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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 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 6-4). The position of the Big Dipper
changes during the year and throughout the course of the night (see Figure 6-4). 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 6-5).
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.
For more information about stars around the south celestial pole, please consult a star atlas.
Figure 6-5
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. 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.
Figure 6-4
The position of the Big Dipper
changes throughout the year
and throughout the night.
