Ecliptic - Wikiparency

# Ecliptic  **Microsim — three.js (Wikitube framework):** *The orbits (Solar System explorer)* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?view=orbits&embed=1" data-title="The orbits (Solar System explorer)"></div> *The Solar System explorer locked on this article's state (`?view=orbits`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.*  *Try: drag the scene until you look along the plane edge-on, and the eight planetary orbits close up into an almost single line, which is the ecliptic plane seen from the side; keep that view and pick out Pluto's orbit crossing it at about 17° and Eris's at about 44°; then set the speed to 1 year/s and follow Earth around one full lap, the motion that makes the Sun appear to travel once around the ecliptic each year.* The **ecliptic** is the great circle that the [[Sun]] appears to trace across the sky over a year, and the **ecliptic plane** is the plane of [[Earth]]'s [[Orbit|orbit]] that projects onto the sky as that circle.[^aa2010][^ned-glossary] Because the Sun's yearly path runs through a fixed band of background stars, the ecliptic became the frame for early astronomy, calendars and astrology, and its name records an ancient observation: eclipses happen only when the [[Moon]] is on or very near this line.[^ball1908] The other planets stay close to the ecliptic in the sky because their orbits lie within a few degrees of Earth's, and the Moon's orbit is tilted to it by only about 5°.[^nasa-moon] Earth's equator, by contrast, is tilted about 23.4° to the ecliptic, the obliquity that produces the seasons and that fixes the equinoxes and solstices where the two circles cross or diverge most.[^expsupp1992] The obliquity slowly changes, the equator precesses around the ecliptic's pole, and the ecliptic itself drifts, so every precise position quoted against it names a date. The explorer at the top of this page shows the ecliptic as the reference plane of the whole scene: the planetary orbits nearly coincide with it, while [[Pluto]], [[Eris_(dwarf_planet)|Eris]] and [[Halley's_Comet|Halley's Comet]] visibly depart from it. ## Sun's apparent motion Seen from [[Earth]], the [[Sun]] moves steadily eastward against the stars and completes one circuit of the ecliptic in a year, because that is how long Earth takes to go once around its orbit.[^expsupp1992] A full circle of 360° shared among about 365.25 days gives just under 1° a day (0.986°, derived). That daily shift has a direct consequence for timekeeping: after one rotation relative to the stars, a sidereal day of about 23 h 56 min, Earth must turn about another degree, roughly four more minutes, before the Sun returns to the same meridian, which is why the solar day is 24 hours (derived).[^nasa-earth] The Sun's pace along the ecliptic is not uniform, because [[Kepler's_laws_of_planetary_motion|Kepler's second law]] makes Earth move faster near perihelion than near aphelion, at about 30.3 against 29.3 km/s.[^nasa-earth] As a result the Sun spends about 185 days a year north of the celestial equator and only about 180 south of it.[^aa2010-c] The same unevenness supplies part of the equation of time, the difference between sundial time and clock time that accumulates through the year.[^expsupp1992-eot] Strictly, the point that follows the ecliptic is the barycentre of the Earth–[[Moon]] pair, not the centre of Earth. Earth circles that barycentre once a month, so the Sun's apparent path, seen from Earth's centre, wobbles about the ecliptic with a monthly period, and the other planets add their own small, irregular displacements of the barycentre from a mean path. In the explorer these small motions are invisible: at the scale of the planetary orbits, Earth's circuit and the plane it defines appear as a single smooth ring.[^jpl-t1] ## Inclination to the plane of the Solar System The planets orbit close to one plane because they formed from a flattened [[Protoplanetary_disk|protoplanetary disk]], in which gas and dust circled the young Sun in nearly the same plane.