Looking at Stars in 3D
		     by Robert J. Chassell
       
Spacechart displays a window onto what amounts to a globe or sphere
which contains stars and whose radius is the view_radius.

The stars within the sphere are located according to their right
ascension, declination, and distance from Sol.

You can rotate the globe using various keys.

(NOTE: (by Miguel Coca) Another way to look at this is thinking of
yourself moving around the sphere, while the stars remain static)

Your point of view as an observer is from the surface of an imaginary
globe that is larger than the one in the display.  Your coordinate
position is as if you were sitting on this surface, looking at Sol (or
looking at whatever is the center of the display-- for this
discussion, we are assuming the center is Sol).

Points on the surface of the globe are located by their Right
Ascension and Declination as measured from the point of view of Sol.

Note, however, that spacechart converts the hours, minutes, and
seconds of Right Ascension to degrees of arc, and refers to this
measure as longitude.

Thus, when the star Sirius is in front of Sol, it is as if I am
looking up at the globe from an Declination of -16 deg and a Right
Ascension of 6 hours 43 minutes (or approximately, 101 degrees) (which is
the Right Ascension and Declination of Sirius).

In the spacechart display, user coordinates are shown in a box in the
lower left.

The user coordinates show you two different pieces of information:

  * The Longitude and Latitude coordinates are the value of Right
    Ascension and Declination of your point of view, looking inwards,
    measured in degrees.  That is to say, if a line were drawn from
    Sol to your eye, the line would have the Longitude and Latitude
    coordinates shown.

  * The direction of North is the degree to which the North pole is
    tipped from being on top, the tipping being along a line 90
    degrees from where you are looking -- along the `edge' of the
    globe, as it were.  The measure tells you how much the globe of
    stars at which you are looking is tipped on its side.

Put another way, the direction from which you are looking -- the
position on the globe which you are sitting, looking inward -- is
stated by the Longitude and Latitude user_coords.

When the direction of North is zero, the North Pole of the globe is at
the top of the screen.  When you increase the value of the north
coordinate, the globe you are looking at tips on its side.  It always
tips so that the North Pole moves along a line that is 90 degrees from
the longitude on which you are sitting, looking inwards.  

Thus, if you increase the value of the the north coordinate from zero
to 60 degrees, the glove tips on its side 60 degrees, so that the
North Pole is now at what used to be a latitude (or declination) of 30
degrees.

If you then rotate the display left or right, the Longitude and
Latitude user_coords will change

If you increase the value of the the north coordinate to 90 degrees,
the globe is, as it were, lying on its side.

In this case, when you rotate the display left or right, the Latitude
changes, but not the Longitude.  This is because the globe is lying on
its side and your point of view is moving left or right, which means
along a line of the same longitude, changing only its latitude.