Ecliptic
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The ecliptic is the apparent path of the Sun traced out along the sky 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 containing the mean orbit of the Earth around the Sun. 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 contributer.
The name ecliptic is derived from being the place where eclipses occur.
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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°4 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 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. 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.
Ecliptic and stars
The constellations through which the ecliptic runs are known as the zodiac. By tradition there are 12 zodiacal constellations, all taking an equal share of the ecliptic, that is 30° of length and an undefined size in latitude. The zodiacal signs are very important to astrologers. Modern astronomers care less, in fact they recognise 13 zodiacal constellations which are of various sizes.
In the Indian tradition there are 27 Nakshatras, 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). Said otherwise the stars roughly shift eastwards measured in both the equatorial and the ecliptic coordinate systems. Therefore the statement that "the constellation of Aries" marks the vernal equinox is only true at a particular time. In about 2200 year precession will move the vernal equinox over 30° in longitude, being one zodiacal constellation. Indeed while the vernal equinox was in the constellation of Aries in the time of the ancient Greeks, by now it is in the constellation of Pisces. Astrologers, however, (to add to the confusion on which they thrive), prefer to work with zodiacal signs, rather the real constellations. The zodiacal signs are fixed to the ecliptic and as such one can say the vernal equinox is per definition the first point of the sign of Aries, even though it is now positioned among the stars of Pisces. Likewise the other equinox, the first point of Libra, is currently in reality among the stars of Virgo, and so forth.
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 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 is reached around 22 June. This is the June solstice or summer solstice in the northern hemipshere and winter solstice in the souther hemisphere. It is also the first point of Cancer, and directly overhead on Earth on the tropic of Cancer so named because the Sun turns around in declination. Ecliptic longitude 180° is reached around 23 September and marks the second equinox or first point of 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° in 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. 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 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 conjuction for example would be much more illustratively described by ecliptic coordinates than equatorial.
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