The Celestial Sphere
The celestial sphere is a model for the two-dimensional direction to the stars, sun and moon as viewed from a particular place on earth. The observer is imagined to be at the center of the sphere, with the metal rim surrounding the sphere as the horizon. The brighter stars and the Milky Way can be painted on the surface of the sphere as their positions relative to each other do not change. The sphere rotates once each sidereal day about the axis defined by the North Star, so that stars appear to rise on the eastern horizon and set on the west. This apparent motion is a consequence of the earth's rotating once on its axis with respect to the reference frame of the stars each day. The angle of the rotation axis above the horizontal is 53 degrees, which was Botjes' latitude in the northern part of the Netherlands (10 degrees to the north of Calvin University).
The sun is represented as a gold disk (see photo) that moves along with the daily motion of the sphere, but which also moves gradually along the sphere, completing one circuit each year along a great circle we call the ecliptic. This apparent motion is a consequence of the earth's revolution about the sun with respect to the reference frame of the stars each year. The ecliptic is tilted by 23.5 degrees to the celestial equator because of the tilt of the earth's spin axis with respect to its orbital axis. This tilt leads to the phenomenon of seasons on earth.
Like the sun, the moon (a silver disk) follows the daily motion of the sphere as well as a slower motion with respect to the sphere (completing one circle each sidereal month, 27.32 days). The circuit of the moon is tilted with respect to the ecliptic by 5.2 degrees. For more information on these two, see the motions of the sun and moon.
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