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Message: <blockquote data-quote="woodelf" data-source="post: 1836386" data-attributes="member: 10201">Well, assuming no further changes (most of what i'm about to list could be "compensated for" if you wanted to):  <ul> <li data-xf-list-type="ul">If everything is in the same plane, you'd have more-frequent [partial] eclipses; if not, you'd never have them (whether the eclipses are full or not depends on the relative sizes of the bodies, and their distances) </li> <li data-xf-list-type="ul">Seasons are caused by the relative angle of the earth's axis and the solar ecliptic changing (due to the orbit), so unless either the sun's orbit wobbled, or the planet wobbled, you wouldn't have seasons. You could give the sun a highly-eccentric orbit (much moreso than most things in our solar system) to compensate for this, of course--or just have seasons happen for reasons other than celestial mechanics, as in most world mythologies (Demeter and Persephone come immediately to mind). </li> <li data-xf-list-type="ul">No retrograde motion. The perception of retrograde motion of the other planets comes from the movement of Earth in its orbit, relative to their orbits. So, unless you introduce epicycles (as Aristotle, Ptolemy, and others did), the planets will always move in one direction in the sky. </li> <li data-xf-list-type="ul">Similarly, planets will always be equally-bright, because their distance won't change. </li> <li data-xf-list-type="ul">No precession of the moons' or planets' orbits. (This is the factor that turns the Moon's 26.something-day orbit into a 28-day cycle as seen from Earth.) </li> <li data-xf-list-type="ul">Relating back to the tilt-of-the-Earth thing, the sun will always rise the same amount north every day of the year, instead of slowly progressing from the Tropic of Capricorn to the Tropic of Cancer, and back again. </li> <li data-xf-list-type="ul">Unless you introduce a rotating "sphere of the heavens" or somesuch, the same stars will be visible every night from a given location, instead of changing over the year.</li> </ul> I think that's all the major stuff--there's lots of little stuff that a good astronomer might be able to pick up, but the above would be noticed by just about anyone, i'd think. Of course, without the frame of reference of our Earth for comparison, they'd not think anything of it.</blockquote>

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