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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.

   Two items need special mention:  the "magnitude parameters". These
are almost always listed with asteroid elements; sadly,  people are
often sloppy about providing the magnitude parameters for comets.  One
method to find the absolute magnitude parameter is to enter a "guess"
of,  say, 10. Then return to Guide and click on the comet; if it is
(for example) 4.5 magnitudes brighter than you would expect (either
from observation or a provided ephemeris),  then return to editing the
comet's data,  and add 4.5 to your initial "guess" for the absolute
magnitude parameter.

   The above process is admittedly quite barbaric.  One alternative is
to send e-mail or call any of the technical support numbers, listed on
page 2,  and ask for data for a particular comet.

   Also,  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.


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