[^armitage2011] That shared plane has a precise dynamical definition: the invariable plane passes through the Solar System's centre of mass perpendicular to its total [[Angular_momentum|angular momentum]].[^souami2012] Against it, Earth's orbit, and so the ecliptic, is tilted by about 1.6°, [[Jupiter]]'s by about 0.3°, and [[Mercury_(planet)|Mercury]]'s, the most tilted of the planets, by about 6.3°.[^souami2012] The planets therefore crowd close to the ecliptic in Earth's sky. The total angular momentum is dominated by the orbital motion of Jupiter, so the orientation of the invariable plane depends on the masses and motions of everything in the system, including bodies known only imperfectly; its position therefore carries a measurable uncertainty.[^souami2012] The ecliptic, on the other hand, is pinned down directly by the Sun's observed motion. That is why astronomers use the ecliptic as the reference plane of the Solar System for both precision and convenience, accepting the one drawback that over geological times the ecliptic moves slowly against the distant stars.[^danby1988][^roy1988] The explorer's orbit view makes the point directly. Its planetary orbits, drawn from JPL mean elements referred to the ecliptic, lie almost flat; seen edge-on, the widest excursion among the planets is Mercury's 7°.[^jpl-t1] The exceptions are small bodies: Pluto is inclined about 17° and Eris about 44°, while Halley's Comet, at about 162°, goes around the Sun in the direction opposite to the planets.[^jpl-sbdb] ### Inclination to the galactic plane The ecliptic is tilted by about 60° to the plane of the [[Milky_Way|Milky Way]], so the Solar System's plane cuts steeply across the band of the Galaxy on the sky.[^swinburne] ## Referencing Earth's equator [[Earth]]'s rotation axis is not perpendicular to its [[Orbit|orbit]], so the equator is tilted to the ecliptic by about 23.4°, an angle called the obliquity of the ecliptic.[^expsupp1992-733] The equator projected onto the sky is the celestial equator, and it crosses the ecliptic at two opposite points, the equinoxes. The Sun passes northward over the celestial equator at the March equinox, which is also called the first point of Aries and is the ascending node of the ecliptic on the equator; it passes southward at the September equinox.[^aa2010-m] ### Celestial reference plane The ecliptic and the celestial equator are the two fundamental reference circles on the sky. The ecliptic is the steadier of the two: its planetary precession is roughly a hundredth of the motion of the equator.[^montenbruck1989] Its poles lie 90° from it, and the north ecliptic pole is the one on the northern side of the equator. Ecliptic longitude and latitude locate a body relative to this circle. Longitude runs eastward from the March equinox, in the direction of the Sun's yearly motion, from 0° to 360°; latitude runs from 0° on the ecliptic to +90° at the north ecliptic pole and −90° at the south. A distance completes the position: astronomical units within the Solar System, Earth radii or kilometres for nearby objects. A rectangular frame is sometimes used as well, with x toward the March equinox, y 90° to the east and z toward the north ecliptic pole.[^expsupp1961-2a] Geocentric coordinates are conventionally written λ, β and Δ; heliocentric ones l, b and r.[^expsupp1961-1g] These coordinates suit [[Planetary_system|planetary]] work because most orbits have small inclinations and so keep near zero latitude. They are not fixed, however: the equinox moves, so every position must name the equinox and ecliptic of a stated date. The *Astronomical Almanac* for 2010, for instance, gives [[Mars]] at 0h TT on 4 January 2010 a heliocentric longitude of 118°09′15.8″, latitude +1°43′16.7″ and distance 1.6302454 [[Astronomical_unit|AU]], referred to the mean equinox and ecliptic of that date.[^aa2010-e14] ## Change of inclination The obliquity is now about 23.4° and is decreasing by about 47″ per century, a change driven by the other planets' pull on [[Earth]]'s orbit.