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   The file is available in two forms:  MPCORB.DAT and
MPCORBCR.DAT.  One is intended for DOS/Windows,  the other for
Unix, but Guide will use either.  If one of these files is in the
Guide directory,  the "Use MPCORB" option will no longer be grayed
out.  Toggling it on will cause Guide to use the elements in MPCORB,
while ignoring the asteroid data built into the Guide disk.

   Switching to MPCORB has advantages and disadvantages,  and it's
important to understand both.  The data on the Guide disk includes
a lot of pre-computed information regarding where each asteroid will
be over a given time and how bright it will get;  this allows Guide
to display asteroids almost immediately (as long as your "location"
is on Earth).  Elements are provided covering a range of several
decades,  so perturbations are modelled with good accuracy over that
entire range.

   But,  of course,  the "built-in" asteroid data can't be rewritten.
You're stuck with the data as it appears on the CD-ROM,  and newly
discovered asteroids (and orbits updated based on new data) will not
be included.  This is where MPCORB has a great advantage;  the MPC
updates it almost daily.

   People engaged in asteroid discovery and astrometry (measurement
of positions) will sometimes find MPCORB to be useful,  or even
essential,  and will accept the fact that it slows Guide down immensely.
Such people are also usually concerned only with "current" information,
so the fact that perturbations are ignored when MPCORB is in use will
not matter much to them.

   Most people,  though,  will instead stick with the built-in asteroid
data in Guide.  And when dealing with dates a year or so into the past
or future,  the fact that the built-in data accounts for perturbations
will usually mean that it is more accurate than MPCORB anyway.
                                                            
   In the "Enter Time" option (hotkey Ctrl-F9) you may set the time
"directly", with the keyboard. Guide accepts times in any of these formats:

'13/6/1987' to reset the date to 13 Jun 1987;
'13/6' to reset the date to 13 Jun of the current year;

'16:45:02' to set the time to 16:45:02 of the current day;
'16:45' to set the time to 16:45 of the current day;
'13/6 16:45:02' to reset the time to 16:45:02 on 13 Jun of
the current year;

   Any similar combination of the first two cases (resetting day)
with the next two cases (resetting time of day);

'+27.3' to advance the current date by 27.3 days;
'-10.4h' to back up the current date/time by 10.4 hours;

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