Yes, your approach yields a result that looks like what I want. I didn't say so earlier, because your code had options I'm not familiar with, so I was waiting til I had time to evaluate it. Now that I see what it evaluates to, that is what I'm after. Thank you.

And thank you for the additional link to Stack Exchange.

Thanks. That looks like what I'm after. I'll study your code.

I'm trying to make a movie about planets interacting under gravity, but when I try this with Mathematica, the movie show the planets getting chopped in half and then disappearing when they leave the boundaries of a "bounding box". I don't want the planets to suddenly disappear. Is there any way to solve this problem? This is not supposed to be a movie about planets getting chopped in half or disappearing.

Thanks. I have done that now, but it has not solved the problem of the unwanted clipping.

Thanks everyone for the advice. I'll see if I can get the results I'd like.

Maybe the option SphericalRegion->True could be of help

Manipulate[
    Graphics3D[
     {
        Sphere[{-Cos[\[Theta] \[Degree]] sep, 0, -Sin[\[Theta] \[Degree]] sep}],
        Sphere[{0, 0, 0}], 
        Sphere[{Cos[\[Theta] \[Degree]] sep, 0, Sin[\[Theta] \[Degree]] sep}]
        },
     Axes -> False, PlotRange -> {{-2, 2}, {-1, 1}, {-1, 1}},
     Boxed -> True, SphericalRegion -> True 
    ],
    {{\[Theta], 0, "Angle around Y"}, -180, 180, Appearance -> "Labeled"},
    {{sep, 1, "Separation"}, 0, 3, Appearance -> "Labeled"}
 ]