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Gravitational differential?

  1. Oct 17, 2003 #1
    Hi Guys/Galls,

    I read this theory somewhere and wondered why it's logic isn't acceptable to physicists. Maybe you can help me in answering this question. It seems so simple to me.

    here it is

    "Gravitational differential.

    The earth and the sun share an attraction called gravity.

    The earth is always in the sun light therefore as the planet spins it is always heating up and cooling down. Sunrise and sunset happening continuously.

    The temperature differential being say approximately 20 degrees C....

    We know that as mass cools it increases it’s density. We infer that an increase in density also increases the mass’s gravitational attraction.

    So therefore on this continuous sunrise (horizon) is a gravitational differential which means that the sunrise ( cooler – more dense) is more attractive than the sunset (Hotter)

    This differential imparts a torsional effort on the planet thus generating it’s rotational state."

    The logic not the theory is what i am questioning.

    The logic of a continuous event horizon and differential attractions

  2. jcsd
  3. Oct 17, 2003 #2


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    And well you should. The logic you're seeing here is basic "perpetual motion" reasoning. There is a falty premise (but a very understandable one) that greater density equals greater gravitational pull. In actuality, gravitational pull results from mass, regardless of density. But this explains only the mechanism by which one side of the planet is not heavier than the other. The underpinning the principal is that an abject going downhill cannot the heavier than that seemed abject going uphill.
  4. Oct 17, 2003 #3
    The logic is wrong because the situation is wrong, and makes some false assumptions.

    Not True: The atmosphere heats and cools, not the ground(on a whole) we typically think of as the earth. Once you dig down a few feet the temperature of the earth is fairly constant irregardless of day or night time. meaning that the earth will not shrink. besides if it is always sunrise and sunset than on the whole there is never any "cooling" or "heating" because on the whole there is never any progression of time.

    Not True: an increase in density doesn't coorespond to an increase in gravity, because the amount of matter doesn't change. You can think of it in terms of "gauss's law" for gravity. An object outside of the reference object(earth) will always feel a gravitational effect as if the earth were a point particle of Earthmass. the true dependence on the gravity you "feel" is determined by the distance from the center of mass of the earth, and the earth is not going to shink at all.

    Astro isn't my field; however, It seems to me that conservation of angular momentum from the cloud of stuff that became the planet would be the culprit here. Besides if that were true than the net angular acceleration would have to be greater than zero to get the earth going, meaning that we would still be accelerating making the days shorter and shorter, until eventually we would be traveling so fast on the edge of the earth that gravity couldn't provide the necessary centripital force to keep use on the ground.
  5. Oct 17, 2003 #4
    Thanks for your responses so far.

    Why the logic can not be accepted is starting to show though.

    I would tend to think the only way to get acceleration would be to increase the differential in this example by increasing the range in the temperature would do this.

    Is this to say that an individual particle of dust doesn't have it's own gravity? And the earth is only a culminant of all those particles.

    Secondly, we quote a theory of gravity that is not completed yet.

    Has any one ever bothered to do the maths reagrding the mass differential of the atmosphere and water due to temperature, not to mention the surface temperature. from sunrise to sun set. I think that may be an awful lot of mass. and possibly enough to get the torsional effect (given the size of the planet)

    (might provide some ineteresting results i think. especially as gravity is such a weak force then maybe a weak differential may make a difference.)

    Gravitationally, I would speculate, the planet would appear to be a sort of egg shape or maybe a tear drop shape to conform with the cooling/warming cycle maintaining it's distorted shape due to the continuuous nature of the sunrise.

    Lurch I find this point to be quite debatable as a theory of gravity is incomplete. And i would think that until it is completed then all the premises are open to despute.


    Really the answer to why the logic can't be accepted could simply be that we rely on incomplete theories. And inadvertantly consider them to be truths
  6. Oct 17, 2003 #5
    An argument someone is making can be wrong due to faulty logic or due to incorrect information. hese are two distinct things. The logic in your argument seems good to me. The problem arises in whether or not you have the facts correct.

    However, I say that without having ever formally studied logic, and could be wrong by a logician's standards. The person to ask is supermentor Tom.
  7. Oct 18, 2003 #6
    I may be mistaken but to me the only fact we really do know is what we have always known and that is that gravity certainly is attractive.

    The theories on gravity may be a little wrong or incredibly wrong...(or Both) we wont know what is what until we know i guess
  8. Oct 18, 2003 #7


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    Yes. People have done atmospheric density tests all the way up to orbit. These results are tabulated in what is called a 'standard atmosphere' chart. Variations from that vary a bit, but those variations have also been studied greatly. There are atmospheric models comparing position and time of day, time of year, sunspot activity, F10.7 solar activity, etc. etc. In all, predictions can be reliably made to within 85% accuracy.

    Seriously, you don't put up multi-billion dollar spacecraft without knowing what the effects of drag on the satellite will be first.

    The planet is a little egg shaped, but not because of atmospheric temperature variations. The mass of the atmosphere is completely insignificant compared to the mass of the planet itself, and as was said above, once you get down a dozen feet or so, the temperature doesn't change much as the days or seasons progress.

