Bedienungsanleitung Bresser AR-90 (Seite 25 von 44) (Englisch)

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Looking at or near the Sun will cause instant and irreversible damage to your eye!

ap p e n d I x a: ce l e s t I a l c o o r d I n a t e s

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 pro-

vides 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 inter-

vals 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.

Fig. 33: Celestial sphere

Earth rotation

0° DEC

South celestial pole

Right Ascension

Star

Celestial Equator

-90° DEC

+90° DEC

Northern celestial pole

(near Polaris)

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