Meade 4504 instruction manual Lining Up with the Celestial Pole, Celestial sphere

On the surface of the Earth,	North
“lines of longitude” are drawn
	Celestial	+90 Dec.
between the North and South
	Pole	Star
Poles. Similarly, “lines of latitude”	(Vicinity

are drawn in an East-West direc-

of Polaris)

D

e

c

1

l

tion, parallel to the Earth's equa-

i

n

a

t

tor. The celestial equator is sim-

i

14 13

12

11

10

o

15

n

ply a projection

of the Earth's

17 16

9

8

Earth’s

7

equator

onto

the

celestial

18

Rotation

56

19

4

Celestial

sphere. Just as on the surface of

20

21

22

23

0

1

2

3

Equator

the The celestial equivalent to

Right Ascension

0 Dec.

Earth

latitude

is

called

South

2

“Declination,” or simply “Dec.” It

is measured in degrees, minutes

Celestial

-90

Dec.

Pole

or seconds north ("+") or south

("-") of the celestial equator. Thus

Fig. 31: Celestial sphere.

any point on the celestial equator

(which

passes,

for

example,

through the constellations Orion, Virgo and Aquarius) is specified as having 0°0'0" Dec. The Dec of the star Polaris, located very near the North Celestial Pole, is +89.2°.

The celestial equivalent to Earth longitude is called “Right Ascension,” or “R.A.” It is measured in hours, minutes, and seconds from an arbitrarily defined “zero” line of R.A. passing through the constellation Pegasus. R.A. coordinates range from 0hr0min0sec up to (but not including) 24hr0min0sec. Thus there are 24 primary lines of R.A., locat- ed at 15 degree intervals along the celestial equator. Objects located further and fur- ther east of the prime (0h0m0s) R. A. grid line carry increasing R.A. coordinates.

With all celestial objects therefore capable of being specified in position by their celes- tial coordinates of R.A. and Dec, the task of finding objects (in particular, faint objects) in the telescope is vastly simplified. The setting circles, R.A. (15, Fig. 2a) and Dec (9, Fig. 2a)of the Meade 4504 Telescope may be dialed, in effect, to read the object coor- dinates, and the object can be found without resorting to visual location techniques. However, these setting circles may be used to advantage only if the telescope is first properly aligned with the Celestial Pole.

Lining Up with the Celestial Pole

Objects in the sky appear to revolve around the Celestial Pole (actually, celestial objects are essentially “fixed,” and their apparent motion is caused by the Earth’s axial rotation). During any 24 hour period, stars make one complete revolution about the pole, making concentric circles with the pole at the center. By lining up the telescope’s polar axis with the North Celestial Pole (or, for observers located in Earth’s Southern Hemisphere, with the South Celestial Pole), astronomical objects may be followed, or tracked, by moving the telescope about one axis, the polar axis.

Virtually all of the required tele- scope tracking will be in R.A. (if the telescope were perfectly aligned with the pole, no Dec tracking of stellar objects would be required). For the purposes of casual visual telescopic obser- vations, lining up the telescope’s polar axis to within a degree or two of the pole is more than suf- ficient.

Little Dipper

Polaris

Big Dipper

Cassiopeia

Fig. 32: Locating Polaris.

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