 nutation.[28] 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.[29]

Equinoxes and solstices

File:Optical effect march sunset - NOAA.jpg
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.
Positions of equinoxes and solstices
  ecliptic equatorial
longitude right ascension
March equinox 0° 0h
June solstice 90° 6h
September equinox 180° 12h
December solstice 270° 18h
Main article: Equinox (celestial coordinates)

The exact instants of equinoxes and solstices are the times when the apparent ecliptic longitude (including the effects of aberration and nutation) of the Sun is 0°, 90°, 180°, and 270°. Because of perturbations of Earth's orbit and anomalies of the calendar, the dates of these are not fixed.[30]

Eclipses

Main article: Eclipse
File:Eclipse vs new or full moons, annotated.svg
As the Earth revolves around the Sun, approximate axial parallelism of the Moon's orbital plane (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, eclipses always occur on or near it. Because of the inclination of the Moon's orbit, eclipses do not occur at every conjunction and opposition of the Sun and Moon, but only when the Moon is near an ascending or descending node at the same time it is at conjunction (new) or opposition (full). The ecliptic is so named because the ancients noted that eclipses only occur when the Moon is crossing it.[3]

In the constellations

File:Constellations, equirectangular plot, Menzel families.svg
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 constellations (the zodiac, except Ophiuchus):[31]

There are an additional twelve constellations that are not on the ecliptic, but are close enough that the Moon and planets can occasionally appear in some of them.[33][34]

Venus is the only planet that occasionally passes through all 25 of these constellations.[34]

Astrology

Main article: 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.[35] Traditionally, this region is divided into 12 signs of 30° longitude, each of which approximates the Sun's motion in one month.[36] In ancient times, the signs corresponded roughly to 12 of the constellations that straddle the ecliptic.[37] 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; it has since moved into Pisces because of precession of the equinoxes.[38]

See also

Notes

  1. ↑ Strictly, the plane of the mean orbit, with minor variations averaged out.
  2. ↑ 2.0 2.1 2.2 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.

References

  1. ↑ USNO Nautical Almanac Office; UK Hydrographic Office, HM Nautical Almanac Office (2008). The Astronomical Almanac for the Year 2010. GPO. p. M5. ISBN 978-0-7077-4082-9.
  2. ↑ "LEVEL 5 Lexicon and Glossary of Terms".
  3. ↑ 3.0 3.1 Ball, Robert S. (1908). A Treatise on Spherical Astronomy. Cambridge University Press. p. 83.
  4. ↑ "The Ecliptic: the Sun's Annual Path on the Celestial Sphere".
  5. ↑ U.S. Naval Observatory Nautical Almanac Office (1992). P. Kenneth Seidelmann (ed.). Explanatory Supplement to the Astronomical Almanac. University Science Books, Mill Valley, CA. ISBN 0-935702-68-7., p. 11
  6. ↑ Astronomical Almanac 2010, sec. C
  7. ↑ Explanatory Supplement (1992), sec. 1.233
  8. ↑ Armitage, Philip J. (2011). "Dynamics of Protoplanetary Disks". Annual Review of Astronomy and Astrophysics. 49 (1): 195–236. arXiv:1011.1496. Bibcode:2011ARA&A..49..195A. doi:10.1146/annurev-astro-081710-102521. S2CID 55900935.
  9. ↑ Souami, D.; Souchay, J. (1 July 2012). "The solar system's invariable plane". Astronomy & Astrophysics. 543: A133. doi:10.1051/0004-6361/201219011.
  10. ↑ 10.0 10.1 Cite error: Invalid <ref> tag; no text was provided for refs named meanplane
  11. ↑ Danby, J.M.A. (1988). Fundamentals of Celestial Mechanics. Willmann-Bell, Inc., Richmond, VA. section 9.1. ISBN 0-943396-20-4.
  12. ↑ Roy, A.E. (1988). Orbital Motion (third ed.). Institute of Physics Publishing. section 5.3. ISBN 0-85274-229-0.
  13. ↑ "Galactic Plane". Swinburne. Retrieved 9 May 2026.
  14. ↑ Explanatory Supplement (1992), p. 733
  15. ↑ Astronomical Almanac 2010, p. M2 and M6
  16. ↑ Montenbruck, Oliver (1989). Practical Ephemeris Calculations. Springer-Verlag. ISBN 0-387-50704-3., sec 1.4
  17. ↑ Explanatory Supplement (1961), sec. 2A
  18. ↑ Explanatory Supplement (1961), sec. 1G
  19. ↑ Astronomical Almanac 2010, p. E14
  20. ↑ Chauvenet, William (1906). A Manual of Spherical and Practical Astronomy. I. J.B. Lippincott Co., Philadelphia., art. 365–367, p. 694–695, at Google books
  21. ↑ 21.0 21.1 Laskar, J. (1986). "Secular Terms of Classical Planetary Theories Using the Results of General Relativity". Astronomy and Astrophysics. 157 (1): 59. Bibcode:1986A&A...157...59L., table 8, at SAO/NASA ADS
  22. ↑ Explanatory Supplement (1961), sec. 2B
  23. ↑ U.S. Naval Observatory, Nautical Almanac Office; H.M. Nautical Almanac Office (1989). The Astronomical Almanac for the Year 1990. U.S. Govt. Printing Office. ISBN 0-11-886934-5., p. B18
  24. ↑ Astronomical Almanac 2010, p. B52
  25. ↑ Newcomb, Simon (1906). A Compendium of Spherical Astronomy. MacMillan Co., New York., p. 226–227, at Google books
  26. ↑ Meeus, Jean (1991). Astronomical Algorithms. Willmann-Bell, Inc., Richmond, VA. ISBN 0-943396-35-2., chap. 21
  27. ↑ Explanatory Supplement (1992), sec. 1.322 and 3.21
  28. ↑ U.S. Naval Observatory Nautical Almanac Office; H.M. Nautical Almanac Office (1961). Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac. H.M. Stationery Office, London. , sec. 2C
  29. ↑ Explanatory Supplement (1992), p. 731 and 737
  30. ↑ Meeus (1991), chap. 26
  31. ↑ Shapiro, Lee (1977). "The Real Constellations of the Zodiac". International Planetarium Society. Archived from the original on 20 September 2018. Retrieved 2 May 2026.
  32. ↑ Serviss, Garrett P. (1908). Astronomy With the Naked Eye. Harper & Brothers, New York and London. pp. 105, 106.
  33. ↑ Kidger, Mark (2005). Astronomical Enigmas: Life on Mars, the Star of Bethlehem, and Other Milky Way Mysteries. The Johns Hopkins University Press. pp. 38–39. ISBN 9780801880261.
  34. ↑ 34.0 34.1 Mosley, John (1999). "The Real, Real Constellations of the Zodiac". International Planetarium Society. Archived from the original on 1 July 2017. Retrieved 21 March 2017.
  35. ↑ Bryant, Walter W. (1907). A History of Astronomy. Forgotten Books. p. 3. ISBN 9781440057922.
  36. ↑ Bryant (1907), p. 4.
  37. ↑ See, for instance, Leo, Alan (1899). Astrology for All. L.N. Fowler & Company. p. 8. astrology.
  38. ↑ Vallado, David A. (2001). Fundamentals of Astrodynamics and Applications (2nd ed.). El Segundo, CA: Microcosm Press. p. 153. ISBN 1-881883-12-4.

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