Meade 114 EQ-ASB Understanding Celestial Movements and Coordinates, Declination Dec. This

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•Check this alignment on a celestial object, such as a bright star or the Moon, and make any refinements necessary, using the method outlined above.

With this alignment performed, objects first located in the wide-field viewfinder will also be centered in the main telescope’s field of view. (Note: The viewfinder presents an image which is upside-down.)

UNDERSTANDING CELESTIAL MOVEMENTS AND COORDINATES

Understanding where to locate celestial objects, and how those objects move across the sky is fundamental to enjoying the hobby of astronomy. Most amateur astronomers adopt the simple practice of “star-hopping” to locate celestial objects by using star charts or astronomical software which identify bright stars and star patterns (constellations) that serve as “road maps” and “landmarks” in the sky. These visual reference points guide amateur astronomers in their search for astronomical objects. And while star- hopping is the preferred technique, a discussion of using setting circles for locating objects is desirable since your telescope is provided with this feature. However, be advised, compared to star-hopping, object location by use of setting circles requires a greater investment in time and patience to achieve a more precise alignment of the telescope’s polar axis to the celestial pole. For this reason, in part, star-hopping is popular because it is the faster, easier way to become initiated in the hobby.

Understanding how astronomical objects move: Due to the Earth’s rotation, celestial bodies appear to move from East to West in a curved path through the skies. The path they follow is known as their line of Right Ascension (R.A.). The angle of this path they follow is known as their line of Declination (Dec.).

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 (see Fig. 3) 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 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 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 (also known as “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

+90° Dec.

0sec to 23hr 59min 59sec. There are

North Celestial Pole

Pôle nord céleste

(Vicinity of Polaris)

Star

24 primary lines of R.A., located at

15-degree

intervals

along

the

D

celestial

equator.

Objects located

é

c

l

i

n

further and further East of the zero

a

i

s

13

12

11

o

15

14

10

n

R.A. grid line (0hr 0min 0sec) carry

9

17

16

8

7

higher R.A. coordinates.

18

Rotation de la Terre

6

19

4

5

20

3

■ Declination

(Dec.): This

celestial

21

22 23

2

0° Déc.

0

1

Celestial

version of latitude is measured in

Ascension droite

Equator

degrees, arc-minutes, and arc-

seconds

(e.g., 15°

27'

33"). Dec.

South

locations

North

of

the

celestial

Celestial

Pôle

-90° Dec.

céleste Sud.

equator are indicated with a plus (+)

Pole

sign (e.g., the Dec. of the North

Fig. 2: Celestial Sphere.

celestial pole is +90°). Dec. locations