Celestron CPC Deluxe 1100 HD Computerized Telescope - Manual (Page 22)

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ASTRONOMY BASICS
Up to this point, this manual covered the assembly and basic operation
of your CPC telescope . However, to understand your telescope more
thoroughly, you need to know a little about the night sky . This section deals with observational astronomy in general and includes information
on the night sky and polar alignment .
>> The Celestial Coordinate System
To help find objects in the sky, astronomers use a celestial coordinate
system that is similar to our geographical coordinate system here on
Earth . The celestial coordinate system has poles, lines of longitude and
latitude and an equator. For the most part, these remain fixed against the
background stars .
The celestial equator runs 360 degrees around the Earth and separates
the northern celestial hemisphere from the southern . Like the Earth’s
equator, it bears a reading of zero degrees. On Earth this would be
latitude . However, in the sky this is referred to as declination, or DEC for
short . Lines of declination are named for their angular distance above
and below the celestial equator . The lines are broken down into degrees,
minutes of arc and seconds of arc . Declination readings south of the
equator carry a minus sign (-) in front of the coordinate and those north
of the celestial equator are either blank (i .e ., no designation) or preceded
by a plus sign (+).
The celestial equivalent of longitude is called Right Ascension, or R .A . for
short . Like the Earth’s lines of longitude, they run from pole to pole and are
evenly spaced 15 degrees apart . Although the longitude lines are separated
by an angular distance, they are also a measure of time . Each line of
longitude is one hour apart from the next . Since the Earth rotates once
every 24 hours, there are 24 lines total . As a result, the R .A . coordinates are marked off in units of time. It begins with an arbitrary point in the
constellation of Pisces designated as 0 hours, 0 minutes, 0 seconds . All
other points are designated by how far (i .e ., how long) they lag behind this
coordinate after it passes overhead moving toward the west .
>> 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 .)
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).
FIGURE 6-2
FIGURE 6-1
2
1 0
3 20
19
18
17
21
22
23
60
-60
-40
40
20
0
The celestial sphere seen from the outside
showing R.A. and DEC
Stars seen near the
north celestial pole Stars seen near the
celestial equator
Stars seen looking in the
opposite direction of the
north celestial pole