There is some convention? how is this 3d to 2d mapping done?
here an example
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The ellipse is a particular way to draw the surface of a sphere - like the sphere of the skies around us, or the surface of the Earth - on a flat piece of paper or screen (because of the curvature of the sphere, it cannot be "flattened" without distortions). This one is called the Mollweide projection and it preserves the areas (and completely sacrifices correct representation of the angles):
The second link enumerates many other ways how to draw the spherical surface.