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'+2356m' to advance the current date/time by 2356 minutes;
'-63s' to back up the current date/time by 63 seconds;
'J2450540.321' to set the current date/time to JD 2450540.321

   Most people outside the US use dots instead of '/'s to separate day,
month,  and year;  so this option will also accept '13.6.1987' and
similar text,  in place of '13/6/1987' and similar text.
                                                           
   By default,  Guide works on "fixed levels":  for example, one goes
from a field of view of 20 degrees (at level 4) to one of 10 degrees (at
level 5),  with no intermediate step.  You can't access a "level 4.5,
15-degree" field of view; the best you can do is to change level 4 or 5
to be equal to 15 degrees,  using the Set Level Size option in the
Settings menu (p. 29).

   The way to get around this clumsiness is to turn Fixed Levels off.
When you do this,  dragging a box open on the screen lets you choose a
continuously varying field of view,  instead of jumping from one level
to another.  You can still "go to level 4" to get a 20-degree field of
view,  and other aspects of zooming in and out are unaffected. The only
real change is that you can get those intermediate fields of view.
                                                     
   The "Line of Variation" function is a very specialized option, added
for use by people trying to recover comets and asteroids with
poorly-determined orbits.  In most such cases,  the object won't be
recovered exactly at the predicted position;  instead, it will be found
on a line passing through the predicted position, called the line of
variation (LOV).

   To show this,  click on the Line of Variation option. Guide will ask
for the length of the line in days; a starting value of one day is
usually a good idea.  (This corresponds to a guess that the object may
be one day "ahead of prediction" or "behind prediction".) Guide will
display a one-day LOV for all asteroids and comets on the screen,
indicating the set of points where they would most probably be found.

   To turn it off,  return to this menu option and enter an LOV length
of zero days.
                                  
   The "Toggle user datasets" opton is slightly misleading.  In reality,
Guide already contains a number of datasets in the user dataset system,    
including catalogs of galaxies,  quasars,  radio objects,  nearby stars,     
the Palomar survey plates,  and so forth.  These were originally
intended as examples,  but they also can be of use by themselves.

   Hitting "Toggle user datasets" brings up a list of these datasets,
plus any you may have added using the instructions on page 63
or by downloading data from the Project Pluto Web site.  You can click
on those you wish to change.  You'll get a small dialog box where you
can turn the dataset on or off,  change its color,  set a magnitude limit,
and so on.

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