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Would anyone be able to check some math/ let me know if I'm thinking of this the correct way? I had an idea of the moon appearing basically stationary in the sky, but in fact it orbits just faster than the rotation speed of the earth. From earth, it'd appear that it takes an entire year for the moon to make a complete orbit (which is obviously incorrect, as the earth is rotating). So if the moon is directly above you at midnight, it stays directly above you all day and to the next midnight, only moving forward an imperceptible amount. So in half a year, you'd see the moon make its journey across the sky. it'd appear at first on the eastern horizon for a couple weeks, then slowly make its way to it's noon (don't know if there's a better word for that) within three months, and then the next three months it'd slowly lower down to the other horizon, and then be gone for six months before you saw it in the east again. So theoretically (assuming a 365 day year), am i correct the moon would orbit earth 366 times a year? And thus it's orbital period would be 1/366 of a year?

And then what other strange differences from Earth might be observed? I know you'd have a ton of solar eclipses and and crazy long tides and whatnot, I'm trying to think of what other effects this would have.

Edit: I used Earth as an example, but I intend said planet to be roughly Neptune's size as a super-Earth, if that changes anything.

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  • $\begingroup$ What size moon is this? Geosynchronous orbit is a little more than 35k kilometers, which is a ninth of tenth of the orbit of Earth's moon. An object the size of earth's moon in a close-to geosynchronous orbit would exert a tremendous gravitational force and reflect a huge amount of sunlight, with consequent effects. $\endgroup$ – Ted Wrigley Dec 5 '19 at 22:05
  • $\begingroup$ Well, the main effect will be that this "moon", if we can even call it that, is tiny. That orbit is well inside the Roche Limit, so it must be small enough to hold together despite tidal forces. This is possible — plenty of man-made satellites are doing it right now — but my guess is that the biggest "moon" you can have in this orbit isn't going to look much different than a star. $\endgroup$ – Matthew Dec 5 '19 at 22:15
  • $\begingroup$ @TedWrigley Well I was using Earth as an example, I intend the planet to be roughly the size of Neptune, and the moon to be proportionate in size to be similar to our moon's apparent size from Earth. And it wouldn't be geosynchronous orbit, but would have an angular speed about 366/365 of the rotation speed of the planet (assuming 365 days per year) $\endgroup$ – StrandsW Dec 5 '19 at 22:22
  • $\begingroup$ That's close enough to geosynchronous to make the the altitudes comparable (though I'll admit I'm using the numbers for trivially massed satellite; a satellite with the kind of mass you're talking about significantly shifts the barycenter, possibly enough to pull it outside the radius of the main body; it's not a classical orbit by any means). Is this planet a gas giant, or a super earth? $\endgroup$ – Ted Wrigley Dec 5 '19 at 22:42
  • $\begingroup$ @TedWrigley It's more of a super-earth $\endgroup$ – StrandsW Dec 5 '19 at 23:12
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Yes it is entirely possible for this situation to occur for a time. However such a situation would probably not be stable long term. Geostationary orbit is well outside of the roche limit for the Earth-Moon system so the Moon should not be disrupted by tidal effects, however the effects on the Earth's tides would be interesting to say the least.

Probably ok for a number of other sized planets as well but calculations would be needed depending on the exact configuration.

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