Meade LX80 instruction manual Appendix D, Polar Alignment, Celestial Coordinates

APPENDIX D:

EQUATORIAL (POLAR) ALIGNMENT

Alignment)

Polar Alignment

In Polar Alignment, the telescope is oriented so that the horizontal and vertical axes of the telescope are lined up with the celestial coordinate system.

In order to Polar align your telescope, it is essential to have an understanding of how and where to locate celestial objects as they move across the sky. This section provides a basic introduction to the terminology of Polar-aligned astronomy, and includes instructions for finding the celestial pole and for finding objects in the night sky using declination and right ascension.

Celestial Coordinates

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

North
Celestial				+90	Dec.
Pole							Star
(Vicinity
						D
of Polaris)
						c
						e
1						l
						a
						i
						n
							t
							i
		14	13	12	11	10	o
	15						n
17 16							9	8 7
	Earthís
18	Rotation								6
19								4	5
20	21					2	3		Celestial
		22	23	0	1
									Equator
Right Ascension
									0 Dec.

2

South

Celestial

Pole -90 Dec.

Figure 46: Celestial Sphere

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 (Fig.

46, 1) 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 (Fig. 46, 1). The celestial equator (Fig. 46, 2) 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 (M57) by its right ascension (18hr) and its declination (+33°).

•Right Ascension (RA): 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. RA coordinates range from 0hr 0min 0sec to 23hr 59min 59sec. There are 24 primary lines of RA, located at 15-degree intervals along the celestial equator. Objects located further and further East of the zero RA grid line (0hr 0min 0sec) carry higher RA coordinates.

•Declination (DEC): This celestial version of latitude is measured in degrees, arc-minutes, 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 constellations of Orion, Virgo and Aquarius) is said to have a declination of zero, shown as 0° 0’ 0

Appendix D: Equatorial ( Polar

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