[^chauvenet1906] Its value is not set by theory alone: it is extracted from long series of observations of Earth and the planets, and each new fundamental ephemeris yields a slightly revised expression.[^laskar1986] Until 1983 the obliquity was computed from Simon Newcomb's analysis of planetary positions up to about 1895: ε = 23°27′08.26″ − 46.845″ T − 0.0059″ T² + 0.00181″ T³, with T in tropical centuries from B1900.0.[^expsupp1961-2b] From 1984 the [[NASA]] Jet Propulsion Laboratory's numerically integrated ephemerides replaced it, and the value based on DE200, fitted to observations from 1911 to 1979, is ε = 23°26′21.45″ − 46.815″ T − 0.0006″ T² + 0.00181″ T³, with T in Julian centuries from J2000.0.[^aa1990] The 2010 almanac adds terms to T⁵, starting 23°26′21.406″ − 46.836769″ T.[^aa2010-b52] Evaluated for 2026, T ≈ 0.26 and ε ≈ 23°26′09″, about 23.436° (derived). Such polynomials are meant for a few centuries either side of their epoch.[^newcomb1906] Jacques Laskar computed a series to T¹⁰ that stays within 0.04″ per thousand years over 10,000 years.[^laskar1986] All of these give the mean obliquity; the true, instantaneous obliquity also includes nutation.[^meeus1991] ### Precession Precession has two parts. The pulls of the other planets slowly turn the plane of Earth's orbit, moving the ecliptic itself; this is planetary precession. Meanwhile the Moon's and Sun's [[Gravity|gravitational]] pull on Earth's equatorial bulge swings Earth's axis, and the equator with it, around the ecliptic pole in about 26,000 years: lunisolar precession. Together, as general precession, they carry the equinoxes westward along the ecliptic by about 50″, some 0.014°, a year.[^expsupp1992-prec] ### Nutation On top of the steady precession, the periodic motions of the Moon and the apparent motion of the Sun make Earth's axis nod in small, short-period oscillations called nutation.[^expsupp1961] Positions that include both precession and nutation are referred to the true equator and equinox; those that include precession alone use the mean equator and equinox.[^expsupp1992-731] ## Equinoxes and solstices The equinoxes and solstices are defined by the Sun's position on the ecliptic. They are the instants when the Sun's apparent ecliptic longitude, including aberration and nutation, is 0°, 90°, 180° and 270°; the corresponding right ascensions are 0h, 6h, 12h and 18h.[^meeus1991-26] | Event | Ecliptic longitude | Right ascension | |---|---|---| | March equinox | 0° | 0h | | June solstice | 90° | 6h | | September equinox | 180° | 12h | | December solstice | 270° | 18h | At the solstices the Sun reaches its greatest distance from the celestial equator, a declination equal to the obliquity, about ±23.4° (derived). Because Earth's orbit is perturbed and the Gregorian calendar only approximates the tropical year of about 365.242 days, the calendar dates and times of these events shift from year to year.[^meeus1991-26][^nasa-earth] The geometry of the seasons follows from the same numbers. At the two equinoxes the Sun stands on the celestial equator, so it rises almost due east and sets almost due west everywhere on [[Earth]], and day and night are close to equal in length. At the June solstice the Sun is overhead at noon at latitude 23.4° N, and the northern hemisphere receives its longest days; at the December solstice the same happens at 23.4° S (derived from the obliquity).[^nasa-seasons] The seasons come from this tilt, not from Earth's changing distance from the [[Sun]]: perihelion falls in the northern winter.[^nasa-seasons] ## Eclipses The [[Moon]]'s orbit is tilted to the ecliptic by about 5.145°, and it crosses the ecliptic at two points, the ascending and descending nodes.[^nasa-moon] A solar eclipse requires the Moon to pass between the Sun and Earth at new moon; a lunar eclipse requires it to pass into Earth's shadow at full moon. Because of the tilt, most new and full moons pass above or below the Sun or the shadow, and an eclipse occurs only when a new or full moon falls near a node. Since the [[Sun]] is always on the ecliptic, eclipses happen on or near that circle, which is the origin of its name.