Theecliptic orecliptic plane is theorbital plane ofEarth around the Sun.[1][2][a] 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 thecelestial sphere over the course of a year traces out a path along the ecliptic against thebackground of stars – specifically theZodiac constellations.[3] The planets of theSolar 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 thateclipses only occur when the Moon is crossing it.[4]
The ecliptic is the apparent path of the Sun throughout the course of ayear.[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[6] 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-minutesidereal 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 thecelestial equator for about 185 days of each year, and south of it for about 180 days.[7] The variation of orbital speed accounts for part of theequation of time.[8]
Because of the movement of Earth around the Earth–Mooncenter of mass, the apparent path of the Sun wobbles slightly, with a period of aboutone month. Because of furtherperturbations by the otherplanets of the Solar System, the Earth–Moonbarycenter wobbles slightly around a mean position in a complex fashion.
Theplane ofEarth'sorbit projected in all directions forms the reference plane known as the ecliptic. Here, it is shown projected outward (gray) to thecelestial sphere, along with Earth'sequator andpolar axis (green). The plane of the ecliptic intersects the celestial sphere along agreat 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 theequinoxes (red), where the Sun seems to cross the celestial equator.
The orientation ofEarth's axis and equator are not fixed in space, but rotate about thepoles of the ecliptic with a period of about 26,000 years, a process known aslunisolarprecession, as it is due mostly to the gravitational effect of theMoon andSun onEarth'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 asplanetary precession. The combined action of these two motions is calledgeneral precession, and changes the position of the equinoxes by about 50arc seconds (about 0.014°) per year.[11]
Once again, this is a simplification. Periodic motions of theMoon and apparent periodic motions of theSun (actually of Earth in its orbit) cause short-term small-amplitude periodic oscillations of Earth's axis, and hence the celestial equator, known asnutation.[12]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 thetrue equator and equinox; the positions without nutation are themean equator and equinox.[13]
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.[14]
The angular value of the obliquity is found by observation of the motions of Earth and other planets over many years. Astronomers produce newfundamental ephemerides as the accuracy ofobservation improves and as the understanding of thedynamics increases, and from these ephemerides various astronomical values, including the obliquity, are derived.
Obliquity of the ecliptic for 20,000 years, from Laskar (1986).[15] 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 fromwork of Newcomb, who analyzed positions of the planets until about 1895:
From 1984, theJet Propulsion Laboratory's DE series of computer-generated ephemerides took over as the fundamental ephemeris of theAstronomical Almanac. Obliquity based on DE200, which analyzed observations from 1911 to 1979, was calculated:
These expressions for the obliquity are intended for high precision over a relatively short time span, perhaps several centuries.[19] J. Laskar computed an expression to orderT10 good to0.04″/1000 years over 10,000 years.[15]
All of these expressions are for themean obliquity, that is, without the nutation of the equator included. Thetrue or instantaneous obliquity includes the nutation.[20]
The general motion and orientation of the Sun, with Earth and the Moon as its Solar System satellites
Top and side views of the plane of the ecliptic, showing planetsMercury,Venus,Earth, andMars. Most of the planets orbit theSun very nearly in the same plane in which Earth orbits, the ecliptic.
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 aprotoplanetary disk. Probably the closest current representation of the disk is known as theinvariable 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 theangular momentum of the entire Solar System, essentially the vector sum of all of theorbital androtational angular momenta of all the bodies of the system; more than 60% of the total comes from the orbit of Jupiter.[21] 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.[22][23]
Inclination of the ecliptic over 200,000 years, from Dziobek (1892).[24] This is the inclination to the ecliptic of 101,800 CE. Note that the ecliptic rotates by only about 7° during this time, whereas thecelestial equator makes several complete cycles around the ecliptic. The ecliptic is a relatively stable reference compared to the celestial equator.
The ecliptic forms one of the two fundamentalplanes used as reference for positions on the celestial sphere, the other being thecelestial equator. Perpendicular to the ecliptic are theecliptic 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 planetaryprecession being roughly 1/100 that of the celestial equator.[25]
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[6] 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 nearEarth,Earth radii orkilometers are used. A corresponding right-handedrectangular coordinate system is also used occasionally; thex-axis is directed toward the March equinox, they-axis 90° to the east, and thez-axis toward the north ecliptic pole; the astronomical unit is the unit of measure. Symbols for ecliptic coordinates are somewhat standardized; see the table.[26]
Ecliptic coordinates are convenient for specifying positions of Solar System objects, as most of the planets' orbits have smallinclinations 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 theprecessional 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 anepoch; the coordinates are referred to the direction of the equinox at that date. For instance, theAstronomical Almanac[28] lists theheliocentric position ofMars at 0hTerrestrial 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 themean equinox of 4 January 2010 0h TTas above, without the addition of nutation.
As the Earth revolves around the Sun, approximateaxial parallelism of the Moon's orbital plane (tilted five degrees to the ecliptic) results in the revolution of thelunar nodes relative to the Earth. This causes aneclipse season approximately every six months, in which asolar eclipse can occur at thenew moon phase and alunar eclipse can occur at thefull moon phase.
Because theorbit 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 everyconjunction andopposition of the Sun and Moon, but only when the Moon is near anascending 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.[4]
AtEarth's poles the Sun appears at the horizon only and all day aroundequinox, marking the change between the half year longpolar night andpolar day. The picture shows theSouth Pole right before March equinox, with the Sun appearing throughrefraction despite being still below the horizon.
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 thirteenconstellations:
The ecliptic forms the center of thezodiac, a celestial belt about 20° wide in latitude through which the Sun, Moon, and planets always appear to move.[33]Traditionally, this region is divided into 12signs of 30° longitude, each of which approximates the Sun's motion in one month.[34] In ancient times, the signs corresponded roughly to 12 of the constellations that straddle the ecliptic.[35]These signs are sometimes still used in modern terminology. The "First Point of Aries" was named when theMarch equinox Sun was actually in the constellationAries; it has since moved intoPisces because ofprecession of the equinoxes.[36]
^U.S. Naval Observatory Nautical Almanac Office (1992). P. Kenneth Seidelmann (ed.).Explanatory Supplement to the Astronomical Almanac. University Science Books, Mill Valley, CA.ISBN0-935702-68-7., p. 11
^abcThe directionsnorth andsouth on the celestial sphere are in the sensetoward the northcelestial pole andtoward the south celestial pole.East isthe direction toward which Earth rotates,west is opposite that.
^Explanatory Supplement (1992), sec. 1.322 and 3.21
^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
^abLaskar, 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
^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.ISBN0-11-886934-5., p. B18
^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.ISBN9780801880261.
