Okay, the earth is attracted by the sun - as everything else in this solar system is. Very nice. There it is, the earth, nicely going around the sun. And the sun attracts the earth ...... why in the name is the earth and the rest still where it is??? It should come closer and closer to the sun no?? Is there an opposite force pushing us lil back or what? Why is there this equilibrium.
Hi c'est moi, Roughly speaking, the centrifugal force and gravitational force nicely compensate in case of the planets. However, earth loses small bits of energy (because of the little drag there is in space, because of tidal motion, ...) so slowly but surely the earth is slowing down and will eventually wind up crashing into the sun. Luckily, the energy disposal is very tiny and it will take several tens of billions of years before we crash into the sun. By that time, the sun will have already gone nova and burnt us all to a ... Crisp.
Crisp I was about to leap into an explanation on pressure density, gravitational fields and Lagrange points. You’ve summed it quite nicely. Btw – “burnt to a … Crisp”. lol Please Register or Log in to view the hidden image!
Crisp, I don't think that you understood what c'est moi was asking. I think what he meant was why in the last 5 billion years didn't the Earth fly into the sun or out of the solar system??? Why is the balance so perfect?? You might say that it's a coincidence that the Earth found a perfect orbit in which the pull of the sun is equal to the centrifugal force of the earth, but it seems very unlikely. It would probably be easier to balance a needle on its tip than to create a perfect orbit. What makes it even weirder is that there are eight other planets with perfect orbits in our solar system as well. I understand the effect of gravity and orbiting objects, but I just can't understand these stable orbits. Is there another factor in this formula or am I missing something?? Tom
Wont the Earth gradually move away from the Sun due to tidal forces? Just as the Moon is moving away from the Earth. We had a similar question at SSSF and, I believe, that over the life of the Earth it has moved a few centimetres away from the Sun.
yes, Joeblow, that's what I initially meant! I have been reading about all this a very very long time ago and the only thing I remembered was that nobody really knew and that it was quite a mystery ... we'll waite for some more replies cause there are some really clever people out here and I always appreciate their effort to try to explain physics stuff to us dummies Please Register or Log in to view the hidden image! so Crisp, Q, James R, Hamster, etc.: give it a shot!
According to theories of solar system development, there were HEAPS of planetary bodies earlier. We the best of what's left.
Hi all, c'est moi, It really depends on how far you want the explanation to go. One way (the most basic and elementary way) to explain why earth (or any other planet) does not leave its orbit is because all forces compensate. There simply is no force that can drive us towards the sun or away. A more decent explanation would be one where you investigate the stability of the solutions for earth's orbit. For just the system earth + sun there is no problem: the solution can easily be proved to be stable (this is a textbook example). When you take in all other planets, then there's a whole different story to tell. For example, simply the system sun-earth-moon is still not solved. It involves chaos theory, and it was Poincaré who was the first to study this problem in a consistent mathematical way. I believe a short while ago the first results about the stability of earth's orbit were in, and if I remember correctly the answer didn't predict much good: the solar system would be unstable. Luckily however, chaos only exhibits itself after a long time (you might want to look up Lyapunov exponents for our solar system, these tell you roughly how long it will take before chaotic behaviour can be observed). And now I come to think of it, I believe the time required before chaos would come into play in our solar system, was again longer than the lifetime of our sun. Bye! Crisp
The Earth's orbit is not "perfect". Nor is is particularly special. If you set something into motion anywhere in the solar system, it will orbit the sun. Depending on its initial energy, that orbit will be either parabolic, hyperbolic or elliptical. All the planets have elliptical orbits. Earth's orbit is nearly circular, but not quite. It is a slightly eccentric ellipse. Why is it nearly circular? The answer is that the angular momentum of the Earth was effectively set at the origin of the solar system, and remains approximately constant. However, the energy has varied due to collisions and so on. The minimum energy configuration for a planet with constant orbital angular momentum is a circlular orbit.
I do not think that there is an accepted theory which explains how the solar system evolved and why the planets are in their current stable orbits. It is my understanding that a correct theory starts with a rotating mass of gas contracting due to gravity. As this mass contracts, it spins faster due to conservation of angular momentum, with most of the mass ending up in the sun. At various stages of contraction, the rotational velocity of mass at particular distances from the center exactly balanced the gravitational force tending to pull that mass to the center. All of the mass at each of these special distances initially formed a ring, which later collected into a planet. I think (am not sure) that the above is a general description of what is believed to have happened. I do not think that anybody has been able to come up with a well worked out theory and good equations to back up the overall concept. It is my guess that a super computer given a proper set of initial conditions might be able to provide a simulation leading to the solar system as it is today, based on a theory somewhat like that described. We might not have enough computing power to do such a simulation. If we had the computing power and a knowledge of the proper equations, it could still be a formidable task. The equations might not be time reversable, requiring us to guess at the initial conditions. You might have to make thousands (millions, billions, more?) of guesses before you hit on a set of conditions leading to a stable solar system. For each guess you would have to run a simulation that could take a lot of computer time. The above is my SWAG, based on various articles I have read and partially understood. If somebody knowledgable in this subject disagrees, I will not argue hard for my views. BTW: For those not familiar with technical jargon, A WAG is a Wild Assed Guess, and a SWAG is a Sophisticated Wild Assed Guess.
