What if Earth had 2 moons? 

Like half moon sized moons, 1 orbiting half the distance of Earth and the moon, and the other orbiting where the moon is now?

Well, first noticable thing would be that, we would see 2 moons in the sky, logically.
Second is that the waves system would be different. If the closer moon, let's name it Moon A, and the other Moon B, is curently seen on the sky, the sea waves would be higher than if the Moon B would be curently on the sky. But, what if both Moons are on the same side? Would the waves be higher? Probably. But it depends on the positions of the Moons.
Third thing is the Eclipse. We would not have the Eclipse what we have now. If the Moon A is blocking the Sun, it would be the same as the Eclipse we have now. If I'm not mistaken, then it would cover the whole Sun. That's because the Moon A (closer one) is double the size of the Original Moon and at the same time it is closer twice than it should. But, the story is different for the Moon B. You can already guess that there wouldn't be the Total Eclipse.
That's my explanation and if you have another related question, ask. And if I missed something, there's always Watsisname that can explain everything.

Cool.
Would the system be realistic and stable over time? What if Earth captured Asteroidal moons and they stayed in place for close orbit? 

If they're half the size of the Moon (and if we suppose the same density), then they should each have about 1/8 of the mass of the Moon.  Tidal force is proportional to mass and 1/r^3, so for Moon A having 1/8 the mass half as far away this actually gives us the same tidal force as the current Moon.  Neat.  Moon B will provide an additional 1/8 the tidal force.  So the lunar tides will vary in strength by about 25%, strongest when they are together or opposite on the sky, and weakest when they're a quarter orbit apart.  The Sun also adds to the tides as before -- about half the strength of the lunar tide.

So basically the tides will be about the same size as before on average, but fluctuate in a more complicated way.

I wonder as well if their orbits would be stable or not.  Someone should simulate this in Universe Sandbox.

If they're half the size of the Moon (and if we suppose the same density), then they should each have about 1/8 of the mass of the Moon.  Tidal force is proportional to mass and 1/r^3, so for Moon A having 1/8 the mass half as far away this actually gives us the same tidal force as the current Moon.  Neat.  Moon B will provide an additional 1/8 the tidal force.  So the lunar tides will vary in strength by about 25%, strongest when they are together or opposite on the sky, and weakest when they're a quarter orbit apart.  The Sun also adds to the tides as before -- about half the strength of the lunar tide.

So basically the tides will be about the same size as before on average, but fluctuate in a more complicated way.

I wonder as well if their orbits would be stable or not.  Someone should simulate this in Universe Sandbox.

Man you really make things cool and interesting. Makes me imagine what effect the 2 moons would have on human history. If only I had Universe sandbox...

If they're half the size of the Moon (and if we suppose the same density), then they should each have about 1/8 of the mass of the Moon.  Tidal force is proportional to mass and 1/r^3, so for Moon A having 1/8 the mass half as far away this actually gives us the same tidal force as the current Moon.  Neat.  Moon B will provide an additional 1/8 the tidal force.  So the lunar tides will vary in strength by about 25%, strongest when they are together or opposite on the sky, and weakest when they're a quarter orbit apart.  The Sun also adds to the tides as before -- about half the strength of the lunar tide.

So basically the tides will be about the same size as before on average, but fluctuate in a more complicated way.

I wonder as well if their orbits would be stable or not.  Someone should simulate this in Universe Sandbox.

Man you really make things cool and interesting. Makes me imagine what effect the 2 moons would have on human history. If only I had Universe sandbox...

Yeah, I know man. He wants to explain something to sound interesting, like we are in school or something. Even schools don't explain that. When I was writing the answer for your question, I was on mobile so it was pretty hard to type. I would something forget what I wanted to type, plus that auto-correct. >:/
And about that Universe Sandbox, I had it. it is good for the alpha. I didn't follow the updates and I stoped updating the game, since I am not buyer. It is hard to find good version and make it work. Uh, again me with off-topic things....
If you have question, I would be glad to answer everything, even if I don't know it, I'll do resarch and hoping for the best outcome.

