Meade LX90GPS instruction manual Polar Alignment, Celestial Coordinates

APPENDIX A: EQUATORIAL (POLAR) 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 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 (1, Fig. 28) 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 (1, Fig. 28). The celestial equator (2, Fig. 28) 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 longi- tude, 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 con- stellation 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 intervals 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, arc-minutes, and arc-seconds (e.g., 15° 27' 33"). Dec. locations north of the celestial equator are indicat- ed 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."

North
Celestial				+90		Dec.
Pole								Star
(Vicinity
of Polaris)							e
						D
							c
1							l
							a
							i
							n
								t
								i
		14	13	12		11	10	o
	15							n
17 16								9	8
	Earth’s									7
18	Rotation										6
19									4	5
20	21						2	3			Celestial
		22	23		0	1
											Equator
Right Ascension
											0 Dec.
South										2
Celestial				-	90	Dec.
Pole

Fig. 28: Celestial Sphere.

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