depth/alpha problem, perhaps

I’ve got a problem which i’m sure i’ve had and solved before, but i can’t remember how…!
I’m rendering a green sphere with no lighting. When i use a gluPerspective projection, i get a slightly lighter green elipse on the surface of the sphere. When i do an orthagonal projection looking from above (i.e. looking down the negative y-axis in GL coords), the sphere appears half dark-green half light-green.
I’m pretty sure i’ve had the is problem before but i can’t remember how i solved it… I’ve a feeling it might be something to do with alpha blending or something, but i could be totally wrong.
Any suggestions from someone who recognises these symptoms would be appreciated!
Thanks in advance,