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Fi
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ure 10- Ali
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uatorial mount to the
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olar axis of the Earth.
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In each hemisphere, there is a point in the sky around which all the other stars appear to rotate. These points are called the celestial
poles and are named for the hemisphere in which they reside. For example, in the northern hemisphere all stars move around the
north celestial pole. When the telescope's polar axis is pointed at the celestial pole, it is parallel to the
Earth's rotational axis.
Figure 11 - The position of
the Big Dipper changes
throughout the year and the
ni
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ht.
Many methods of polar alignment require that you know how to find the celestial pole by identifying
stars in the area. For those in the northern hemisphere, finding the celestial pole is not too difficult.
Fortunately, we have a naked eye star less than a degree away. This star, Polaris, is the end star in the
handle of the Little Dipper. Since the Little Dipper (technically called Ursa Minor) is not one of the
brightest constellations in the sky, it may be difficult to locate from urban areas. If this is the case, use
the two end stars in the bowl of the Big Dipper (the pointer stars). Draw an imaginary line through them
toward the Little Dipper. They point to Polaris (see Figure 12). The position of the Big Dipper changes
during the year and throughout the course of the night (see Figure 11). When the Big Dipper is low in
the sky (i.e., near the horizon), it may be difficult to locate. During these times, look for Cassiopeia (see
Figure 12). Observers in the southern hemisphere are not as fortunate as those in the northern
hemisphere. The stars around the south celestial pole are not nearly as bright as those around the north.
The closest star that is relatively bright is Sigma Octantis. This star is just within naked eye limit
(magnitude 5.5) and lies about 59 arc minutes from the pole.
The north celestial pole is the point in the northern hemisphere around which all stars appear to rotate. The
counterpart in the southern hemisphere is referred to as the south celestial pole.
Definition
+” sign.
Figure 12
The two stars in the front of the bowl of the Big Dipper point to Polaris which
is less than one degree from the true (north) celestial pole. Cassiopeia, the
“W” shaped constellation, is on the opposite side of the pole from the Big
Dipper. The North Celestial Pole (N.C.P.) is marked by the “
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