Meade 4504 Instruction Manual download pdf (Page 40)

page 40

On the surface of the Earth,

“lines of longitude” are drawn

between the North and South

Poles. Similarly, “lines of latitude”

are drawn in an East-West direc-

tion, parallel to the Earth's equa-

tor. The celestial equator is sim-

ply a projection of the Earth's

equator onto the celestial

sphere. Just as on the surface of

the The celestial equivalent to

Earth latitude is called

“Declination,” or simply “Dec.” It

is measured in degrees, minutes

or seconds north ("+") or south

("-") of the celestial equator. Thus

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.

Earth’s

Rotation

0 Dec.

South

Celestial

Pole

Right Ascension

Star

Celestial

Equator

-90 Dec.

+90 Dec.

North

Celestial

Pole

(Vicinity

of Polaris)

Fig. 31: Celestial sphere.

Polaris

Little Dipper

Big Dipper

Cassiopeia

Fig. 32: Locating Polaris.

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Meade 4504 Instruction Manual Page 40

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