Editing Ecliptic

The '''ecliptic''' is the apparent path of the Sun traces out along the sky &mdash; independent of Earth's rotation &mdash; in the course of the year. More accurately, it is the intersection of the [[celestial sphere]] with the '''ecliptic plane''', which is the geometric [[plane (mathematics)|plane]] containing the mean [[orbit]] of the [[Earth]] around the [[Sun]]. It should be distinguished from the [[Invariable_plane|invariable ecliptic plane]], which is the vector sum of the [[angular momentum|angular momenta]] of all planetary orbital planes, to which [[Jupiter (planet)|Jupiter]] is the main contributor.

The name ecliptic is derived from being the place where [[eclipses]] occur.

==Ecliptic and equator==
As the rotation axis of the Earth is not perpendicular to its orbital plane, the [[equatorial plane]] is not parallel to the ecliptic plane, but makes an angle of about 23°44 which is known as the [[obliquity of the ecliptic]]. The intersections of the equatorial and ecliptic plane with the celestial dome are [[great circles]] known as the celestial equator and the ecliptic. The intersection line of the two planes results in two diametrically opposite intersection points, known as the [[equinoxes]]. The equinox which the Sun passes from [[Cardinal directions|south]] to [[Cardinal directions|north]] is known as the [[vernal equinox]] or [[first point of Aries]]. Ecliptic [[longitude]], usually indicated with the letter '''λ''', is measured from this point on 0° to 360° towards the [[Cardinal directions|east]]. Ecliptic [[latitude]], usually indicated with the letter '''β''' is measured +90° to the north or -90° to the south. The same intersection point also defines the origin of the equatorial coordinate system, named [[right ascension]] measured from 0 to 24 hours also to the east and usually indicated with '''α''' or ''R.A.'', and [[declination]], usually indicated with '''δ''' also measured +90° to the north or -90° to the south. Simple rotation formulas allow a conversion from α,δ to λ,β and back (see: [[ecliptic coordinate system]]).

==Ecliptic and stars==
The ecliptic serves as the center of a region called the [[zodiac]] which constitutes a band of 8° on either side. Traditionally, this region is divided into 12 signs of 30° longitude each. By tradition, these signs are named after 12 of the 13 [[constellations]] straddling the ecliptic. The zodiac signs are very important to [[astrologers]]. Modern [[astronomers]] typically use other coordinate systems today (see below).

In the Indian tradition there are 27 [[Nakshatra]]s, which cover 13°20’ of the ecliptic each. Each Nakshatra is divided into quarters or ''padas'' of 3°20’.

The position of the vernal equinox is not fixed among the stars but due to the [[lunisolar precession]] slowly shifting westwards over the ecliptic with a speed of 1° per 72 years. A much smaller north/southwards shift can also be discerned, (the planetary precession, along the instantaneous equator, which results in a rotation of the ecliptic plane). Said otherwise the stars shift eastwards (increase their longitude) measured with respect to the equinoxes (in other words, as measured in [[Ecliptic_coordinate_system |ecliptic]] coordinates and (often) also in [[Equatorial_coordinate_system|equatorial]] coordinates.

Using the current official [[IAU]] constellation boundaries &mdash; and taking into account the variable precession speed and the rotation of the ecliptic &mdash; the equinoxes shift through the constellations in the [[Astronomical Julian calendar]] years (in which the year 0 = 1 BC, -1 = 2 BC, etc.) as follows:<ref>J. Meeus; Mathematical astronomical morsels; ISBN 0-943396-51-4</ref>
*The March equinox passed from Taurus into Aries in year -1865, passed into Pisces in year -67, will pass into Aquarius in year 2597, will pass into Capricorn in year 4312. It passed along (but not into) a 'corner' of Cetus on 0°10' distance in year 1489.
*The June solstice passed from Leo into Cancer in year -1458, passed into Gemini in year -10, passed into Taurus in December year 1989, will pass into Aries in year 4609.
*The September equinox passed from Libra into Virgo in year -729, will pass into Leo in year 2439.
*The December solstice passed from Capricorn into Sagittarius in year -130, will pass into Ophiuchus in year 2269, and will pass into Scorpius in year 3597.

