As I prepare to pack for Germany, I'm going through the hand-outs I use when teaching writing. One of these is a nifty monograph by Poul Anderson and Stephen L. Gillett called "How To Build a Planet." It starts with the Hertzsprung-Russell diagram of stars, with instructions on choosing one likely to have viable planets, and then goes through how to figure out such planetary factors as gravity, axial tilt, energy received per square foot, etc. These, of course, translate into seasons, weather, plausible biomass, and everything else that makes a believable world on which to set an SF story.
Poul Anderson published the first version of this article long ago in the SFWA Bulletin. Many authors wrote the Bulletin to express gratitude for the article. One person, however, sent the following letter, which was included in the next issue:
Dear Mr. Anderson,
That is not the way I do it.
Well, maybe not. But since God's methods seem unavailable to most writers, we do it any way we can. The updated Anderson/Gillett article is a great help; I recommend it. And it doesn't even take seven days and seven nights.