Celestial sphere

The celestial sphere is the imaginary sphere of infinite radius, centered on the observer, onto which all bodies are projected as if fixed to its inside surface. It carries the celestial equator (the projection of the Earth’s equator), the celestial poles above the terrestrial poles, the ecliptic (the Sun’s apparent yearly path), and the first point of Aries where they cross. Positions on it are given by declination (angle north or south of the celestial equator) and hour angle or right ascension measured along it. The sphere appears to rotate westward once per sidereal day; this model underlies every hour-angle and altitude calculation.