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The daily motion of the Sun across the sky is familiar to even the most casual observer. This daily trek is not the Sun
moving as early astronomers thought, but the result of the Earth's rotation. The Earth's rotation also causes the stars to do
the same, scribing out a large circle as the Earth completes one rotation. The size of the circular path a star follows depends
on where it is in the sky. Stars near the celestial equator form the largest circles rising in the east and setting in the west.
Moving toward the north celestial pole, the point around which the stars in the northern hemisphere appear to rotate, these
circles become smaller. Stars in the mid-celestial latitudes rise in the northeast and set in the northwest. Stars at high
celestial latitudes are always above the horizon, and are said to be circumpolar because they never rise and never set. You
will never see the stars complete one circle because the sunlight during the day washes out the starlight. However, part of
this circular motion of stars in this region of the sky can be seen by setting up a camera on a tripod and opening the shutter
for a couple hours. The processed film will reveal semicircles that revolve around the pole. (This description of stellar
motions also applies to the southern hemisphere except all stars south of the celestial equator move around the south
celestial pole.)
All stars appear to rotate around the celestial poles. However, the appearance of this motion
varies depending on where you are looking in the sky. Near the north celestial pole the stars
scribe out recognizable circles centered on the pole (1). Stars near the celestial equator also
follow circular paths around the pole. But, the complete path is interrupted by the horizon.
These appear to rise in the east and set in the west (2). Looking toward the opposite pole, stars
curve or arc in the opposite direction scribing a circle around the opposite pole (3).
Figure 6-2
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