A celestial sphere is an imaginary sphere of infinite extent on which the stars, the planets, and other heavenly bodies appear to be located to an observer standing at the celestial sphere's center.
The properties of a celestial sphere include:
The stars, the Sun, the Moon, and the planets are all projected onto a celestial sphere.
- Angular measurements at the sphere do not depend on the celestial sphere's radius.
- The center of a celestial sphere may be selected randomly. Each observer is free to choose his own center, while there may be many observers.
A celestial sphere includes
An imaginary vertical line crosses the surface of a celestial sphere in two points: the upper point Z (zenith) and the lower point Z' (nadir).
- Mathematical horizon
- Celestial equator
- Celestial meridian
A mathematical horizon is a large circle on the celestial sphere, with its plane perpendicular to the vertical.
N-point of the mathematical horizon is known as the North point. S-point of the mathematical horizon is called South point. NS line is known as the meridian line.
The Earth rotates around the line known as axis of the globe. It crosses the celestial sphere at the points that are called poles. Celestial equator is a large circle that is perpendicular to the axis of the globe. Celestial equator intersects with mathematical horizon in East and West points E and W.
Celestial meridian is a large circle that passes through zenith Z, celestial pole P, south celestial pole P', and nadir Z'.
An ecliptic is the visible annual part of the solar disk that is moving along the celestial sphere. The annual rotation of the Earth around the Sun induces the solar motion along the ecliptic. Solar disk center crosses the celestial equator twice every year: in March and in September. Ecliptic intersections with the celestial equator are called the spring and autumnal equinox points. The Sun passes from the southern hemisphere of the celestial sphere at the spring equinox point on March 21. The Sun passes from the northern hemisphere to the southern hemisphere at the autumn equinox point on September 21.
Summer solstice occurs on June 22nd, when the Sun is in Cancer. At that point, the Sun is at its maximal declination of δ = +23?26'. Winter solstice occurs on December 22nd, when the Sun is in Capricorn. The Sun therein is at its minimal declination of δ = -23?26' (December 22). Solstice and equinox days may slightly fluctuate on different years, since a year lasts slightly longer than 365 days.
Solstice points are located at 90? from equinox points.
This model is an interactive illustration of the basic lines and points of a celestial sphere. Integral lines and points of the celestial sphere that have not been selected appear in red. Lines and points that have been selected appear as bright green.