If I recall, early astronomers observed what they believed was retrograde motion in the other planets, largely because they were fixated on the idea of an Earth-centric universe. They invented complicated looping orbits for these planets to make their observations fit the idea of Earth-centricity. It was only when looked at from a heliocentric view that those apparent backward loop-de-loops resolved into circular (well, elliptical) orbits.
So what kind of apparent motion would the other planets seem to have if the astronomers had lived on the moon?* How would they know if they revolved around the earth or vice-versa and would it matter? Anybody ever figure out a lunarcentric system?
*presuming first an atmosphere and biosphere, of course.
The quick answer is that the motion of the other planets looks pretty muxch the same as from the earth – the moon is going around the sun with the earth, after all, and the quarter of a million miles distance between them is small potatoes next to planetary distances. When Mars seems to be going retrograde as seen from the Earth, it looks as if it’s going retrograde from the Moon, too.
Let’s not confuse our vantage point with the model. You can imagine that the universe is centered on the Earth, but if we have the planets going around the sun (which is going ariound the earth), it all still looks the same as if the sun is still and the earth is moving. In fact, people effectively do that for a lot of figuring about where they expect planets to be. You could do the same with the moon at the center. It’s just a matter of switching the center mathematically from the place where calculations are simplest to somewhere that’s not at the center, and moving.
This is different from making a model that assumes that you’re at the center and that everything is either directly moving around you, or is attaxched to thinmgs moving around you, or attached to things that are attached to thingsd that …you get the idea. The only reason the complexity of eccentrics and equants and such were necessary was that a lot of those early philosophers were convinced that everything was reducible to circles. Actually, they did suprisingly well, in some cases. But ellipses made a muich better fit, once Kepler had thrown off the commitment to the Circle as All.
If someone werere trying to do the asame with the moon as center, I think they’d come up with a very similar model for the planets, but they’d have to throw in an extra deal for the sun to account for the discrepancies introduced by the moon’s orbit around the earth.
It’d be pretty to notice that, from a ‘fixed moon’ point of view, there was this kinda big bluey-green globe that never moved too much relative to your own position on the moonscape, just kind of sat there in the sky and wiggled back and forth a bit.
That object would go through phases from new earth to full earth and back again over a period of about 29 days. That would be the same period as from local midday to local midday on the moon would, however, be about 29 days, i.e., the Sun would spend a bit over 14 days above the horizon and a bit over 14 days below the horizon.
You would also see the continents and oceans on the earth go round with a full apparent rotation taking about 25 hours (a bit more than that actual rotation period of 24 hours, because the Moon is orbiting round the Earth). This is assuming that you’re on the Earth-ward side of the Moon: on the other side, you would never see the Earth.
Note that given the radius and period of the Moon’s orbit around the Earth, if you were to trace out just the Moon’s path around the Sun it would be very slightly wobbly. In fact, the near circle would be everywhere convex! (So no Spirograph™ type looping at all.)
I don’t think a lunar civilization at a level that would believe in a lunar-centric solar system would have the tech necessary to measure such small differences in lunar orbital path vs. the Earth’s.
Actually, if you take any point inside a wheel or on its edge and run it around a larger wheel for one “orbit”, you won’t get any “looping” either. A point on the edge of a wheel rolling over a flat surface will trace out a series of cycloids that won’t intersect themselves.
– CalMeacham, who played with his spirograph a lot.
