50
Looking at or near the
Sun
will cause
irreversable
damage to your eye. Do not point this telescope at or near the
Sun. Do not look through the telescope as it is moving.
Polar Alignment
In Polar Alignment, the telescope is oriented so that the horizontal and vertical axes of the
telescope are lined up with the celestial coordinate system.
In order to Polar align your telescope, it is essential to have an understanding of how and where
to locate celestial objects as they move across the sky. This section provides a basic
introduction to the terminology of Polar-aligned astronomy, and includes instructions for finding
the celestial pole and for finding objects in the night sky using Declination and Right Ascension.
Celestial Coordinates
A celestial coordinate system was created that maps an imaginary sphere surrounding the
Earth upon which all stars appear to be placed. This mapping system is similar to the system
of latitude and longitude on Earth surface maps.
In mapping the surface of the Earth, lines of longitude are drawn between the North and South
Poles and lines of latitude are drawn in an East-West direction, parallel to the Earth’s equator.
Similarly, imaginary lines have been drawn to form a latitude and longitude grid for the
celestial sphere. These lines are known as
Right Ascension
and
Declination
.
The celestial map also contains two poles and an equator just like a map of the Earth. The
poles of this coordinate system are defined as those two points where the Earth’s north and
south poles (
i.e., the Earth's axis), if extended to infinity, would cross the celestial sphere. Thus,
the North Celestial Pole (
1, Fig. 28
) is that point in the sky where an extension of the North Pole
intersects the celestial sphere. The North Star, Polaris is located very near the North Celestial
Pole (
1, Fig. 28
). The celestial equator (
2, Fig. 28
) is a projection of the Earth’s equator onto
the celestial sphere.
So just as an object's position on the Earth’s surface can be located by its latitude and longi-
tude, celestial objects may also be located using Right Ascension and Declination. For
example, you could locate Los Angeles, California, by its latitude (+34°) and longitude (118°).
Similarly, you could locate the Ring Nebula (M57) by its Right Ascension (18hr) and its
Declination (+33°).
•
Right Ascension (R.A.):
This celestial version of longitude is measured in units of hours (hr),
minutes (min), and seconds (sec) on a 24-hour "clock" (similar to how Earth's time zones are
determined by longitude lines). The "zero" line was arbitrarily chosen to pass through the con-
stellation Pegasus — a sort of cosmic Greenwich meridian. R.A. coordinates range from 0hr
0min 0sec to 23hr 59min 59sec. There are 24 primary lines of R.A., located at 15-degree
intervals along the celestial equator. Objects located further and further East of the zero R.A.
grid line (0hr 0min 0sec) carry higher R.A. coordinates.
•
Declination (Dec.):
This celestial version of latitude is measured in degrees, arc-minutes,
and arc-seconds (
e.g., 15° 27' 33"). Dec. locations north of the celestial equator are indicat-
ed with a plus (+) sign (
e.g., the Dec. of the North celestial pole is +90°). Dec. locations south
of the celestial equator are indicated with a minus (–) sign (
e.g., the Dec. of the South
celestial pole is –90°). Any point on the celestial equator (such as the the constellations of
Orion, Virgo, and Aquarius) is said to have a Declination of zero, shown as 0° 0' 0."
14
15
16
17
18
19
20 21
22
23
0
1
12
11
10
9
8
7
5
6
4
3
2
13
Earth’s
Rotation
0 Dec.
South
Celestial
Pole
Right Ascension
Star
Celestial
Equator
-90 Dec.
+90 Dec.
North
Celestial
Pole
(Vicinity
of Polaris)
D
e
cli
n
a
tio
n
1
2
Fig. 28: Celestial Sphere.
APPENDIX A: EQUATORIAL (POLAR) ALIGNMENT
