A world with a moon orbiting much closer than ours

The world I am envisioning is a rocky planet with oceans, plate tectonics, atmosphere, and several other similarities to our globe. The mass and size is roughly equal, but its moon is orbiting much closer around it than our moon revolves around Earth. For the sake of clarity: the moon has roughly the same size and mass as our moon.

I am wondering what the consequences would be if it were to orbit the planet at a distance of 1/20th the distance our moon orbits the Earth (let's say 20 000 km as an approximation).

The questions I have are:

1. Would such a system be stable?

2. If yes:

   a) what orbital speed would be necessary for the moon to stay in geocentric orbit?
   b) would an eccentricity close to zero be possible, or would the trajectory have to look different in order for the system to be stable (if yes: what would it look like)?
   c) what sidereal day would the moon have at the altitude of 20 000 km and the necessary speed?

3. How many degrees of the sky would it take up when seen from the world's surface? (I would love to simulate for myself with the software from spaceengine.org, but I lack hardware spec for that and they lack a Linux version)

4. What would the tidal effects on the oceans look like with such a close orbit (as an average, I might add; I am well aware that tidal patterns are greatly affected by local geography, depth of the sea, shape of the sea bottom, etc)?

   Side note: I read that a moon 20 times closer to the Earth would mean tidal effects 400 times stronger, but I do not have the mathematical knowledge to double check the numbers myself.

5. Would a geocentric orbit by an object of this mass allow for an axial tilt of zero degrees for the planet, or would that be impossible?

6. What would the moons pull on the planet mean in terms of plate tectonics (earthquakes, volcanoes, etc)? I assume it would be increased, but is it possible to calculate how much more increased it would be, or are there too many unknown factors involved for that?

I hope I have been specific enough, and I look forward to your answers and thoughts on the subject!

posted over 9 years ago

fantasia‭

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