[^ball1908] The Sun, moving along the ecliptic, passes each of the two nodes once a year, so eclipses cluster in two eclipse seasons that fall roughly six months apart (derived). A 5° tilt, at the Moon's mean distance of about 384,400 km, amounts to a displacement of about 34,000 km from the ecliptic plane at maximum, several times Earth's radius, which is why eclipses need the nodal alignment (derived).[^nasa-moon] ## In the constellations With the modern constellation boundaries, the ecliptic runs through thirteen constellations: the twelve traditional zodiac constellations and Ophiuchus, which the Sun crosses in late November and early December.[^shapiro1977] Because the [[Moon]] and planets can stray several degrees from the ecliptic, they also appear in twelve further constellations that the ecliptic does not touch, such as Cetus, Orion, Auriga, Hydra and Sextans.[^kidger2005][^mosley1999] [[Venus]], whose orbit is inclined 3.4° and which comes close to Earth, can reach latitudes large enough to visit all 25 of them.[^mosley1999][^nasa-ven] ## Astrology The zodiac of astrology is a band about 20° wide centred on the ecliptic, inside which the [[Sun]], the [[Moon]] and the planets always appear.[^bryant1907] It is divided into twelve signs of 30° of longitude, each roughly the Sun's motion in a month, so that the sign of a date simply encodes the Sun's ecliptic longitude.[^bryant1907-4] When the system was set up in antiquity, the signs corresponded approximately to twelve constellations lying across the ecliptic.[^leo1899] The names survive in astronomical usage: the March equinox is still called the first point of Aries, though precession has since carried it into Pisces.[^vallado2001] ## See also - [[Formation_and_evolution_of_the_Solar_System]] - [[Protoplanetary_disk]] - [[Orbit]] - [[Zodiacal_light]] - Invariable plane · Celestial coordinate system · Analemma ## Notes Ecliptic coordinates of date change continuously with precession and nutation; values quoted here without an epoch are rounded mean values near J2000.0. ## References [^aa2010]: U.S. Naval Observatory Nautical Almanac Office; HM Nautical Almanac Office (2008). *The Astronomical Almanac for the Year 2010*. U.S. Government Printing Office, p. M5. ISBN 978-0-7077-4082-9. [^ned-glossary]: NASA/IPAC Extragalactic Database. "Level 5 lexicon and glossary of terms: E". https://ned.ipac.caltech.edu/level5/Glossary/Glossary_E.html [^ball1908]: Ball, R. S. (1908). *A Treatise on Spherical Astronomy*. Cambridge University Press, p. 83. https://archive.org/details/atreatiseonsphe00ballgoog [^nasa-moon]: Williams, D. R. "Moon Fact Sheet". NASA Goddard Space Flight Center (fetched 2026-09-18). https://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html [^expsupp1992]: Seidelmann, P. K. (ed.) (1992). *Explanatory Supplement to the Astronomical Almanac*. University Science Books. ISBN 0-935702-68-7. [^nasa-earth]: Williams, D. R. "Earth Fact Sheet". NASA Goddard Space Flight Center (fetched 2026-09-18). https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html [^aa2010-c]: *The Astronomical Almanac for the Year 2010* (2008), section C. [^expsupp1992-eot]: *Explanatory Supplement to the Astronomical Almanac* (1992), sec. 1.233. [^jpl-t1]: JPL Solar System Dynamics. "Approximate Positions of the Planets", Table 1. https://ssd.jpl.nasa.gov/planets/approx_pos.html [^armitage2011]: Armitage, P. J. (2011). "Dynamics of protoplanetary disks". *Annual Review of Astronomy and Astrophysics* 49: 195–236. https://doi.org/10.1146/annurev-astro-081710-102521 [^souami2012]: Souami, D.; Souchay, J. (2012). "The solar system's invariable plane". *Astronomy & Astrophysics* 543: A133. https://doi.org/10.1051/0004-6361/201219011 [^danby1988]: Danby, J. M. A. (1988). *Fundamentals of Celestial Mechanics*. Willmann-Bell. ISBN 0-943396-20-4. [^roy1988]: Roy, A. E. (1988). *Orbital Motion*. Institute of Physics Publishing. ISBN 0-85274-229-0. [^jpl-sbdb]: JPL Small-Body Database (elements fetched 2026-09-18): 134340 Pluto, 136199 Eris, 