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Animation Dialog and start animating.  The effect of this option will
be immediately obvious:  while animating,  the moving target you
selected (Jupiter) stays at the center of the screen.  In RA/dec mode,
watching Jupiter's moons in animation can be annoying,  because the
planet wanders off the screen; but the Moving option forces a "moving
target",  such as a planet, asteroid,  comet, or satellite,  to stay at
the center of the chart.

   One drawback will be immediately apparent:  if Jupiter is to stand
still, stars have to be redrawn at each step.  That takes some
computing power,  and the results can be somewhat jerky.  (This is also
true of the remaining two options.)

   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.

   To demonstrate the "Proper Motion" option,  click "Go To... Nearby
Star",  and select Barnard's Star.  Zoom down to Level 6 (five-degree
field of view). Increase the animation step size to about 8 years/step.
Click on the "proper motion" radio button,  and start animating.

   In this particular field,  Barnard's Star is the only one with really
large proper motion.  You'll see it slowly drift to the north,  at about
10 arcseconds/year.

   Using this option for wider fields usually requires a much larger step
size to get much in the way of visible motion.  This is just another way
of saying that most stars are quite close to being "fixed".
                                                                 
   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

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