Why are planets round? How thick is the earth?  Are black holes a proven fact?
469675
Planets came from clouds of spinning gases that later solidified.   So they are pretty round, but not exactly spherical.  The earth is around 8,000 miles in diameter.  Black holes are proven.  They were proven using gravity experiments on nearby bodies and light experiments from stars "behind" them when viewed from earth.
1025906
The Earth is far too small to form a black hole spontaneously. You need an object of about 3-5 times the mass of our sun to cause a gravitational collapse that would generate a black hole.
43539
Planets seem to be spheres, not cubes or cylinders or oddball rocky shapes.  Some smaller bodies such as asteroids or Mars moons Phobos and Deimos, do have odd shapes, but larger bodies like the nine planets and most of their moons do look like spheres.  That's because of the nature of gravity.  You can think of gravity as a force that points inward toward the center of the planet so that every part of the surface is pulled evenly toward the center, resulting in a spherical shape. \nOf course, planets are not perfect spheres because mountains and valleys and even skyscrapers are all deviations from the spherical shape.  However, as planets get larger, gravity gets stronger, until eventually large objects on the surface are crushed under their own weight.  That's why we don't have mountains that are 50 miles high or skyscrapers that are 2,000 stories tall.  Planets stay basically spherical because any large deviations get crushed. \n\nAlthough gravity keeps planets close to spherical, there are other forces that cause deviations from the basic spherical shape.  For example, the rotation of the earth once every 24 hours, causes an apparent centrifugal force which creates a bulge at the equator.  In fact the earth's diameter at the equator is 7,926 miles while the diameter between the poles is only 7,900.
791325
Most are actually more ellipsoid in shape (i.e. not perfect spheres).\n\nI'm no astronomy/physics expeert, but I think it has something to do with gravity and rotational forces.
450478
We live on a very interesting planet. This planet actually hasn't completely cooled down yet and there is a lot of activity hundreds of miles below the Earth's crust (ther crust is where we are). The center of the Earth is very very very hot and a lot of substances (a lot of it being Iron (Fe) )are in molten form down there......Have you ever seen a volcano errupt (on a documentary or in a film?)? Well that stuff coming out is Molten Lava and is a mixture of substances (mostly rock) in liquid form. You cant get too close or you'll burn from the heat. I would go on telling you about this but then we'll be talking about another subject.\n\nThe gravitational pull of the Earth is because of its mass AND because of its rotation around the sun and itself. All bodies in the universe have a gravitational force and this force effects every other "body" in the universe (a bit like a draw in a tug-o-war) (This also explains why matter never can be completely annihilated and other aspects like the existence of dark matter and so forth....but that's advanced Astro-physics) \nThis gravitational pull is measured in NEWTONS (Newton is the guy that laid down the laws of gravity after an apple fell on his head.....) You'd weigh differently on each planet due to its MASS. For example if you where to lift a matchbox on Jupiter you would need a lot of help because it would weigh a lot more then it does here.....and so would you. So much so that your skeleton would not be able to support you and you would probably crumple into a meat heap......(i'm not going into detail because this will just go on and on to knew subjects.....you can find this info on the internet.....it's more fun to find out for yourself)\n\nI hope I've been helpful.......
425798
Our planet earth and for that matter all planets, moons and stars are generally spheres, not cubes or cylinders or odd shapes. Some small galactic bodies such as asteroids comets do have odd shapes, but planets, suns and stars and even moons look like spheres. That's because, when they were formed millions of years ago, they were essentially molten liquid. Gravity and meniscus forces (surface tension) cause the object to take on a rounded shape--and it cools in that position. Do you remember seeing astronauts squeezing out juice or other liquids, they immediately become round in space.  It is the nature of gravity too pull uniformily toward the center--as the distance from the center increases, so does the gravitational force. You can think of gravity as a force that points inward toward the center of all things that have mass-- so that every part of the surface (especially liquids)is pulled evenly toward the center, resulting in a spherical shape.\n\nPlanets are not perfect spheres because there are other forces acting on the planets too which are trying to stretch them--rotational forces, etc. Because they were once hot moltent liquid, they buldge in the middle for these secondary centrifigal forces. Earth and the moon reflect this phenomina. Although gravity keeps planets close to spherical, there are other forces that cause deviations from the basic spherical shape. For example, the rotation of the earth once every 24 hours, causes an apparent centrifugal force which creates a bulge at the equator. In fact the earth's diameter at the equator is 7,926 miles while the diameter between the poles is only 7,900. \n\nNote that planets and moons had to be a liquid or they would not have been able to escape the gravitational force of the nearest larger body. Remember that liquid will seek the path of least resistance as it moves. In otherwords, an impact on the earth would cause a splash of moltent material that could escape the earth's atmosphere at the exact angle (path of least resistance) to be able to exit the planet's atmosphere without burning up. Once it exits into space, the gravitational pull of the larger body will pull it into orbit. It will stay in orbit until it is acted upon by a larger force. Fortunately, we haven't seen any larger forces since the earth was created and therefore, the conditions will remain for millions of years into the future.
