The "OpenGL Vertex Program" does not show "Ambient Light"
To word it another way, if "Ambient Light" is turned on (e.g. set to "Medium") and if the "Render path" is set to be "OpenGL Vertex Program" (the 4th of the 5 paths available) then the object being viewed is illuminated as if "Ambient Light" were set to "None": the night side of a planet is bkack instead of lit. The other 4 render paths do light up the dark.
Here is a (link to a larger) picture of Mars using the 3rd render path, "NVIDIA Combiners":
Here is a (link to a larger) picture of Mars using the 4th render path, "OpenGL Vertex Program":
Notice how much darker the right side of the planet is in the second picture.
And here is a Cel:// URL to put Mars into this orientation:
Martian Volcanoes
You'll have to type Control-V to step through the rendering paths.
Images taken of Mars at this viewpoint using all 5 rendering paths can be found at http://www.lns.cornell.edu/~seb/celestia/gallery-001c.html#6ab
This problem is seen on this system:
256MB, 500MHz P3, WinXP Pro Sp1
Nvidia Geforce4, Ti4200, Detonator 41.09 (v6.13.10.4109)