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the planet wanders off the screen,  much like a telescope without a
telescope drive; but the Moving option forces a "moving target",
such as a planet, asteroid,  comet, or satellite, to stay at the
center of the chart.

   To see how the "Horizon" option works,  go to a Level 4 (20-degree)
field of view.  Click on "Go To... Horizon" and select "NE" (move to
Northeast horizon).  Go to the Inversion Dialog and select Zenith Up.

   You will see the horizon as a dark blue line near the bottom of the
chart (and/or as a filled-in ground area,  or with objects such as
trees, if you have selected those options in the Background dialog).
Go back to the Animation Dialog,  set a 5-minute stepsize,  click on
the "Horizon" radio button,  and start animating.

   As the name suggests,  the horizon will now stay fixed while
everything else moves (stars rise above the northeast horizon).  In
general, the idea is that if you go to a particular altitude and
azimuth (in this case,  about 8 degrees above the horizon and at
azimuth=45 degrees),  that point stays fixed while stars,  planets,
and so forth rotate by.
                                                                 
   Next in the Animation menu are the "Add a Trail" and "Make
Ephemeris" options.  The process for adding a trail may take some
practice.  Let's take an example.  Suppose you would like to create a
trail (or an ephemeris) describing the motion of Mars starting at 10
Nov 1993,  and running for 100 days after that.  You need to make sure
that the time (in the Time Menu) is set to 10 Nov 1993,  and that your
location is set correctly (in the "Location" option in the Settings
menu).  Then click on Mars with the RIGHT mouse button;  this will
immediately result in the usual brief summary of data concerning Mars.

   When you click on a solar system object,  the Add A Trail and Make
Ephemeris options are not gray any more:  you can make a trail or
ephemeris for that object now.  If you click on a non-solar system
object, you can't make a trail or ephemeris,  and these two options
remain grayed out.
                                                                       
   When you click on "Add a Trail",  you'll see this dialog box:

Step size:   [ 10 min]
[x] Round to nearest step
[x] Index marks
Index freq     [ 2     ]
[x] Time labels on
Time label freq [  6   ]
Number of steps [ 100  ]
Add a Trail...
Clear Trails    Cancel

   With the above settings,  you would be telling Guide to show the

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