But, if magnetic forces are 1036stronger than gravity, does that mean that we can create some sort of anti-gravity craft?

It is easy to use magnetic forces to levitate things, but the difficult part is keeping stability, or levitating more than a very small distance (the magnetic force drops as 1/r^3).  You can stabilize a paramagnetic levitator with spin, or you can use diamagnetic materials.  A fun demonstration of totally stable magnetic levitation is through the Meissner effect with superconductivity.  We also use magnetic levitation for high speed trains. 

If one desires a more general kind of flying craft that balances the gravitational force at any desired altitude?  That's more difficult to imagine from the physics.  Levitation works not by producing opposing gravitational forces, but by producing opposing magnetic forces.  There isn't a convenient magnetic field around the Earth to use for this -- Earth's field is very weak and oriented in different directions at different latitudes.

Let's do the math.  Magnetic pressure is [tex]P_B = \frac{B^2}{2\mu_0}[/tex]

Pressure is force per area, and we want the magnetic force produced by opposing Earth's magnetic field to balance the gravitational force on us and our craft which is mg.

The Earth's magnetic field at the surface ranges from 25 to 65 microtesla.  Let's pretend it is 65 microtesla everywhere and oriented vertically.  What surface area on an opposing magnet would you need in order to levitate a 60kg person against this field?

[tex]Area = \frac{2\mu_0 mg}{B^2}[/tex] = 350000 m² = 35 hectares.

That's more than 6 times the area under the great pyramid of Giza!  Pretty impractical, not to mention that this size of magnet on your levitating craft would add to the weight which will add to the size requirement.  In other words, this will never work.

In order to levitate a 60kg human on something about 1 square meter in size, the field strength must be about 40 millitesla, or about 1000 times stronger.  We can achieve this very easily with modern magnetic materials or electromagnets, but of course, it requires having the same magnetic base on the ground below you everywhere you go to produce the opposing field, and by Earnshaw's Theorem, you have to use something more complex than just static paramagnets to maintain stability.

So perhaps Tesla himself figured out a way to build a stable magnetic levitator back in his day, but by the physics, it cannot be a thing that acts like a flying car and solves all our transportation problems.  Sadly. =(

In the end, it's probably easier to build a hovercraft that uses air. If you do it right, you can trap the air beneath you and glide very efficiently centimeters above the ground.

If they're half the size of the Moon (and if we suppose the same density), then they should each have about 1/8 of the mass of the Moon.  Tidal force is proportional to mass and 1/r^3, so for Moon A having 1/8 the mass half as far away this actually gives us the same tidal force as the current Moon.  Neat.  Moon B will provide an additional 1/8 the tidal force.  So the lunar tides will vary in strength by about 25%, strongest when they are together or opposite on the sky, and weakest when they're a quarter orbit apart.  The Sun also adds to the tides as before -- about half the strength of the lunar tide.

So basically the tides will be about the same size as before on average, but fluctuate in a more complicated way.

I wonder as well if their orbits would be stable or not.  Someone should simulate this in Universe Sandbox.

Man you really make things cool and interesting. Makes me imagine what effect the 2 moons would have on human history. If only I had Universe sandbox...

Yeah, I know man. He wants to explain something to sound interesting, like we are in school or something. Even schools don't explain that. When I was writing the answer for your question, I was on mobile so it was pretty hard to type. I would something forget what I wanted to type, plus that auto-correct. >:/
And about that Universe Sandbox, I had it. it is good for the alpha. I didn't follow the updates and I stoped updating the game, since I am not buyer. It is hard to find good version and make it work. Uh, again me with off-topic things....
If you have question, I would be glad to answer everything, even if I don't know it, I'll do resarch and hoping for the best outcome.

Would these 2 moons be stable over time? How bright and big would they be in the sky?

Would these 2 moons be stable over time? How bright and big would they be in the sky?

