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Many ancient societies believed that the stars were equidistant from the Earth and that this sphere was a real model of the universe. This model is a useful abstraction, but not correct. Everything we see in the sky is so very far away that their distances are impossible to gauge just by looking at them. Since their distances are indeterminate, you only need to know the direction toward the object to locate it in the sky. In this sense, the celestial sphere model is a very practical tool for positional astronomyPositional astronomy is the study of the positions of celestial objects. This is the oldest branch of astronomy and dates back to antiquity. Observations of celestial objects are important for religious and astrological purposes, as well as for timekeepin.

As the Earth rotates on its axis, the objects on the celestial sphere will appear to rotate around the celestial poles every 24 hours, this is diurnal motionDiurnal motion is an astronomical term referring to the apparent daily motion of stars in " orbit" around the Earth, caused by the Earth's rotation around its axis. It is a rotation around the axis between the two celestial poles. It takes Earth 23 hours,. For example the Sun will typically appear to rise in the east and set in the west, as will the stars, planets and moon. On each subsequent night, a given star will rise ~4 minuteA minute is: a unit of time equal to 1/60th of an hour and to 60 seconds. Some rare minutes have 59 or 61 seconds; see leap second. a unit of angle, 1/60th of a degree. It is then also known as a minute of angle or minute of arc and can further be divideds earlier than it rose the previous night. Superimposed on diurnal motion is; intrinsic motion as the objects change their relative positions, with respect to Earth. For example, over the course of a year the Sun, relative to the background stars, will follow a bisecting great circleA great circle is a circle on the surface of a sphere that has the same diameter as the sphere, dividing the sphere into two equal hemispheres. Equivalently, a great circle on a sphere is a circle on the sphere's surface whose center is the same as the ce (known as the eclipticThe ecliptic is the geometric plane that contains the orbit of the Earth. The orbits of most planets in the Solar System lie very close to it. Seen from the Earth, this is a bisecting great circle, superimposed upon the celestial sphere, which contains th).

The celestial sphere is divided by projecting the equatorIn geography, the equator is an imaginary line drawn around a planet, halfway between the poles, where the surface of the roughly spherical planet is parallel to the axis of rotation. The equator divides the surface into the Northern Hemisphere and the So into space. This divides the sphere into the north celestial hemisphere and the south celestial hemisphere. Likewise, one can locate the Celestial Tropic of CancerThe Tropic of Cancer is one of the five major circles of latitude that mark maps of the Earth. This is the parallel of latitude that runs 23° 26' 22" north of the Equator, and is the farthest northern latitude at which the sun can appear directly overhead, Celestial Tropic of CapricornThe Tropic of Capricorn is one of the five major circles of latitude that mark maps of the Earth. This is the parallel of latitude that runs 23° 26' 22" south of the Equator, and is the farthest southern latitude at which the sun can appear directly overh, North Celestial Pole, and South Celestial Pole.

As the earth rotates from west to east, the celestial sphere appears to rotate from east to west. If a star is sufficiently near the celestial pole that is above the horizon, the star is also always above the horizon, encircling the pole; such stars are circumpolar.

The directions toward various objects in the sky can be quantified by constructing a celestial coordinate system.