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Finding the 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 north-
ern 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 bow] of the Big Dipper (the pointer stars). Draw an imaginary
line through them toward the Little Dipper. They point to Polaris (see figure
5-5). The position of the Big Dipper changes during the year and throughout
i
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the course of the night (see figure 5-4). When the Big Dipper is low in the
Figure 5-4
sky (i.e., near the horizon), it may be difficult to locate. During these times,
look for Cassiopeia (see figure 5-5).
The position of the Big
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Observers in the southern hemisphere are not as fortunate as those in the
and throughout the
northern hemisphere. The stars around the south celestial pole are not
night.
This is due to
nearly as bright as those around the north. The closest star that is relatively
the Earth's motion
bright is Sigma Octantis. This star is just within naked eye limit (magnitude
around the Sun and the
5.5) and lies about 59 arc minutes from the pole. For more information about
Earth's rotation on its
stars around the south celestial pole, please consult a star atlas.
axis.
NE
M E
Sm
EE
Figure 5-5
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. Cas-
siopeia, 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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Astronomy Basics
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