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APPENDIX A: RIGHT ASCENSION AND DECLINATION Right ascension (RA) and declination are the celestial versions of longitude and latitude on the Earth. The celestial versions of the poles are found by taking the Earth's axis of rotation (which, of course, passes through the North and South poles) and extending it into infinity. The celestial equator is found by extending the Earth's equator into infinity, projecting it onto the sky. On Earth, latitude is a measure, in degrees, of how far one is from the equator. Thus, the north pole is at +90 degrees; the south pole is at -90 degrees. This translates readily to the concept of declination in the sky: Polaris, the North Star, is at (close to) declination +90. You can express a declination as you would any angle: in decimal degrees, in degrees plus decimal minutes of arc, or in degrees, minutes, and decimal arcseconds. (The format Guide uses in displaying declination, and RA, can be specified; see page 31.) RA is similarly analogous to longitude. The celestial version of the Prime Meridian is the Sun's location at the vernal equinox (the place where it crosses the celestial equator near 22 Mar). RA is measured from this point, all the way around the sky until the vernal equinox is reached again. There are two key differences between RA and longitude. In the first place, longitude is (usually) measured from -180 to +180 degrees. RA is never negative; it's similar to measuring from zero to 360 degrees (which is how some people do measure longitude.) The second difference is that instead of being measured in degrees, RA is measured in units of time. The entire "distance" around the sky is not 360 degrees, but one day. A fraction of this distance can be measured in hours, minutes, and seconds. As with declination, this can be either decimal hours, hours and decimal minutes, or hours plus minutes plus decimal seconds. Guide will always accept input in any of these forms, and will figure out which method you used and thereafter show all positions in that format. There is one twist on this situation. If you find the latitude and longitude of a point on the Earth, those values won't change significantly over time. The North Pole stays under the Arctic icecap, and Greenwich, England doesn't move. The situation is not so tidy for RA and declination, as described in the following appendix. APPENDIX B: PRECESSION AND EPOCHS EXPLAINED If all you wish to do is to look at stars, or possibly find out what that bright planet you see each night at dusk is, you can probably skip this Appendix. If you need fairly precise positions, or wish to match a chart printed in Guide with one from another source, you probably should know the following: The positions of stars and planets are usually described in terms of right ascension (RA) and declination (see the preceding appendix). One
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