That great circle which is the projection of the Earth's orbit onto the celestial sphere, and therefore is the apparent path of the Sun across our sky. Positions of the planets as viewed from Earth are generally very close to the ecliptic.
The ecliptic is the apparent path the Sun traces out along the sky — independent of Earth's rotation — in the course of the year. It should be distinguished from the invariable ecliptic plane, which is the vector sum of the angular momenta of all planetary orbital planes, to which 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°27' 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 equinox which the Sun passes from south to 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 east. Ecliptic latitude, usually indicated with the letter β is 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 9° on either side. By tradition, these signs are named after 12 of the 13 constellations straddling the ecliptic.
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 coordinates and (often) also in equatorial coordinates.
Using the current official IAU constellation boundaries — and taking into account the variable precession speed and the rotation of the ecliptic — 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:
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. 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. 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).
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 actual date and time varies from year to year because of the occurrence of leap years.
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. Ecliptic longitude 180°, right ascension 12 hours is reached around 23 September and marks the second equinox or first point of Libra. 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 around 22 December.
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.
The intersection line of the ecliptical plane and another planet's orbital plane is called the nodal line of that planet, and the nodal line's 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.
Ecliptic and Moon
The orbit of the Moon is inclined by about 5° on the ecliptic. The moon crosses the ecliptic about twice per month.
Ecliptic and star coordinates
Up to the 17th century, starmaps and positions in star catalogues were always given in ecliptical coordinates. A planetary conjunction for example would be much more illustratively described by ecliptic coordinates than equatorial.
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