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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 30.)

   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
difficulty with using this system is that RAs and declinations are in
constant change over time.  The position of an object expressed in 1950
coordinates may be .7 degrees different from its 2000 coordinates.

   The reason for this is that declination and RA are measured from
the Earth,  and the Earth doesn't stand still.  The axis it rotates on
slowly describes a circle in the sky,  completed once every 25,800
years.  This is why Polaris won't always be and hasn't always been the
"North Star".  This slow motion of the Pole is called precession.

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