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It's similar to what happens with a top; you've probably noticed that
while the top spins rapidly, it also has a slower,  "wobbling" motion.
This motion makes a RA and declination alone are slightly ambiguous;
you also need to know the year for which that position is valid.  That
year is called the epoch.

   Most catalogs are in a "standard" epoch.  Standard epochs are
separated by 50 years;  some catalogs are still in the B1950.0 epoch,
while most have been switched to J2000.0.  If you read about the
position of an object in a book or magazine,  make sure you also get
its epoch.  This is especially important for dim objects. If you
mistakenly looked for an object as dim as Pluto,  say, in a J2000.0
position when it was really given as B1950.0,  you would have no hope
of finding it.  (The difference is usually around half a degree.)

   By default,  Guide shows you positions and accepts positions in
J2000.0.  If you wish to change this,  click on the epoch shown in
the legend,  or hit Alt-E.

  You can set the epoch in which grids,  ticks,  hatches,  and/or side
labels are shown separately;  there is a menu item in the Spacings menu
that lets you do this.

   A brief note:  You may wonder what the "B" before "1950.0" and the
"J" before "2000.0" mean.  The answer is:  in terms of finding
something,  not much.  In 1950,  epochs were measured from the start
of the Besselian year,  which is 365.2421988 days long.  In 1984,
the International Astronomical Union decided to switch to the start
of Julian years,  which are exactly 365.25 days long.  The actual
difference in the sky is always well under .1 second of arc,  and you
can usually ignore these prefixes with very little harm.  With the
exception of Hipparcos and Tycho stars,  and of planets and natural
satellites in "full precision" mode,  no object in Guide can claim
positional precision of .1 second or better.

APPENDIX C:  ALTITUDE AND AZIMUTH EXPLAINED  

   You will note that all objects,  when clicked on,  show times of
rising and setting,  plus their "alt"  and "azim". This refers to the
object's "altitude" and "azimuth",  which tell you where in the sky to
look for an object.

   Altitude and azimuth are not reckoned from either the Earth's poles
or the celestial poles.  They are reckoned from the observer's
position:   the point straight overhead has an altitude of +90 degrees;
that straight underneath, an altitude of -90 degrees.  Points on the
horizon have 0 degree altitudes.  An object halfway up in the sky has
an altitude of 45 degrees.

   Thus,  looking at altitude tells you immediately if the object is
even visible.  A negative altitude means the object is below the

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