First, I'll explain the stability. As Moon A is in the middle, it would be on the constant pressure. Gravity from Earth is pulling Moon A towards the planet, but gravity from Moon B is doing the opposite. That only counts if they're on the same side of Earth. Even tho the gravity isn't that strong from the Moons, it has big impact on Earth. But, of course that they won't collide in the future. That's because there isn't any resistance in space that would slow down the orbit, if that's true. There's a video that visualised the orbits and gravity. Let's imagine space full of air. Orbits would drastically change. Resistance from the air is slowing down the moons at the rate that over the time, they would look closer, and at the end, they would collide with Earth.
About the brightness of the Moons, there wouldn't be noticeable differences for the Moon A as it is twice the size and closer twice,so it would look the same as the original Moon. Moon B would look smaller, as expected. As sunlight isn't different, moons would have same luminosity, brightness. Maybe there would be some differences, but they would be minor.
Conclusion: Stability would change over time because of the gravity from Earth and Moons. As for the look and brightness, Moon A is same, Moon B is smaller in the sky.
I hope that you're satisfied with the answer! :3 Curiosity is the thing that keeps asking questions. It loves finding answers in everything.         

Would these 2 moons be stable over time? How bright and big would they be in the sky?

First, I'll explain the stability. As Moon A is in the middle, it would be on the constant pressure. Gravity from Earth is pulling Moon A towards the planet, but gravity from Moon B is doing the opposite. That only counts if they're on the same side of Earth. Even tho the gravity isn't that strong from the Moons, it has big impact on Earth. But, of course that they won't collide in the future. That's because there isn't any resistance in space that would slow down the orbit, if that's true. There's a video that visualised the orbits and gravity. Let's imagine space full of air. Orbits would drastically change. Resistance from the air is slowing down the moons at the rate that over the time, they would look closer, and at the end, they would collide with Earth.
About the brightness of the Moons, there wouldn't be noticeable differences for the Moon A as it is twice the size and closer twice,so it would look the same as the original Moon. Moon B would look smaller, as expected. As sunlight isn't different, moons would have same luminosity, brightness. Maybe there would be some differences, but they would be minor.
Conclusion: Stability would change over time because of the gravity from Earth and Moons. As for the look and brightness, Moon A is same, Moon B is smaller in the sky.
I hope that you're satisfied with the answer! :3 Curiosity is the thing that keeps asking questions. It loves finding answers in everything.         

So if we had 2 moons, we'd be dead if there was air in space, there isn't. So the system is 'stable'. I'd imagine Moon A could potentially have volcanoes and be like Io to an extent. Imagine looking up in the sky and seeing that. That'd be pretty.
What if Venus and Earth were a binary system, where Venus was at the same distance the moon is, moon doesn't exist in this hypothetical excersize, and both bodies are tidally locked. What would it be like? 

So if we had 2 moons, we'd be dead if there was air in space, there isn't. So the system is 'stable'. I'd imagine Moon A could potentially have volcanoes and be like Io to an extent. Imagine looking up in the sky and seeing that. That'd be pretty.
What if Venus and Earth were a binary system, where Venus was at the same distance the moon is, moon doesn't exist in this hypothetical excersize, and both bodies are tidally locked. What would it be like? 

That's an interesting imagination and creativity you have! Our Moon having volcanoes as Io has. Would be watching that Moon whole night, lol.
Now on to the Venus question! It is common for the stars to have binary systems, but for the planets, I don't think so. It is indeed an interesting question. Our Earth is very similar to Venus by size. So they would be orbiting each other. That is called barycenter. Where they would be orbiting each other on the same point in space. Usually, larger objects would have smaller orbits rather than those objects that are smaller and would have bigger orbits.

Since we already know the distance between Earth and Venus, we know where the barycenter (center of mass) would be. It would be a little bit closer to the Earth, since it is bigger a bit than Venus. That would be somewhere around 200.000 km since the Moon (Venus for this explanation) is somewhere around 400.000 km.

