The celestial sphere is a sphere of undefined radius, the centre of which is occupied by the Earth. On the inner surface by convention are placed all stars and planets. These are so far away that we can arbitrarily consider them as if they were all at the same distance from the Earth, exactly on the sphere, as mentioned above for the fixed stars.
Only two points of the sky remain fixed, without motion: they are the celestial poles, created by the intersection of the axis of rotation of the Earth with the celestial sphere and have the names of the North celestial pole and South celestial pole.
The Earth's equator projected on the sphere become the celestial equator and divides the sphere into two hemispheres North and South ones or Boreal and Austral.
The Ecliptic is the imaginary plane on which the Earth moves in its orbit around the Sun. This plan is projected onto the celestial sphere and it also creates the celestial Ecliptic, a circular route that for a terrestrial observer indicates the circular path traveled by the Sun during the year.
This plan does not coincide with the Earth’s plane of rotation, as it is obtained from the plane perpendicular to the Earth axis, which is tilted by 23° 27' with respect to the orbital plane.
The name Ecliptic originates from eclipses occurrence, when the full or new Moon meets the Sun on this path.