"I suppose I could make a sphere with the HDR image as a texture around the scene but that seems like a poor man's workaround."

Actually, this is the sophisticated solution, not the poor man's workaround. You can parent a light (for the sun) to the sphere and then rotate it, keeping the light in sync with however you want it spun to produce the effect you want. For Sci Fi you can parent more than one light or a planet or whatever. You can use a more sophisticated shader (which I provide) to do special effects, like turn day into night, or clear weather into a storm.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)