312029
As moses said very well indeed, Nobody knows.\nThe major problem I have with a black hole is why it is called a black hole. If this thing is so dense and gravity is so powerful it would be bending light around at the event horizon so that you'd not even know the black hole is there. The other way to confirm this "black hole" would be to observe the planets remaining in the solar system still rotating around a sun which is not there any more. What you would actually confirm would be, according to the laws of gravitational attraction that two massive bodies are attracting each other. One is the planet you see and the other is the "black hole.\nWe know we can change things by sqeezing them, coal into diamonds etc, but there are some materials such as water that are supposed to be incompressible.\nWhat we are supposed to think is the acceleration gradient is so massive that atoms are stripped into their components.\nThe thing is, if the graviation is so massive beyond the event horizon that things are moving at the speed of light.\nThis is a contradiction according to the relativistic E=MC^2 equation.\nIn order for something to travel at the speed of light it must have no mass.\nWe know light has no rest mass.. I.e. stop some light and you get energy but nothing to look at.\nTherefore we are supposed to assume that a massive body changes into pure energy to reach the speed of light and then turn back to mass when it gets to the centre of the black hole.\nThe conclusion there is neither mass nor energy according to the laws of physics currently at the centre of a black hole.
280298
you are stupid. would a computer say that. yes, the earth is round.
1115991
The ancient greeks knew the earth was round.  They knew it because they watched ships sail off.  Someone with good eyes could see that the ship was going over the horizon, the bottom of the ship dissappeared first.  Also, they knew that an eclipse of the moon showed the shadow of the earth on the moon.  Erastothenes calculated the diameter of the earth in about 200 bc.  He was accurate to about 3%.
540447
Let's just talk about Earth. The earth's diameter is 12756 km. Its circumference is pi x 12756 = 40074 km. The earth spins 1 round in 24 hours. That's 1700 km/h, or 472 m/s. If it rotates in an OPPOSITE direction, it's like the Earth is rotating at 944 km/s and suddenly stops. What happens? You fly through the air at 944 km/s. All the houses fly through the air at 944 km/s. Whole forests fly at 944 km/s. Even whole mountain ranges and mountain tips fly at 944 km/s. The ocean's waters fly backward at 944 km/s. Talk about a uber tsunami. I don't know what the friction of air against these object would do to them. Can anyone solve that? Even the Earth's crust might become detached with its outer mantle, I think. Lava breaks out everywhere, global earthquakes... In short: All hell breaks loose. Man, that's even worse than the Death Star. Superman actually does that to Earth in one of his movies. He's a major nut!
12606
Our world, Earth, is a planet, and planets are round because their gravitational field acts as though it originates from the center of the body and pulls everything toward it. With its large body and internal heating from radioactive elements, a planet behaves like a fluid, and over long periods of time succumbs to the gravitational pull from its center of gravity. The only way to get all the mass as close to planet's center of gravity as possible is to form a sphere. The technical name for this process is "isostatic adjustment."\n\nIf the shape of the Earth were due solely to gravity it would be a perfect sphere (round in 3 dimensions) but it's not, the Earth is shaped like what is called an oblate spheroid. As the Earth spins on it's axis, it produces a centrifugal force that pulls away from the Earth (in opposition to gravity)  at 90 degrees from the spin axis, this corresponds to the equatorial regions. So the Earth bulges out a little around it's middle.
993320
Planets are not "round" but in fact "spherical".  Planets are spherical because gravity acts upon each part of the planet, pulling it closer to the center.  A sphere is the most efficient shape to contain the greatest volume with the smallest dimensions.  In the case of a sphere, the radius is a smaller distance from the center of the planet than any other shape could manage, thus, matter naturally forms this shape in the absence of other factors.\n\nIt is important to note that the planets were not always as cool and ridged as they are now.  When the planets were born, they were made of molten elements, and did not solidify until their shape had already taken.  In some other cases, where planets were formed through the collision of multiple bodies, the heat generated by the impact liquified sufficient amounts of the bodies for a spherical shape to form.