Here's a formula for a precise distance: r1 = a*m2/(m1+m2) =a/1+m1/m2

If you want to know what would it be like here on Earth, it is hard to answer. But you are here for the answer, right? Because the gravity of Venus is 8.87 m/s², and is tidal locked, well unlucky the side it is on. There would be devastation. Floods would be the main problem and waves would be enormous. And if I am not wrong. we would fell lighter. Because there's gravitational forces pulling us towards the barycenter. But, what about the other side? You can guess that the effect would be the opposite. Like we would feel heavier and tides would be lower. 
This is all just speculations, but there's no limits for the imagination and questions. 

I actually calculated the barycenter and it is 179.610 km away from Earth. That would mean that Venus is 220.390 km away from barycenter.
Yeah, I calculated it.

So if we had 2 moons, we'd be dead if there was air in space, there isn't. So the system is 'stable'. I'd imagine Moon A could potentially have volcanoes and be like Io to an extent. Imagine looking up in the sky and seeing that. That'd be pretty.
What if Venus and Earth were a binary system, where Venus was at the same distance the moon is, moon doesn't exist in this hypothetical excersize, and both bodies are tidally locked. What would it be like? 

That's an interesting imagination and creativity you have! Our Moon having volcanoes as Io has. Would be watching that Moon whole night, lol.
Now on to the Venus question! It is common for the stars to have binary systems, but for the planets, I don't think so. It is indeed an interesting question. Our Earth is very similar to Venus by size. So they would be orbiting each other. That is called barycenter. Where they would be orbiting each other on the same point in space. Usually, larger objects would have smaller orbits rather than those objects that are smaller and would have bigger orbits.

Since we already know the distance between Earth and Venus, we know where the barycenter (center of mass) would be. It would be a little bit closer to the Earth, since it is bigger a bit than Venus. That would be somewhere around 200.000 km since the Moon (Venus for this explanation) is somewhere around 400.000 km.

Here's a formula for a precise distance: r1 = a*m2/(m1+m2) =a/1+m1/m2

If you want to know what would it be like here on Earth, it is hard to answer. But you are here for the answer, right? Because the gravity of Venus is 8.87 m/s², and is tidal locked, well unlucky the side it is on. There would be devastation. Floods would be the main problem and waves would be enormous. And if I am not wrong. we would fell lighter. Because there's gravitational forces pulling us towards the barycenter. But, what about the other side? You can guess that the effect would be the opposite. Like we would feel heavier and tides would be lower. 
This is all just speculations, but there's no limits for the imagination and questions. 

I actually calculated the barycenter and it is 179.610 km away from Earth. That would mean that Venus is 220.390 km away from barycenter.
Yeah, I calculated it.

Impressive. Although I was asking if they had formed together, not all of a sudden thing if you didn't know. Or I could be completely wrong.
 I'd imagine if Earth and Venus were formed together, orbiting eachother both planets would be extremely different. No landmasses as we know them, since Venus's gravity would affect the plates of the earth, I think. Venus would not have the 92 pressure atmosphere it has, It could be a world like ours if things went right. 
I'm more for having a hot hellworld in the spot of our moon though. Those clouds of venus would reflect alot of light, and it would light up the sky at night. Making astronomy harder, but in place of that we get to look at a beautiful ball of churning clouds and poisonous gasses. 

Venus has always fascinated me. I love it. Its a planet I would like to see up and close, but I'm sure everyone would.
Man I have alot more questions. I like your answers. 
What if Jupiter had Impressive rings like Saturn? Think a large opaque disk, and having the moons further out from Jupiter so they would not effect the rings of Jupiter. Say....a few million years ago...Io, a large asteroid struck the moon, pulling it closer to its mother planet, the roche limit tearing the moon to fine granules. Would Jupiter be brighter in the sky? 
Another thing, lets say in this same hypothetical universe, Jupiter and its moons are closer to the sun, sucking up the asteroid belt as we know them. 