==Ecliptic and Sun==
Due to perturbations to the Earth's orbit by the other planets, the ''true'' Sun is not always exactly on the ecliptic, but may be some arcseconds north or south of it. It is therefore the centre of the ''mean'' Sun which outlines its path. As the Earth revolves in one year around the Sun, it appears that the Sun also needs one year to pass the whole ecliptic. With slightly more than 365 days in the year, the Sun moves almost 1° eastwards every day (direction of increasing longitude). This annual motion should not be confused with the [[diurnal motion|daily motion]] of the Sun (and the stars, the whole celestial sphere for that matter) towards the west in 24 hours and along the equator. In fact where the stars need about 23h56m for one such rotation to complete, the [[sidereal day]], the Sun, which has shifted 1° eastwards during that time needs 4 minutes extra to complete its circle, making the [[solar day]] just 24 hours.

The mean Sun crosses the equator around 21 March in the vernal equinox, its declination, right ascension, and ecliptic longitude are all zero then (the ecliptic latitude is always). The March equinox marks the onset of spring in the northern hemisphere and autumn in the southern. As such the term "spring equinox" should be avoided. The actual date and time varies from year to year because of the occurrence of [[leap years]]. It also shifts slowly over the centuries due to imperfections in the [[Gregorian calendar]].

Ecliptic longitude 90°, at right ascension 6 hours and a northern declination equal to the obliquity of the ecliptic (23.44°), is reached around 22 June. This is the June [[solstice]] or summer solstice in the northern hemipshere and winter solstice in the southern hemisphere. It is also the first point of [[Cancer (astrology)|Cancer]] and directly overhead on Earth on the [[tropic of Cancer]] so named because the Sun [[tropics|turns around]] in declination. Ecliptic longitude 180°, right ascension 12 hours is reached around 23 September and marks the second equinox or first point of [[Libra (astrology)|Libra]]. Due to perturbations to the Earth orbit, the moment the real Sun passes the equator might be several minutes earlier or later. The southern most declination of the sun is reached at ecliptic longitude 270°, right ascension 18 hours at the first point of the sign of [[Capricorn (astrology)|Capricorn]] around 22 December.

In any case it must be stressed that although these traditional ''signs'' (in western [[tropical astrology]]) have given their names to the solstices and equinoxes, in reality, (as from the list in the previous chapter) the cardinal points are currently situated in the ''constellations'' of Pisces, Taurus, Virgo and Sagittarius respectively.

==Ecliptic and planets==
Most planets go in orbits around the sun which are almost in the same plane as the Earth's orbital plane, differing by a few degrees at most. As such they always appear close to the ecliptic when seen in the sky. Mercury with an orbital [[inclination]] of 7° or Pluto with 17° are exceptions. Many [[minor planets]] have large inclinations too. The intersection line of the ecliptical plane and the orbital plane is called the nodal line, and the intersection points on the celestial sphere are the [[ascending node]] (where the planet crosses the ecliptic from south to north) and the diametrically opposite [[descending node]]. Only when an [[inferior planet]] passes through one of its nodes a transit over the Sun can take place.

Inclination and nodal lines, as almost all other orbital elements, change slowly over the centuries due to [[perturbation (astronomy)|perturbations]] from the other planets. 

==Ecliptic and Moon==
The orbit of the [[Moon]] is inclined by about 5° on the ecliptic. Its nodal line is not fixed either, but regresses (moves towards the west) over a full circle every 18.6 years. This is the cause of [[nutation]] and [[lunar standstill]]. The moon crosses the ecliptic about twice per month. If this happens during [[new moon]] a [[solar eclipse]] occurs, during [[full moon]] a [[lunar eclipse]]. This was the way the ancients could trace the ecliptic along the sky; they marked the places where eclipses could occur.

==Ecliptic and star coordinates==
Up to the 17th century, starmaps and positions in star catalogues were always given in ecliptical coordinates. It was not until astronomers started to use telescopes to measure star positions that equatorial coordinates came in use, and so exclusively that nowadays ecliptical coordinates are no longer used. This is not always desirable. A planetary [[conjunction (astronomy)|conjuction]] for example would be much more illustratively described by ecliptic coordinates than equatorial.

Also see [[Zodiac#Zodiac_celestial_coordinate_systems|zodiac]]al coordinates.

==References==
<references/>
== External links ==
* [http://www-spof.gsfc.nasa.gov/stargaze/Secliptc.htm NASA: "The Path of the Sun, the Ecliptic"]
* [http://hyperphysics.phy-astr.gsu.edu/hbase/eclip.html Orbits and the Ecliptic Plane]
* [http://www.dur.ac.uk/john.lucey/users/solar_year.html The Ecliptic: the Sun's Annual Path]

==See also==
* [http://en.wikipedia.org/wiki/Ecliptic Ecliptic on Wikipedia]

[[Category: Glossary]]
[[Category: Celestial mechanics]]
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