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PostPosted: Sun Nov 08, 2015 8:29 am 
Sanci
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Pretty sure this is the right forum for this.

Will the more hard-science-minded among you please answer a few questions? I've been googling but can't find exactly what I'm looking for.

First, is there a reason that our moon rotates on its axis and orbits the earth in the exact same amount of time, or is it just a coincidence?

Next, supposing an Earthlike planet and a moonlike satellite, if the moon rotated on a different schedule than its orbit, how would that affect the planet? Would there be an effect on the tides, or not really because the gravity would be the same?

Finally, I know moons could be many different colors other than white, just as planets could, but how likely is it for a moon to be multiple colors? That is, that it would have a surface composed of different materials. Since the moon is covered in a lot of kicked up dust, would it all just mix into an average color?


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PostPosted: Sun Nov 08, 2015 9:24 am 
Sanci
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sam wrote:
Pretty sure this is the right forum for this.

Guess so; this is conworlding.

Quote:
First, is there a reason that our moon rotates on its axis and orbits the earth in the exact same amount of time, or is it just a coincidence?

This is called tidal locking; it happens when a small body orbits a larger one. Over time, the orbits change so that the satellite's day matches its year, with the same face always facing the planet. A similar phenomena is orbital resonance, which has happened with mercury and the sun. Instead of a 1:1 day : year ratio like the moon, Mercury has a 3:2 ratio. The length of time it takes a body to become orbitally locked depends on mass and radius etc. The wikipedia page should have a formula to allow you to calculate it; if not, it's easy to find on the internet. You could then check if the moon in your conworld should be locked or if it is plausible for it to still have a different day length.

Quote:
Next, supposing an Earthlike planet and a moonlike satellite, if the moon rotated on a different schedule than its orbit, how would that affect the planet? Would there be an effect on the tides, or not really because the gravity would be the same?

No, no effect on tides. As you said, the gravitational pull remains the same no matter how fast the moon spins, and it's gravity that causes the tides. The length of the moon's orbit will affect the interval between high tides of course.

One effect of tidal locking is reduced tidal forces on the moon; in the same way that the moon's gravity rhythmically pulls water over the planet, the planet's gravity has a similar pull on the moon. This causes the moon's surface rock to stretch, and causes volcanoes, moonquakes, internal heating and other things. Once the moon is tidally locked, this stops (it's equivalent to the moon staying in one fixed place in the sky: high-tide would be permanently in the same place), so there are less volcanoes etc. This tidal heating is the reason for Jupiter's moons having liquid water oceans under the ice and geysers and all sorts of interesting things, whereas our moon is just dead unchanging rock.

Quote:
Finally, I know moons could be many different colors other than white, just as planets could, but how likely is it for a moon to be multiple colors? That is, that it would have a surface composed of different materials. Since the moon is covered in a lot of kicked up dust, would it all just mix into an average color?

Hmmm. Not sure. The dust might not even be the biggest problem. Our moon seems white because it reflects sunlight very brightly - the surface is actually a dark grey colour, similar to the tarmac/bitumen surface of a road. This "bright shiny effect" might drown out any colours actually present on the surface. Perhaps if there were large patches of very strong contrasting colours, and the overall reflectivity (called albedo) of the moon was lower (to reduce the glare effect), then the colours would be visible.

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PostPosted: Sun Nov 08, 2015 9:38 am 
Lebom
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1. There is a reason that the moon rotates around its own axis in the same amount of time that it orbits the Earth (tidally locked). The reason is called tidal drag, and is explained better than I ever could in this youtube video: https://www.youtube.com/watch?v=6jUpX7J7ySo

2. The answer to the second question is also answered in the video, though less directly. In a short timespan (less that a couple of thound years) the moon would be dragged into a tidally locked situation. I believe that this would not have any serious effect on the parent body, and certainly the size of tides wouldn't be affected.

3. Many bodies have surfaces that are multiple colours, usually due to geological activity, either current or in the past. Take a look at the moon and there are clearly lighter areas and darker areas (the lunar maria (seas)). The dark areas are basaltic plains that are the ploduct of ancient volcanic eruptions. If your conmoon has never had geological activity the tough luck finding a plausible explanation.

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PostPosted: Sun Nov 08, 2015 10:16 am 
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Mâq Lar wrote:
This tidal heating is the reason for Jupiter's moons having liquid water oceans under the ice and geysers and all sorts of interesting things, whereas our moon is just dead unchanging rock.

Small addendum. A major component in the Jovian and Saturnian moons is interaction among the moons as well as tidal interactions with the parent planets. This is particularly true of the Galilean moons, which are tidally locked to Jupiter and in orbital resonances with each other.

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PostPosted: Sun Nov 08, 2015 11:06 am 
Avisaru
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The Moon is actually slightly colourful which you can see if you take a picture of it and pull the colour saturation up. You can see an example here. The different colours show areas with different geology.

In reality the lunar soil is quite dull in colour. I think the reason for this was that it's poor in oxidised minerals; there's no free oxygen around on the Moon. However, exceptions do occur. They found some orange soil on the Moon during the Apollo 17 mission.


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PostPosted: Sun Nov 08, 2015 2:14 pm 
Lebom
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Distance is the biggie missing from the above explanations. Move the moon much further in, and you'll put it inside the Roche limit, where it'll be torn apart and form a ring system. Move it further out and the tidal force (difference in the effect of gravity of a body on the two sides of another body due to the distance between the sides) won't be enough to tidally lock it.

You'll still have tides - remember that the sun also effects the tides - but much smaller and weaker. Note that this will also make Earth less tectonically active.

Also, the tidal force has slowed Earth's rotation by about 6 hours over 4 billion years. Move the moon far enough out to mot he tidally locked and the day will be shorter. Off the top of my head, w/o doing any calculations, we'd likely have a 20 hour or so day.

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PostPosted: Sun Nov 08, 2015 6:08 pm 
Sanci
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Hi,

I agree with the answer above.
I would add that there is tidal heating in the Earth moon system. That is a hypothesis that explains why the moon core is not completely crystallized.
Plus, our moon maybe still volcanically active, the last eruption dates back 18 million years, which is nothing.

If you had a bigger moon by your people, you could have some volcanoes visible.

There is an equation that allows you to compute waiting time before tidal locking, found it on wikipedia.


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