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manual abstract
(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.). Right Ascension and Declination is analogous to the Earth-based coordinate system of latitude and longitude. Understanding celestial coordinates: Celestial objects are mapped according to the R.A. and Dec. coordinate system on the “celestial sphere” (Fig. 2), the imaginary sphere on which all stars appear to be placed. The Poles of the celestial coordinate system are defined as those 2 points where the Earth's rotational axis, if extended to infinity, North and South, intersect the celestial sphere. Thus, the North Celestial Pole is that point in the sky where an extension of the Earth's axis through the North Pole intersects the celestial sphere. In fact, this point in the sky is located near the North Star, or Polaris. 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 direction, parallel to the Earth's equator. The celestial equator is simply a projection of the Earth's equator onto the celestial sphere. Just as on the surface of the Earth, imaginary lines have been drawn on the celestial sphere to form a coordinate grid. Celestial object positions on the Earth's surface are specified by their latitude and longitude. The celestial equivalent to Earth latitude is called “Declination,” or simply “Dec,” and is measured in degrees, minutes and seconds north ("+") or south ("-") of the celestial equator. Thus any point on 1415 16 17 18 19 20 21 22 23 0 1 12 11 10 9 8 7 5 6 4 3 2 13 Rotation de la Terre 0° Dec. Ascension droite ° ° Declinaison Fig. 2: Celestial Sphere. North Celestial Pole (Vicinity of Polaris) +90° Dec. Star Celestial Equator -90° Dec. South Celestial Pole – 9– the celestial equator (which passes, for example, through the constellations Orion, Virgo and Aquarius) is specified as having 0°0'0" Declination. The Declination 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.” and is measured in hours, minutes and seconds from an arbitrarily defined “zero” line of R.A. passing through the constellation Pegasus. Right Ascension coordinates range from 0hr 0min 0sec up to (but not including) 24hr 0min 0sec. Thus 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 prime (0h 0m 0s) Right Ascension grid line carry increasing R.A. coordinates. With all celestial objects therefore capable of being specified in position by their celestial coordinates of Right Ascension and Declination, the task of finding objects (in particular, faint objects) in the telescope is vastly simplified. The setting circles, R.A (27, Fig. 1) and Dec. (28, Fig. 1) of the Polaris 114 EQ-D telescope may be dialed, in effect, to read the object coordinates and the object 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 North 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 Souther...