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Astronomy Basics • 35
For 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 of the 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 relatively easy. 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 (see figure 4-5). Since the Little Dipper (technically
called Ursa Minor) is not one of the brightest constellations in the sky, it may be
difficult to locate, especially from urban areas. If this is the case, use the two end
stars in the bowl of the Big Dipper. Draw an imaginary line through them (about
five times the distance between these two stars) toward the Little Dipper. They
will point to Polaris. The position of the Big Dipper will change during the year
and throughout the course of the night (see figure 4-4). When the Big Dipper is
difficult to locate, try using Cassiopia.
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 59 arc minutes from the pole. For more information about stars around the
south celestial pole, please consult a star atlas.
While it may seem that pointing at the pole star produces a parallax effect,
especially for observers near the equator, this effect is negligible since Polaris
is so far away.
Definition: The north celestial pole is the point in the northern sky around which all stars
appear to rotate. The counterpart in the southern hemisphere is referred to as
the south celestial pole.
Finding the Pole
Figure 4-4
The position of the Big
Dipper changes through-
out the year and through-
out the night.
Figure 4-5
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. Cassiopia, 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.