Celestron CGE925 Instruction Manual download pdf (Page 38)

Finding the North Celestial 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 5-5). The position of

the Big Dipper changes during the year and

throughout the course of the night (see Figure 5-4).

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 5-5).

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.

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.

Polar Aligning the Mount

Latitude Scales

The easiest way to polar align a telescope is with a latitude scale. Unlike other methods that require you to find the celestial

pole by identifying certain stars near it, this method works off of a known constant to determine how high the polar axis

should be pointed. The CGE Pro mount can be adjusted from 15 to 65 degrees (see figure 5-3).

The constant, mentioned above, is a relationship between your latitude and

the angular distance the celestial pole is above the northern (or southern)

horizon; The angular distance from the northern horizon to the north

celestial pole is always equal to your latitude. To illustrate this, imagine

that you are standing on the north pole, latitude +90°. The north celestial

pole, which has a declination of +90°, would be directly overhead (i.e., 90

above the horizon). Now, let’s say that you move one degree south —

your latitude is now +89° and the celestial pole is no longer directly

overhead. It has moved one degree closer toward the northern horizon.

This means the pole is now 89° above the northern horizon. If you move

one degree further south, the same thing happens again. You would have

to travel 70 miles north or south to change your latitude by one degree. As

you can see from this example, the distance from the northern horizon to

the celestial pole is always equal to your latitude.

Figure 5-3

Latitude

Scale

Latitude

Adjustment

Knob

Definition

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. 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.

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