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Message: <blockquote data-quote="fuindordm" data-source="post: 6990307" data-attributes="member: 5435">Any departure from real-world physics is going to require magic to do some heavy lifting to keep the world in place. This is not a problem, and I think you have the right idea.The ancient Greeks did debate the question of whether the Earth went around the Sun or vice versa. The proof they looked for was parallax in the stars: if the Earth goes around the Sun, they reasoned, then they should be able to observe the apparent position of the stars changing slightly due to the different viewing angle. Since they could observe no parallax, they (that is, the majority of philosophers arguing about such things at the time) concluded that the Earth was fixed. More precisely, they calculated a minimum distance for the stars that would make the parallax unobservable. This number was so much larger than the diameter of the Earth that they thought it nonsensical. Of course, modern telescopes are able to observe parallax to the closest stars, and now we have come to terms with the scale of the universe, but I feel sympathy for the Greeks who balked at the idea.The answer to your question depends also on how complicated you want the sky to be. A sunlike moon orbiting the Earth could indeed keep it warm. You could explain seasonal variations with an elliptical orbit as you suggest, or as a regular fluctuation in the sun's output, like a beating heart.  Are there other planets in the sky? A regular moon? A bright Jupiter roaming the sky, or a Venus-like evening/morning star? A blood-red Mars? The scientists of your world can easily verify that they are the center by observing the phases of the other celestial orbs as they change their position with respect to the Sun; indeed, the first nail in the coffin of the heliocentric model in our world was Galileo's observation of the phases of Venus.So there you have it--the key observations available to middle-age astronomers are parallax (if the Earth is moving) and phases (whether or not it is moving). What they are probably most interested in, however, is predicting the movements of any other planets across the backdrop of fixed stars. But maybe your world doesn't have stars--maybe the sky is completely black except for the moon, or maybe the backdrop is a gorgeous nebula or galaxy, or an iridescent glow like the inside of a clamshell. Enjoy making your setting!Cheers,Ben</blockquote>

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