1P/Halley. https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html [^swinburne]: Swinburne University of Technology. "Galactic plane". *COSMOS – The SAO Encyclopedia of Astronomy*. https://astronomy.swin.edu.au/cosmos/G/Galactic+Plane [^nasa-seasons]: NASA Space Place. "What causes the seasons?". https://spaceplace.nasa.gov/seasons/ [^expsupp1992-733]: *Explanatory Supplement to the Astronomical Almanac* (1992), p. 733. [^aa2010-m]: *The Astronomical Almanac for the Year 2010* (2008), pp. M2, M6. [^montenbruck1989]: Montenbruck, O. (1989). *Practical Ephemeris Calculations*. Springer-Verlag. ISBN 0-387-50704-3. [^expsupp1961-2a]: HM Nautical Almanac Office; U.S. Naval Observatory (1961). *Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac*. HM Stationery Office, sec. 2A. [^expsupp1961-1g]: *Explanatory Supplement to the Astronomical Ephemeris* (1961), sec. 1G. [^aa2010-e14]: *The Astronomical Almanac for the Year 2010* (2008), p. E14. [^chauvenet1906]: Chauvenet, W. (1906). *A Manual of Spherical and Practical Astronomy*, vol. I. J. B. Lippincott. https://books.google.com/books?id=yobvAAAAMAAJ [^laskar1986]: Laskar, J. (1986). "Secular terms of classical planetary theories using the results of general relativity". *Astronomy and Astrophysics* 157: 59–70. Bibcode 1986A&A...157...59L. https://ui.adsabs.harvard.edu/abs/1986A&A...157...59L [^expsupp1961-2b]: *Explanatory Supplement to the Astronomical Ephemeris* (1961), sec. 2B. [^aa1990]: U.S. Naval Observatory Nautical Almanac Office; HM Nautical Almanac Office (1989). *The Astronomical Almanac for the Year 1990*. U.S. Government Printing Office. ISBN 0-11-886934-5. [^aa2010-b52]: *The Astronomical Almanac for the Year 2010* (2008), p. B52. [^newcomb1906]: Newcomb, S. (1906). *A Compendium of Spherical Astronomy*. Macmillan. https://archive.org/details/acompendiumsphe00newcgoog [^meeus1991]: Meeus, J. (1991). *Astronomical Algorithms*. Willmann-Bell. ISBN 0-943396-35-2. [^expsupp1992-prec]: *Explanatory Supplement to the Astronomical Almanac* (1992), secs. 1.322 and 3.21. [^expsupp1961]: *Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac* (1961). HM Stationery Office. [^expsupp1992-731]: *Explanatory Supplement to the Astronomical Almanac* (1992), pp. 731, 737. [^meeus1991-26]: Meeus, J. (1991). *Astronomical Algorithms*, chap. 26. Willmann-Bell. ISBN 0-943396-35-2. [^shapiro1977]: Shapiro, L. (1977). "The real constellations of the zodiac". International Planetarium Society. https://www.ips-planetarium.org/page/a_shapiro1977 [^kidger2005]: Kidger, M. (2005). *Astronomical Enigmas: Life on Mars, the Star of Bethlehem, and Other Milky Way Mysteries*. Johns Hopkins University Press, pp. 38–39. ISBN 978-0-8018-8026-1. [^mosley1999]: Mosley, J. (1999). "The real, real constellations of the zodiac". International Planetarium Society. http://www.ips-planetarium.org/?page=a_mosley1999b [^nasa-ven]: Williams, D. R. "Venus Fact Sheet". NASA Goddard Space Flight Center. https://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html [^bryant1907]: Bryant, W. W. (1907). *A History of Astronomy*, p. 3. ISBN 978-1-4400-5792-2. [^bryant1907-4]: Bryant (1907), p. 4. [^leo1899]: Leo, A. (1899). *Astrology for All*. L. N. Fowler & Co., p. 8. https://archive.org/details/bub_gb_8CwSAAAAYAAJ [^vallado2001]: Vallado, D. A. (2001). *Fundamentals of Astrodynamics and Applications*. Microcosm Press, p. 153. ISBN 1-881883-12-4. ## External links - NASA Space Place: "What causes the seasons?" — https://spaceplace.nasa.gov/seasons/ - U.S. Naval Observatory, Astronomical Applications: equinoxes, solstices and Earth's seasons — https://aa.usno.navy.mil/data/Earth_Seasons - JPL Solar System Dynamics: planetary mean elements referred to the ecliptic — https://ssd.jpl.nasa.gov/planets/approx_pos.html ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Ecliptic) : [Wikitube](https://en.wikitube.io/wiki/Ecliptic) · pinned revision [1372548438](https://en.wikipedia.org/w/index.php?oldid=1372548438) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-042 · explorer state `?view=orbits`.*