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Finding the North Celestial Pole
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.
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 7-5). The position of the Big Dipper changes during the year and throughout
the course of the night (see Figure 7-5). 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 7-6). 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
Figure 7-5
The position of the Big
Dipper changes throughout
the year and the night.
Figure 7-6
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 “+” sign.
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