​For this story, I imagined a specific setting: an earth-like planet, with a large, ruddy moon dominating the sky. The moon, and the mythical figure that it represented, played a central role in life on this planet. To a visitor, the red light pervading the sky would emphasize the strangeness of the world. To the inhabitants, the stately 40-day phases of the moon measure out their time.

It's all very well and good to imagine such a place, but can it really exist? We can turn to the basic laws of motion worked out by Kepler for some answers.