```

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.
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,  use the Settings... Formats
dialog (described on page 31.)

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.

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