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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.
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 7-4).
The position of the Big Dipper changes during the year and throughout the
course of the night (see Figure 4-3). 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 4-4). 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 4-3 –
The position of the Big Dipper
changes throughout the year
and the night.
Figure 4-4
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.