Impressive. Although I was asking if they had formed together, not all of a sudden thing if you didn't know. Or I could be completely wrong.
 I'd imagine if Earth and Venus were formed together, orbiting eachother both planets would be extremely different. No landmasses as we know them, since Venus's gravity would affect the plates of the earth, I think. Venus would not have the 92 pressure atmosphere it has, It could be a world like ours if things went right. 
I'm more for having a hot hellworld in the spot of our moon though. Those clouds of venus would reflect alot of light, and it would light up the sky at night. Making astronomy harder, but in place of that we get to look at a beautiful ball of churning clouds and poisonous gasses. 

Venus has always fascinated me. I love it. Its a planet I would like to see up and close, but I'm sure everyone would.
Man I have alot more questions. I like your answers. 
What if Jupiter had Impressive rings like Saturn? Think a large opaque disk, and having the moons further out from Jupiter so they would not effect the rings of Jupiter. Say....a few million years ago...Io, a large asteroid struck the moon, pulling it closer to its mother planet, the roche limit tearing the moon to fine granules. Would Jupiter be brighter in the sky? 
Another thing, lets say in this same hypothetical universe, Jupiter and its moons are closer to the sun, sucking up the asteroid belt as we know them. 

There's a lot of answers and speculations. It depends on mine or yours imagination. That's what makes us special. But, if it's tide-locked then we wouldn't see Venus on the night sky. Or I am completely wrong. Thing is the same as our moon. It's tide-locked, so we see same side always.
There's just tons of questions and, I would like to say: Butterfly Effect Space Events. Where something could happen in space, but didn't.
I don't believe in Big Bang theory and billions of years of formation. But, if we would discuss about formation and positions of, let's say Jupiter, as you asked. It already has rings, but they aren't as magnificent as Saturns. If it had one, then it would be the best planet of our Solar System. By the moons, size of planet, and even rings. Did you know that even every single piece of rock in rings have effect on other rocks and moons? Every piece has it's own gravity. You probably knew that, but wanted to mention it.
If an asteroid hit the Io, and went on the way to Jupiter, while also going into Jupiters rings, Io would be bombarded with those rocks. Probably would be nice watching it. And when it would hit the Jupiter, it would cause the impact that never happened before to Jupiter. As Io would go to the Jupiter, it would also pull the rings into the Jupiter. It would be indeed nice watching.
Now on that hypothetical universe. You also probably know this, but it is speculated that there was long ago a planet in that asteroid belt. Then something happened, and it got destroyed. I think that planet X (Nibiru) destroyed it. But that's again off-topic. If Jupiter got there somehow with those moons, it would be the same bombardment as with the rings and Io. Moons would get off Jupiters track and would change orbits and some moons would maybe escape and, maybe would be headed to the Earth... Well that's dark. And I think with this scenario Jupiter would be brighter in the sky.
Space is fascinating, a lot of questions, but little answers. I would now use mu homemade unfinished telescope to look at the Moon and trying to get the best view.
I also have questions, like, what if this what we know now isn't correct? Those measurements, and other things.
I was fascinated about light. And I would thing of all possible things that people just take for granted. What if what we know about light isn't correct? I would always question myself about everything. Mostly at the night time. Like now. It is 2.15 as I am writing this sentence.
Hope you are enjoying questioning and finding answers, like I does. 

Impressive. Although I was asking if they had formed together, not all of a sudden thing if you didn't know. Or I could be completely wrong.
 I'd imagine if Earth and Venus were formed together, orbiting eachother both planets would be extremely different. No landmasses as we know them, since Venus's gravity would affect the plates of the earth, I think. Venus would not have the 92 pressure atmosphere it has, It could be a world like ours if things went right. 
I'm more for having a hot hellworld in the spot of our moon though. Those clouds of venus would reflect alot of light, and it would light up the sky at night. Making astronomy harder, but in place of that we get to look at a beautiful ball of churning clouds and poisonous gasses. 

