Hypothetical moon questions

[Kyle]

Question: is it theoretically possible to have a moon that has a geosynchronous orbit around its planet (I don’t see why not), and if so- what would it’s phases look like and how would it look from different places on the planet?

[Mike]

It all depends on the size of your planet and the length of your day etc. For example, if you slowed down the rotation of the earth so that a day on earth took 29.5 days, then our moon would be in geosynchronous orbit while moving and behaving exactly as it does now.

But let's go the other way. Put our moon in geosynchronous orbit above the earth, and it would be much closer. Normal geosynchronous orbit is around 36000 km, but I think it would be a little farther for the moon since it provides so much gravitational attraction of its own.

So it would appear about 10 times as wide in the sky (so around 100 times the visible area). It would still have phases like it does now, but the full cycle would be 24 hours long. Let's say this geosynchronous orbit is also geostationary--that is, it circles the earth directly above the equator--and it happens to be directly over Singapore (which is only a degree off the equator)

Singapore would always see the moon directly overhead. People as far west as central Europe and Africa would see the moon perched permanently on the eastern horizon. On the other side, Hawaii would see it on the western horizon. North America, South America, the entire Atlantic, plus western Europe and Africa would never seen the moon at all.

Every day at Singapore noon would be a new moon. Then by Singapore midnight, the moon would be full. The phase of the moon is determined solely by Singapore time, and all places that can see the moon see the same phase at the same time.

Twice a year, on the equinoxes, you would have perfect solar eclipses in Singapore, followed by perfect lunar eclipses 12 hours later. The weeks before and after the equinoxes would see partial eclipses every Singapore noon and midnight, slowly increasing each night until the equinox and then slowly decreasing in the days after. I can't calculate how long these partial eclipse periods would last, but the lunar eclipses would start days sooner and last days longer.

[Tahlvin]

And how that would mess with the tides!

[Eliahad]

When the moon first formed 4.5 billion years ago, it was actually closer than Geosynchronous orbit, and it DID wreak havoc with the tides. In fact, it was the tidal effect that started slowing the moon down and slowly pushed it away to its current distance.

[Mike]

[Kyle]

What if we suddenly got a second moon that was 50 percent bigger than our moon? And I want it to orbit as an opposite counterpart to our current moon. Meaning- when one is setting, the other is rising. Is that possible?

[El Jefe]

Yes, it is. But I'd have a lot of questions about the tech level available, because that's quickly going to determine how feasible.

It's going do some real fun things to tidal dynamics if 50% bigger and at the same distance as the existing one.

[Mike]

Except that if they are exactly opposite each other in the same orbit, then they would cancel each other out, so that tidal effects would be similar to having a single half-size moon where the new moon is located.

NOTE--I'm assuming that "size" refers to mass.

[Kyle]

Size refers to mass.

[poorpete]

Dear moon experts

If somebody switched out the moon overnight, with one double the diameter, with the same density, but farther from us and at a faster speed to maintain the tides near to the equator, how would we notice the difference on Earth? Would the moon look visually larger or smaller?

[Mike]

If the moon was twice the diameter, it would have to be twice as far away to appear to be the same size. But it would be eight times the mass, so it would have to be about 2.8 times as far away (square root of 8) to have the same gravitational (tidal) effect on the earth. The time it takes for something to orbit the earth depends almost solely on the radius of its orbit. Therefore, at 2 to 3 times the distance from earth, the new super-moon would be taking much much much longer to orbit the earth (I dont know the exact calculation off the top of my head). So now matter where you put it, the double sized moon will be immediately and obviously different.

[FlameBlade]

Simplify the computation. If the distance between Earth and Moon doubles, then circumference doubles, pretty much. I know. Supposed to compute for elliptical motion, but I'm using circle to make back of envelope estimate.


However, gravitation. Geez. GMm/r^2.

So, new distance and mass would be something like.

G8Mm/4r^2. Which is twice as much gravitation as old one. Now what I have yet to compute is how much faster the moon needs to be to overcome stronger gravity. Don't think periodicity is going to be the same