Celestron Ultima 80 - Manual (Page 8)

Motion of the Stars
The daily motion of the Sun across the sky is familiar to even the most casual observer. This daily trek is not the Sun
moving as early astronomers thought, but the result of the Earth's rotation. The Earth's rotation also causes the stars to do the
same, scribing out a large circle as the Earth completes one rotation. The size of the circular path a star follows depends on
where it is in the sky. Stars near the celestial equator form the largest circles rising in the east and setting in the west.
Moving toward the north celestial pole, the point around which the stars in the northern hemisphere appear to rotate, these
circles become smaller. Stars in the mid-celestial latitudes rise in the northeast and set in the northwest. Stars at high
celestial latitudes are always above the horizon, and are said to be circumpolar because they never rise and never set. You
will never see the stars complete one circle because the sunlight during the day washes out the starlight. However, part of
this circular motion of stars in this region of the sky can be seen by setting up a camera on a tripod and opening the shutter
for a couple hours. The processed film will reveal semicircles that revolve around the pole. (This description of stellar
motions also applies to the southern hemisphere except all stars south of the celestial equator move around the south celestial
pole.)
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 (see figure 11).
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 9
All stars appear to rotate around the celestial poles. However, the appearance of this motion
varies depending on where you are looking in the sky. Near the north celestial pole the stars
scribe out recognizable circles centered on the pole (1). Stars near the celestial equator also
follow circular paths around the pole. But, the complete path is interrupted by the horizon.
These appear to rise in the east and set in the west (2). Looking toward the opposite pole, stars
curve or arc in the opposite direction scribing a circle around the opposite pole (3).
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