Astronomy Basics • 29
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 that 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 north-
east 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
motion also applies to the southern hemisphere except all stars south of the
celestial equator move around the south celestial pole.)
Motion of the Stars
Figure 4-2
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).