Venus has always fascinated me. I love it. Its a planet I would like to see up and close, but I'm sure everyone would.
Man I have alot more questions. I like your answers. 
What if Jupiter had Impressive rings like Saturn? Think a large opaque disk, and having the moons further out from Jupiter so they would not effect the rings of Jupiter. Say....a few million years ago...Io, a large asteroid struck the moon, pulling it closer to its mother planet, the roche limit tearing the moon to fine granules. Would Jupiter be brighter in the sky? 
Another thing, lets say in this same hypothetical universe, Jupiter and its moons are closer to the sun, sucking up the asteroid belt as we know them. 

There's a lot of answers and speculations. It depends on mine or yours imagination. That's what makes us special. But, if it's tide-locked then we wouldn't see Venus on the night sky. Or I am completely wrong. Thing is the same as our moon. It's tide-locked, so we see same side always.
There's just tons of questions and, I would like to say: Butterfly Effect Space Events. Where something could happen in space, but didn't.
I don't believe in Big Bang theory and billions of years of formation. But, if we would discuss about formation and positions of, let's say Jupiter, as you asked. It already has rings, but they aren't as magnificent as Saturns. If it had one, then it would be the best planet of our Solar System. By the moons, size of planet, and even rings. Did you know that even every single piece of rock in rings have effect on other rocks and moons? Every piece has it's own gravity. You probably knew that, but wanted to mention it.
If an asteroid hit the Io, and went on the way to Jupiter, while also going into Jupiters rings, Io would be bombarded with those rocks. Probably would be nice watching it. And when it would hit the Jupiter, it would cause the impact that never happened before to Jupiter. As Io would go to the Jupiter, it would also pull the rings into the Jupiter. It would be indeed nice watching.
Now on that hypothetical universe. You also probably know this, but it is speculated that there was long ago a planet in that asteroid belt. Then something happened, and it got destroyed. I think that planet X (Nibiru) destroyed it. But that's again off-topic. If Jupiter got there somehow with those moons, it would be the same bombardment as with the rings and Io. Moons would get off Jupiters track and would change orbits and some moons would maybe escape and, maybe would be headed to the Earth... Well that's dark. And I think with this scenario Jupiter would be brighter in the sky.
Space is fascinating, a lot of questions, but little answers. I would now use mu homemade unfinished telescope to look at the Moon and trying to get the best view.
I also have questions, like, what if this what we know now isn't correct? Those measurements, and other things.
I was fascinated about light. And I would thing of all possible things that people just take for granted. What if what we know about light isn't correct? I would always question myself about everything. Mostly at the night time. Like now. It is 2.15 as I am writing this sentence.
Hope you are enjoying questioning and finding answers, like I does. 

Tidal locking means that one side would face Venus, one side of Earth would face venus, the other would never see it. Both objects are quite similar, so both would be tidal locked to one and another. One side of venus would face Earth. So one side of earth would see venus, would be interesting to see how civilizations would interact from venus facing side, to anti venus side.
Back in the formation of the solar system, If I do remember correctly there was an event in the solar system. Called the heavy bombardment caused by the resonances of Jupiter and Saturn. Uranus and Neptune switched places in this event, dislodging quite a bit of debris and planetoids, probably creating the Kuiper belt. Let say, that this happens, and during this interaction Jupiter winds up closer to the sun, orbiting where the asteroid belt lies. 
You would probably end up seeing the moons of jupiter with the naked eye. Thats how close Jupiter would be. Jupiter's influence would throw out the asteroid belt, towards the inner system and beyond. More objects would hit the inner planets, leaving those inner planets with more craters. Earth would survive, and we would still probably exist. Moon would have more craters and might look different, same with mars, and mercury. 
(Leaving Venus and earth not binary in this situation)
But with a different system.
With jupiter closer in, it would probably dislodge the impacter that hit venus, and stopped its rotation. Venus may very well be more milder in this alternate timeline and not the hellworld it is in our universe.
This means no asteroid belt. We would probably still see Asteroids, but not as much. Jupiter's moons, the main 3 that would exist would be moon  like worlds, pockmarked with alot of craters. The rest would be asteroids orbiting Jupiter. This means Europa could be different and more rocky.

