The Celestial Sphere
The celestial sphere is an imaginary sphere surrounding the Earth onto which the stars, planets, constellations, and other celestial objects are projected. Many of the constellations are shown here. You can move an arbitrary point to show how right ascension and declination relate to specific points on the celestial sphere. The vernal and autumnal equinoxes can be seen as the intersection of the celestial equator and the ecliptic. The ecliptic is the intersection of the plane of the solar system and the celestial sphere. The celestial equator is the projection of the Earth's equator onto the celestial sphere. Constellations that lie along the ecliptic are known as the zodiacal constellations.