Great Circles on the Celestial Sphere

A great circle is a circle on a sphere obtained by intersecting the sphere with a plane that passes through the center of the sphere. There are certain important great circles on the celestial sphere.
1. Celestial Equator
The celestial equator is a circle that cuts the celestial sphere into two equal halves, with respect to the poles. It lies above the terrestrial equator.
2. Horizon
The horizon is a circle that cuts the celestial sphere into two halves: one which is visible, and one which is not. It is the “ground level” of the observer. The plane of this great circle is perpendicular to a line joining the point of an observer on the Earth and the zenith, and cuts through the centre of the Earth.

3. Ecliptic
The ecliptic is the annual path of the Sun, with respect to the stars, as seen from a geocentric perspective. The plane of this great circle always makes an angle of 23.5° with the plane of the celestial equator. This tilt is of great importance, as it is responsible for the seasons on Earth.
4. Central Meridan
The central meridan is an imaginary arc which cuts through the north point on the horizon, the zenith, and the south point on the horizon.
5. Prime Vertical
The prime vertical is an imaginary arc which cuts through the east point on the horizon, the zenith and the west point on the horizon.

6. Equinoxes and Solstices
Two great circles intersect at two points diametrically opposite each other. For the celestial equator and the ecliptic, these are the equinoxes. The solstices are the highest and lowest points on the ecliptic, with respect to the celestial equator. We will also use these terms for the times when the Sun is at these points. The spring or vernal equinox
falls on March 21, while the autumnal equinox is on September 23. At the equinoxes, the days and nights are of equal length. The highest point, the summer solstice, falls on June 21, and is the longest day of the year for the northern hemisphere. The lowest point, the winter solstice, is on December 22, and coincides with the shortest day of the year for the northern hemisphere.