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Enter location name
[ ] Use geocentric position
 Humidity: 20% 
 Temperature: 20 C
 Pressure: 760 mm Hg
                                                            
   The very first item tells you from where you are looking at the
sky. If you click on it, you will get a long list of Solar System
objects. You can select one, and after that, all planets,  moons,
and so on will be drawn from that planet (or satellite). You can see
what Jupiter and its moons look like from one of Jupiter's moons, or
what the Earth looks like from the Moon or other planets. (The view
of the inner moons of Saturn as viewed from Japetus is particularly
recommended.) This can be very interesting and educational, although
most Guide users cannot travel outside Earth and near-Earth orbits.

   Most Guide users can set their viewpoint on the Earth with the "Enter
location name" button.  Click on this,  and enter the name of your
city or town,  and Guide will (usually) figure out the corresponding
latitude and longitude.  In the US,  this can be a name plus two-letter
state abbreviation:  "Augusta, ME",  or "Houston, TX";  or it can be
a five-digit postal (ZIP) code,  such as "04008".  Outside the US,  it
can only be a name,  and unfortunately,  the number of recognized cities
is not very great.                               
                                                 
   So most people will have to enter a latitude/longitude position.
This can be found on most maps,  or with a GPS receiver.  You usually
do not need to be nit-pickingly precise about this. Guide uses your
place on the earth to calculate rise and set times and to get better
accuracy on planetary positions. (For example,  a solar eclipse visible
on one part of the earth may not be visible at another, because the
moon isn't exactly in the same spot in the sky.) An accuracy to a
degree (meaning about 111 kilometers or 70 miles) will get rise and set
times to within about four minutes.  But if you want to get good
positions for artificial satellites,  or times for eclipses and
occultations,  you will want an accuracy of at least one kilometer.

   Systems of latitude and longitude are also defined for other planets.
The center of the visible side of the moon (as seen from Earth) is
defined to be at latitude 0,  longitude 0.  If you select "Luna" as your
home planet,  and set those values, the Earth will be shown very near
the zenith.  (It wanders a bit around that point,  due to librations,
as described on page 15.)
                                                   
   The next item,  "Alt 100 meters",  tells you that your point of view
is 100 meters above sea level.  Once again,  you need usually be only
approximately correct here.

   Clicking on the "Use geocentric position" causes Guide to ignore the
lat/lon/altitude values;  instead,  your viewpoint will be from the center
of the earth (or whatever home planet you have selected).

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