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. 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: the North, or Boreal, hemisphere and the South, or Austral, hemisphere.
Image: the celestial equator and ecliptic (copyright and source: Joshua Cesa User / Wikimedia Commons).
The Ecliptic is the imaginary plane on which the Earth moves in its orbit around the Sun. This plane 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 travelled by the Sun during the year. This plane 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.
Consequently, the hemisphere that is tilted toward the Sun is more enlightened and warmed. During the year with the revolution around the Sun the hemisphere facing the Sun changes. This is the main cause of the alternance of the seasons. When a hemisphere is tilted toward the Sun, the days are longer and the nights shorter. The axial inclination is causing not only the variation of the daylight hours, but also the angle at which light reaches the Earth, more vertical in summer, less in winter.