Meade StarNavigator 90mm Instruction Manual - Page 40

APPENDIX A Celestial Coordinates It is helpful to understand how to locate celestial objects as they move across the sky. 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 EastWest direction, parallel to the Earth's equator. Similarly, imaginary lines have been drawn to form a latitude and longitude grid on the celestial sphere. These lines are known as Right Ascension and Declination. North Celestial Pole (Vicinity of Polaris) 1 +90 Dec. Star Declinatio 17 16 15 14 13 Earth's 12 11 10 18 19 Rotation 20 21 22 23 012 Right Ascension South Celestial Pole -90 Dec. 98 7 6 5 3 4 Celestial Equator 0 Dec. 2 Fig. 30: Celestial Sphere. 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. 30) is that point in the sky where an extension of the North Pole intersects the celestial sphere. This point in the sky is located very near the North Star, Polaris. The celestial equator (2, Fig. 30) 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 constellation Ursa Major (the Big Dipper) by its Right Ascension (11hr) and its Declination (+50°). • 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 constellation 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 (Altitude): This celestial version of latitude is measured in degrees, minutes, and 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 the constellations of Orion, Virgo, and Aquarius) is said to have a Declination of zero, shown as 0° 0' 0." Locating the Celestial Pole To get basic bearings at an observing location, take note of where the Sun rises (East) and sets (West) each day. After the site is dark, face North by pointing your left shoulder toward where the Sun set. To precisely point at the pole, find the North Star (Polaris) by using the Big Dipper as a guide (Fig. 31). IMPORTANT NOTE: For almost all astronomical observing requirements, approximate settings are acceptable. Do not allow undue attention to precise alignment of the telescope to interfere with your basic enjoyment of the instrument. Little Dipper Polaris Big Dipper Cassiopeia Fig. 31: Locating Polaris. Looking at or near the Sun will cause irreversible damage to your eye. Do not point this telescope at or near the Sun. Do not look through the telescope as it is moving. 38