Dinosaur, Such simulations already exist, and they produce solar systems similar to ours when you run them. We know all the relevant equations. Simple Newtonian physics is sufficient.
A slightly more philosophical approach.. Please Register or Log in to view the hidden image! Our solar system and the stable orbit of Earth is probably a very improbable configuration for solar systems in general (correct me if I'm wrong here). But that doesn't mean that there is some theory we have missed out on that should explain this, for it doesn't matter if its very improbable. The reason is simple. We could not be here to ask ourselves these questions if the orbit of Earth was elliptic or somehow unstable, because we would not exist. So its very logical that Earth has an almost circular orbit and its nothing to ponder about. Just a thought.
hum, but my thought is following: a satellite which orbits around the earth is really "falling" all the time around it. BUT, if it gets to close to the earth it falls ON the earth; IF it gets to far, the attraction of the earth is not strong enough anymore and will not orbit the earth anymore ............. do you see where I'm going?? I project this line of thinking to the sun: We are orbiting the sun as well as for example Pluto ... and Pluto is so so so much further. If you replace the sun here with the earth and those two planets with two sattelites, then I think you will all understand what I mean. Or Pluto shouldn't be orbiting the sun and the earth should fall on the sun, or the earth orbits the sun and Pluto not (and flies away or just does nothing). Maybe I'm just too stupid - that's the other possibility!
cest moi It would make sense that if the Earth loses a bit of energy due to drag as Crisp mentioned , then it is entirely possible that the Earth IS gradually moving closer to the Sun. But remember, the Sun is also losing energy and therefore losing mass. If it gradually loses mass, it's gravitational influence on the planets lessens. I'm not saying that these losses of energy are in equilibrium, but it would help somewhat to equalize the orbits and conserve angular momentum.
cest moi Earth and Pluto? We need to apply Keplers Laws of Planetary Motion. In other words, orbital mechanics, which requires some calculus. Basically, for an object to remain in a stable orbit, the centripetal forces must balance the gravitational forces of the object orbited. Once orbital velocity has been achieved, the orbit should remain stable. Anomolies such as drag and the change in landscape of the orbited object will influence the orbit over time and a correction in velocity may be required. This change in velocity is known as Delta V.
"Basically, for an object to remain in a stable orbit, the centripetal forces must balance the gravitational forces of the object orbited." I don't know what a "centripetal force" exactly means.
Centripetal force is the perpendicular force applied to an object in motion forcing the object to follow a circular path. The force is constant in that the object is always accelerating and is always directed towards the center of the curvature. Centripetal literally means 'center-seeking.'
Okay, but then the centripetal force and the gravitational force point BOTH towards the sun in this case. They can't be cancelling each other out like you suggested. If gravity is *ever attracting* and the other force is making it go around a certain point, then I still don't get it why we are not smashing against the sun and why pluto is still nicely orbitting around the sun instead of drifting of. After all, with the earth it is easy to see that once a satellite is too far from earth it won't be orbitting earth anymore or once it is too close it will simply fall on earth. But maybe I'm just getting stupid here ... when an object turns around a point very rapidly, it tends flying away from the centre no? Is that what you mean with the centripetal force? So the force of wanting to fly away cause of the speed and the force of gravity wanting to keep it, must be a little in balance and cancel each other out so that the object's orbit is stable. Does this mean that cause gravity is much weaker far away where Pluto is, Pluto's orbiting speed must also be much lower? and the speed of mercurius must be very very high ---> does in the evolution of the solar system it must have been spinning very very hard, much harder than the others
Whoops, missed that c’est moi ASKED for hamster input. Can’t let such an opportunity pass. Let’s see… Crisp already covered two-body Newtonian solutions and the lack of general solutions for more than two bodies. Also covered the chaotic nature of solar system dynamics. Yep, the Earth “could” be the next object to impact Jupiter. (If “could” were stretched to cover such low probability events.) James R. gives reasons why relatively stable circular orbits arise. And he refers to existing solar system simulators. Dinosaur provides a very good SWAG. This hamster suspects both James R. and Dinosaur are correct. Simulations do exist that model the present solar system configuration very well. And predict future orbital positions very well. And some model solar system generation. However this hamster guesses Dinosaur is also correct that no existing simulation captures the full complexity of the birth of the solar system. (Given the difficulty of the task and the vagueness of the hamster statement how could it be wrong. Hehe.) Nor does any existing simulation begin to include the vast number of celestial bodies in the solar system. Itchy brings up the good philosophical point that the requirements for life have potentially filtered out many possible stellar system configurations. So the Solar System could be very unusual. Science might wrongly favor explanations and models that predict the Solar System as a likely outcome. (This should be resolved as distant stellar systems are observed.) Sorry C’est moi. Nothing this hamster can add.
