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(Non-solar system objects are not affected.  The parallax effect is
ignored for them.)

   The Moon can be shifted by up to 2 degrees if the wrong lat/lon
is used.  Asteroids and comets passing very near to the Earth can
be still more seriously affected. Planets are usually not affected
by more than a few arcseconds.

   TIME.  If the time is 13 days off from its expected value,  make
sure you are using the Gregorian calendar. Check the Time Zone setting.
Remember that Guide uses a 24-hour clock;  8:13 PM becomes 20:13.
(This is one reason it's a good idea to have the time and time zone
displayed in the legend area,  as they are by default.)

   Finally,  it's worth considering the precision of the source used
for the "expected" value.  Perhaps the most common source of error is
the assumption that some software has generated a "precise" value,
and that any difference must be due to errors in Guide.  It may be
flatly stated that,  properly used,  Guide will indeed generate data
to the precisions listed in the next section.

APPENDIX E:  NOTES ON ACCURACY      

   The subject of the accuracy of any astronomy software is a very
complex one. The accuracy of positions shown by Guide varies from
about a thousandth of an arcsecond (for stars in the Hipparcos
catalog) to being as poor as several degrees (for some asteroids whose
orbits have not been thoroughly studied,  or artificial satellites
that have maneuvered or haven't been updated recently).  For moving
objects, the accuracy is also a function of time; that of positions
of planets within a century of the year 2000 is of the order of
milliarcseconds, but probably no better than arcminutes for very
distant dates.

   There is a very thorough description of accuracy at

http://www.projectpluto.com/accuracy.htm

   These describe what level of accuracy can be expected for
various data in Guide,  how that level was determined,  and what
factors can affect that level of accuracy. If your sole need is
to be able to find objects visually or telescopically,  then the
accuracy given by Guide will always be far more than sufficient.

APPENDIX F:  ADDING NEW COMETS               

   Guide is shipped with a fairly complete list of comets and with a
set of over 158,000 asteroids.  New comets and asteroids are always
being discovered,  and in some cases, new observations allow the
precision of existing orbits to be improved.  Guide will be fairly up
to date on comets when you receive it;  as new ones are discovered,
they are added to COMETS.DAT.  Guide will also show some future

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