Filter
Top Products
Astronomy Basics • 31
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 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 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. The position of the Big
Dipper changes during the year and throughout the course of the night. When
the Big Dipper is low in the sky (i.e., near the horizon), it may be difficult to
locate.
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.
Definition: 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.
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. 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.
Spring
Winter
Summer
Fall