I don't believe in Big Bang theory and billions of years of formation.

Why?  This is by far the most successful theory of cosmology ever developed, and all astronomers use it.  I even implicitly use it in my own research (I use Lambda-CDM model with established values of cosmological parameters), since without it I would compute distances and sizes of galactic clusters incorrectly.

Also a word of caution on determining the stability of the two moons -- the orbital mechanics that can lead to instability are more complicated than "is there air or not?"  Mutual interaction between the moons will be important, as well as between the moons and the Sun (this is already important for the current Moon), and the tidal interaction (which can cause orbits to expand or contract over time, also important for the current Moon.)  Simulating the tidal interaction is probably beyond our ability to do here, but we can at least see the effects of the direct gravitational interactions with an n-body simulator.

You also probably know this, but it is speculated that there was long ago a planet in that asteroid belt. Then something happened, and it got destroyed. I think that planet X (Nibiru) destroyed it

This is a popular idea, but it doesn't fit with observations.  For example, if the asteroid belt was once a planet, then it would have differentiated, producing a dense metallic core surrounded by mantle and crust.  If it was then shattered, then every asteroid independent of size would show compositions related to that differentiated structure.  But this is not what we observe.  Instead, every large asteroid (e.g. Ceres) is differentiated on its own, while the smaller asteroids are more homogeneous mixtures.  What this indicates is that the asteroid belt is a bunch of primordial material left over from the protoplanetary disk, and the gravitational effects of Jupiter prevented them from accreting into a single object.

I also have questions, like, what if this what we know now isn't correct? Those measurements, and other things.

This is a great and fundamental question, actually.

Science is based on building models which attempt to explain and predict what we observe.  Sadly, we can never know if a model is "right" -- science just doesn't work that way -- but we can test between different models and see which one works best.  For example, Newtonian gravity makes sufficiently good predictions in most circumstances, but in other circumstances you have to use General Relativity to be more accurate. 
 
With any model, you can find out if it begins to diverge from reality by making observations.  In the Newton vs. General Relativity example, you can find out that Newton is wrong in strong gravitational fields by looking at how much light is deflected when passing near the Sun.  Or you can find out that it is wrong in weak gravitational fields (like here on Earth) if you have very high precision.  But if you only care about motions in weak gravity, speeds much slower than light, and you don't have 10+ decimal points of precision, then you're fine to use Newton.

There is a whole field of science known as "Error Analysis", which deals with the gory details of how we figure out "how well we know something" (how confident we are in a measurement, or a model). It's a fascinating topic!

if it's tide-locked then we wouldn't see Venus on the night sky. Or I am completely wrong. Thing is the same as our moon. It's tide-locked, so we see same side always.

It will still be visible at night even if both worlds are mutually tidally locked.  You just won't ever be able to see it from one side of the planet.  From where you can see it, it will appear fixed in place in the sky, while the Sun and stars revolve around.  Try it out in Space Engine -- there are some mutually tidally locked binary planets out there.

I hope this post didn't come across as being nitpicky or anything.  Your answers are really good and a joy to read!  I just wanted to correct any misconceptions, as they can also be interesting to learn and talk about, or lead to new questions.  In my opinion learning is one of life's great pleasures.

If the acceleration of the universe continues to increase and in the far future it exceeds the force required to pull apart subatomic particles, what happens if it rips apart quark pairs generating more quark pairs?

If the acceleration of the universe continues to increase and in the far future it exceeds the force required to pull apart subatomic particles, what happens if it rips apart quark pairs generating more quark pairs?

[youtube]dfHrXsGJ8mo[/youtube]
(I truly don't know for sure.  Nice job blowing my mind with this question!) :shock: