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   Next,  you'll get a menu allowing you to adjust and save all the
elements labeled above.  Because orbital data tends to confuse the
uninitiated (and,  at times,  the initiated), there is a paragraph or
two provided on the screen to clue you in to what the variable in
question means.

   After entering all the elements,  you can save the results, and
Guide will add that comet or asteroid to its list.  Keep in mind that
the new object obeys the same rules as any comet:  if you have comets
turned off in the Data Shown dialog,  or if the comet is not brighter
than the limiting magnitude for comets, it will not show up on the
chart.

   Be aware that a particular set of elements is good for a limited time
only (usually a few months around the epoch). Over time, gravitational
effects of other planets will change the elements and, therefore,  the
object position.  After a few months,  if the object doesn't come too
close to a planet, errors of a few arcseconds will accumulate;  over a
few years, the errors will grow to a few arcminutes.

APPENDIX G:  ASTRONOMICAL MAGNITUDES         

   The creator of the first star catalog,  Hipparchos,  was also the
creator of the system used to measure the brightness of stars.  He
assigned a magnitude to each star:  the brightest were magnitude 1
stars,  those slightly fainter magnitude 2, and so on,  down to
magnitude 6.  This was based strictly on looking at the star and
guessing how bright it was.

   The invention of the telescope revealed objects fainter than mag 6,
so the scale was extended to higher numbers.  It was eventually decided
that five magnitudes should represent a change in brightness of a
hundred-fold;  that is,  a mag 2 star should be 100 times brighter than
a mag 7 star.  This also means that each magnitude represents a 2.512
fold change in brightness. This is a somewhat unwieldy number,  but
we're stuck with it now.

   This system means that one can measure fractions of a magnitude
(Polaris,  for example,  is a mag 2.02 star) and that really bright
objects can have negative magnitudes (Sirius is mag -1.58, Venus can be
as bright as mag -4.4,  the Sun is magnitude -26). It is still true,
however,  that most humans cannot see an object fainter than mag 6,
though this limit varies among humans.

APPENDIX H:  COPYRIGHT AND LIABILITY NOTICES       

   This program and data are copyrighted (except when otherwise
specified) @1993-2011 by Project Pluto.  They are distributed under
the GNU General Public License.  See the file 'license.txt' in the
root folder of the DVD for details.  Note that not all source code is
available yet,  though it is hoped that this will gradually be fixed.
Please let us know if particular parts would be useful to you.

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