Table of Contents
Advertisement
Up to this point, this manual covered the assembly and basic operation of your 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.
T
h
e
C
e
l
e
s
t
i
a
l
C
o
T
h
e
C
e
l
e
s
t
i
a
l
C
o
To help find objects in the sky, astronomers use a celestial coordinate system that is similar to our geographical co-
ordinate 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.
M
o
t
i
o
n
o
f
t
h
e
S
t
a
M
o
t
i
o
n
o
f
t
h
e
S
t
a
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 timed exposure 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.)
o
r
d
i
n
a
t
e
S
y
s
t
e
m
o
r
d
i
n
a
t
e
S
y
s
t
e
m
The celestial sphere seen from the outside showing R.A. and DEC
r
s
r
s
Figure 4-1
.
Hide quick links:
Advertisement
Table of Contents
