Appendix A:
Celestial coordinates
For a sufficient tracking of an celestial object, the telescope mount
has to be aligned with the celestial pole.
By doing this, the mount’s axes are orientated in this way that they fit
to the celesial sphere.
If you want to align the telescope’s mount to the celestial pole, you
need some knowledge in which way an object at the sky can be
located while it is steadily moving across the sphere. This chapter
provides a basic knowledge about equatorial coordiates, the celestial
pole and how objects can be found by their coordinates. You will also
get used to the meaning of “Right aszension” and “Declination”
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 elestial sphere.
Thus, the North Celestial Pole (1, Fig. 34) 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.
The celestial equator (2, Fig. 34) is a projection of the Earth’s equator
onto the celestial sphere.
Just as an object's position on the Earth’s surface can be located by
its latitude and longitude, 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 constellation 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, arcminutes, and arc-seconds (e.g., 15° 27' 33"). Dec.
locations north of the celestial equator are indicated 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.
Looking at or near the Sun will cause instant and irreversible damage to your eye!
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APPENDIX A: CELESTIAL COORDINATES
Abb. 33: Celestial sphere
