The ecliptic is the apparent path that the Sun takes in a year with respect to the background of the celestial sphere. This is the intersection of the celestial sphere with the geometric plane, called the ecliptic plane or the ecliptic plane, on which the earth's orbit lies. It is therefore the great circle of the geocentric celestial sphere with a radius equal to the distance between the center of the Sun and the center of the Earth. astronomical phenomenon. The ecliptic plane should be distinguished from the invariable ecliptic plane which is perpendicular to the sum vector of the angular moments of all planetary orbital planes, among which the angular momentum of that of Jupiter is the main contributor in the solar system. Currently the ecliptic plane is inclined with respect to the invariable ecliptic plane of about 1.5 °.
Ecliptic and celestial equator
Since the axis of rotation of the Earth is not perpendicular to its orbital plane, the equatorial plane is not parallel to the plane of the ecliptic, but forms an angle with it of about 23 ° 27 'known as the inclination of the ecliptic.
The intersections of the two planes with the celestial sphere are great circles known as the celestial equator and ecliptic. The line of intersection between the two planes defines two diametrically opposite equinoctial points on the celestial sphere. The equinox where the Sun passes from south to north of the celestial equator (i.e. the vernal equinox) is called the vernal point, γ point or first point of Aries. This nomenclature refers to when the vernal equinox fell within the constellation of Aries. The equinoctial colure passes through this point and the celestial poles.
The ecliptic longitude is typically indicated with the letter λ, it is measured from this point from 0 ° to 360 ° towards the east. The ecliptic latitude, usually indicated with the letter β, is measured from + 90 ° in the north to -90 ° in the south. The same point of intersection also defines the origin of the equatorial coordinate system, called right ascension measured from 0 to 24 hours always towards the east and typically indicated with α or R.A., and the declination, typically indicated with δ always measured from + 90 ° north to -90 ° south. Simple rotation formulas allow a conversion from α, δ to λ, β and vice versa (see ecliptic coordinate system).
The position of the equinoctial points on the celestial sphere varies slowly due to the precession of the earth's axis, for this reason called the precession of the equinoxes, and its nutation.
Ecliptic and stars
The ecliptic serves as the center of a region called the zodiac which forms a 9 ° band on both sides. Traditionally, this region is divided into 12 signs, each of 30 degrees of longitude. According to tradition, these signs are named after 12 of the 13 constellations that straddle the ecliptic. Modern astronomers typically use other coordinate systems today (see below).
The position of the vernal equinox is not fixed among the stars, but determined by the lunisolar precession that slowly moves west on the ecliptic with a speed of 1 ° every 72 years. A much smaller north / south shift can also be perceived, (the planetary precession, along the equator, resulting from a rotation of the ecliptic plane). In other words, stars move east (increasing their longitude) with respect to the equinoxes - in other words, with respect to the coordinates of the ecliptic and (often) also to the equatorial coordinates.
Using the current official boundaries of the IAU constellation - and taking into account both the variable rate of precession and the rotation of the ecliptic - the equinoxes move through the constellations in the years of the Julian astronomical calendar (where the year 0 1 BC, - 1 2 BC, etc.) as follows:
The March equinox p