The ecliptic is the apparent path that the Sun traces out in the sky during the year, appearing to move eastwards on an imaginary spherical surface, the celestial sphere, relative to the (almost) fixed stars. In more accurate terms, 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. (The ecliptic plane should be distinguished from the invariable plane of the solar system, which is perpendicular to the vector sum of the angular momenta of all planetary orbital planes, to which Jupiter is the main contributor. The present ecliptic plane is inclined to the invariable plane by about 1.5°.)
The name ecliptic arises because eclipses occur when the full or new Moon is very close to this path of the Sun.
Ecliptic and equator
As the rotational 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°26', which is known as the axial tilt (or obliquity of the ecliptic).
The intersections of the equatorial and ecliptic planes with the celestial dome are great circles known as the celestial equator and the ecliptic respectively.
The intersection line of the two planes results in two diametrically opposite intersection points, known as the equinoxes. The equinox that 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 ‹φ› 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).
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