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Desription
Celestial spheres are a means of describing, visually, what people see in the sky from the Earth's surface, day or night, any time of year. A celestial sphere is a clear plastic model of the night sky. It has rotational axis, equator and meridian markings as well as indicators for the Milk Way, other galaxies and constellations. A sun model on a movable arm tracks the sun's course across the sky. Instead of a rotating Earth and a stationary sky, the Earth remains immobile at the sphere's center, and the sky spins.
Celestial Axis, Equator and Meridian
The Earth's axis extends outward to intersect the celestial sphere. Celestial north and south poles are located at the junctures where the axis and the sphere intersect. Earth's latitudes and longitudes are projected outward to create celestial latitudes and longitudes. Two large circles around the center of the sphere indicate the celestial equator (latitude) and the celestial meridian (longitude).
Motion
The celestial sphere tilts on an axis at the same angle as the Earth, and the stars on its surface move in arcs based around the Earth's polar axis. The sphere spins on its celestial axis to demonstrate what a particular area of the Earth observes --- nighttime or day, in the northern or southern hemisphere.
Ecliptic Plane
The ecliptic plane --- the movement of the planets around the sun --- intersects the sphere along the ecliptic --- the solar system's route across the celestial sphere. This path wobbles north and south of the celestial equator depending on the season of the year.
Finding Celestial Objects
To find which nightly celestial objects (stars, Milky Way, sun) will be visible from a particular location on Earth, a person can align his or her latitude with a horizontal metal band marked with hour, day, month and year. Then peering through the back of the sphere (180 degrees behind that position), that night's visible constellations will appear on the celestial sphere over the person's location. The time when the stars rise and set in the night sky can also be determined by spinning the sphere from west to east, which gives the appearance of rising in the east and setting in the west.