1037575
The Earth is about 12,750 kilometers (km) in diameter.\n\nHere are some answers to some more of your questions:\n\nHow old is the earth?\nThe earth is about 4.5 billion years old. That is 45,000,000,000 years. The sun, the moon, the planets, and nearly everything in our solar system was formed at the same time, so they are all 4.5 billion years old. \nHow does the earth spin?\nIf you look around at planets, stars, and galaxies in the universe you will see that EVERYTHING spins! This is because planets like Earth (and stars and galaxies) formed from a large cloud of gas and dust. As gravity pulled the cloud together, any little bit of spin gets multiplied. This is known to scientists as the Conservation of Angular Momentum. You can try this out yourself. Find a chair that spins easily and sit on it. Start out by spinning with your arms outstretched, then pull them in to your body. Did you notice that the chair started to spin faster when you pulled your arms in? The same thing happens as the planet forms — as its gas, dust, and rocks fall together the planet spins up faster and faster. Later other things can affect the spin. Earth and the moon pull on each other, affecting both their spins and the orbit of the moon. It's quite a dance! \n\nHow fast is the earth spinning and why can't we feel it spin?\nThe circumference of (distance around) the earth is 2 x PI x radius, which turns out to be about 24,000 miles. Since the earth turns once per day (or 24 hours), that is 1,000 miles per hour! We don't feel that because everything else is moving with us and because the motion is very, very smooth. It's kind of like sitting in a car and reading a book or playing a game. If the car is going very smoothly, with no stops or bumps, and you don't look out the window, you wouldn't know you were moving. The only reason we know the earth is turning is because the sun and stars move across the sky. \n\nWhat is the exact tilt of the earth? How much does it change throughout the year?\nThe tilt of the earth is 23.45 degrees. This is measured as the tilt of the earth's equator compared to its orbit around the sun. The amount of the tilt doesn't change with time, but the direction that the earth's poles are pointed does. This is called precession. It is just like the wobbling of a spinning top. The direction that the North Pole (and South Pole) is pointed moves in a circle in the sky very slowly, completing each circle once every 26,000 years. \n\nWhy does only Earth have so much oxygen?\nThat's a very good question — no other planet in the solar system has so much oxygen. The answer is — life! Earth did not originally have a lot of free oxygen in the atmosphere. The oxygen was mostly combined with other elements to make carbon dioxide, water, or in rocks. But as early life evolved, especially plants, well they give off oxygen. So the reason we have so much oxygen in our atmosphere — and we are here and can breathe it! — is due to plant life on the earth! \n\nHow can rockets get through the earth's atmosphere, but comets can't?\nObjects hitting our atmosphere usually burn up because they are hitting it so fast. Usually they are moving at hundreds or thousands of miles per second. Rockets and the shuttle move more slowly (although pretty fast compared to cars on the highway). Here's a comparison. If you jump into a swimming pool from the high board and hit the water really hard, it stings and you make a big splash. But if you step into the water slowly, it feels fine and you make only a little ripple. The same thing happens to meteors — they hit the earth's atmosphere real hard and go splat! \n\nIf a comet hit the earth, what would happen?\nIt could cause a big explosion. The nucleus of a comet is thought to be a mile or two in size. The nucleus is made up of ice and rock, but we aren't sure how solid it is. Something that big hitting the earth from space would be bigger than
370
The Earth is round because it is a large body, and gravity tends to make large bodies round because that is the gravitational efficient configuration for the matter to be in.\n\nThere are a few reasons you can encounter non-round bodies in space:\n\n1) the object is small (for example asteroids), so it's gravity has less say proportionally on the shape\n\n2) It has experienced a catastrophic collision with a large body\n\n3) It is under very strong rotational or tidal forces\n\n4) It is expelling matter/energy in the form of cosmic jets, nova ejecta, etc.
884879
Because the earth is round, and if you made your flat map a oval(circle), the oceans would connect and become one!
195652
the earth is a oblate spheroid....we do not fall to the edges because of the gravitational pull...
341419
Yes, Planets are generally spheres, not cubes or cylinders or odd shapes. Some small galactic bodies such as asteroids comets do have odd shapes, but planets, suns and stars and even moons look like spheres. That's because, when they were formed millions of years ago, they were essentially molten liquid.  Gravity and meniscus forces (surface tension) cause the object to take on a rounded shape--and it cools in that position.  In addition, the nature of gravity will pull uniformily toward the center--as the distance from the center increases, so does the gravitational force. You can think of gravity as a force that points inward toward the center of all things that have mass-- so that every part of the surface (especially liquids)is pulled evenly toward the center, resulting in a spherical shape.\n\nPlanets are not perfect spheres because there are other forces acting on the planets too which are trying to stretch them.  Because they were once hot moltent liquid, they buldge in the middle.  Earth and the moon reflect this phenomina.   Although gravity keeps planets close to spherical, there are other forces that cause deviations from the basic spherical shape. For example, the rotation of the earth once every 24 hours, causes an apparent centrifugal force which creates a bulge at the equator. In fact the earth's diameter at the equator is 7,926 miles while the diameter between the poles is only 7,900. \n\nNote that planets and moons had to be a liquid or they would not have been able to escape the gravitational force of the nearest larger body.  Remember that liquid will seek the path of least resistance as it moves.  In otherwords, an impact on the earth would cause a splash of moltent material that could escape the earth's atmosphere at the exact angle (path of least resistance) to be able to exit the planet's atmosphere without burning up.  Once it exits into space, the gravitational pull of the larger body will pull it into orbit. It will stay in orbit until it is acted upon by a larger force.   Fortunately, we haven't seen any larger forces since the earth was created and therefore, the conditions will remain for millions of years into the future.