    The longitudinal distortions of the planet contribute to what is called East-West 'tesserals' in the orbits of satellites. An orbit will oscillate E-W about 75.3 degrees East longitude or 255.3 West longitude. Again, this is entirely due to the oblateness of the Earth.
  9. Oct 18, 2003 #8
    Enigma, thanks for all that information and direction. I think possibly the dynamic-ness of the differential may be an issue.

    If you have any differential and i mean any, what effect does that have on a mass floating in a vacuum.

    say the differential is 100 lbs on a diameter of 25000 miles. what effect would this have considering there is no resistance to rotation.

    Ok...a little extreme...but say you have 1 million lbs when extended from the coldest (on the horizon) reducing untill it gets to zero differential (= the sunset temp) say a approx 1/3 of the circumferance facing the sun....wouldn't this assist in rotation and add to the cons. of ang. mom?

    Does the theory of angular conservation allow for this effect?

    If the differential effect is added to Con.of ang. Mom. would this not lead to a state of continuuous accelleration in rotation. I am assuming that the theory of C.O.A.M. is absolute in maths. ( i haven't studied it as I don't have the maths background.

    Am i being silly again
    Last edited by a moderator: Oct 18, 2003
  10. Oct 18, 2003 #9


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    No, it's not obvious. I'm in a senior level aerospace engineering course covering pertubations (changes to optimal orbits due to differential forces) right now, and we're only getting a very shallow overview of some of the variations. Many PhDs spend their entire lives studying them.

    It will change its orbit. Usually the effects are very small. For a decently high altitude orbit (greater than 300-400km or so. Space Shuttle usually operates around 250+km), you have to use less than 20m/s delta V per year to maintain your orbit. That is next to nothing considering you had to spend well over 7000 m/s delta V to get into orbit to begin with.

    Do you mean that the Earth's mass is greater by 100lbm (pounds mass), or the force of gravity is greater on a satellite at a circular 25000 mile orbit by a constant 100lbf (pounds force) toward the center of attraction?

    In either case, the orbit will change. The 100lbf will have the greater effect (that's close to the force which a medium to large sized cold-gas maneuvering thruster puts out). If the force is pulling the sat. in toward the planet greater than it was before, then the speed of the satellite will have to increase to stay in the same orbit. Unless the sat. is able to speed up, it will fall into a lower orbit, converting some of its gravitational potential into speed, stabilizing in a lower altitude orbit.
  11. Oct 18, 2003 #10
    Enigma....you may have missed my update on the last post.

    hmmmm.... I am sorry to confuse.

    I am refering to the earth and used a fictional figure or 25000 miles diameter as I can't remember the actual.

    Think of the earth as a not to symetrical sphere of x miles diameter

    for X varies as the planet is not symetrical.

    at any time we have X East and X West on this sphere. Both are horizons. Sun rise and sun set. At X east we have a greater attraction to the sun than we have at X West.

    If the planet was actually stationary at the start both X's would be equal.

    But because we are rotating with X east and X west staying geometrically still relevant to the sun, are in fact different. Now I am suggesting that even if the difference was miniscule it would cause the planet to accellerate if the theory of COAM was fully founded.

    SO if we can agree that there is a differential then COAM may need adjustment. or other. I'm not sure what the ramifications to theories would be.

    I might add given your background etc I really appreciate your thoughts on this.
    Last edited by a moderator: Oct 18, 2003
  12. Oct 18, 2003 #11


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    Radius of the Earth is just over 6378km. That works out to a diameter of roughly 7975 miles.

    You need to be careful. East and West are a direction, not a position. Looking from the north pole, if you're going counterclockwise, you're going east. There isn't an east 'place' on the sphere. If you assume that the Earth's orbit around the sun is completely circular (it's close, so it's suitable for back of the napkin calculations), then the morning horizon will always be in the direction the planet is travelling in its orbit.

    Let's talk coordinate systems really quick, so we're on the same page. If you draw a point with a circle around it. We'll call the point the sun, and the circle is the orbit of the earth. Assume you're looking down from the top of the solar system. If you draw a line from the sun to a point on the orbit, we'll call that direction R for radial. If you're looking from the top, the orbit of the earth will proceed counterclockwise. If you draw a line tangent to the circle designating the direction the Earth travels, we'll call that direction S for satellite track. The third direction, up out of the page, we'll call direction W. In this case, morning horizon has the coordinates (0,1,0) RSW in earth radii (ER) from the center of the Earth. Evening Horizon has coordinates (0,-1,0) ER from the center of the Earth.

    Why? What causes the greater attraction? They are the same distance from the sun. The mass of the satellite (the Earth in this case) also doesn't affect the acceleration it feels. The only things which change the acceleration are distance from the gravitating body and the mass of the gravitating body.

    F = -G *M1*M2/r^2 r

    I think I can see what you're trying to get at, but the physics you're describing doesn't exactly gel with reality. If I'm understanding what you're meaning, the oblateness of the Earth won't affect its orbit around the sun. It will affect its rotation though. The different pulls of gravity from the sun on different points of the Earth do cause some changes, however. The most significant points are (1,0,0) ER and (-1,0,0) ER for this. The reason is, the first is farthest from the sun, and the second is closest to the sun. The phenomena which is caused by the difference over time is called 'tidal lock', and is the reason that the moon always faces us.
  13. Oct 18, 2003 #12

    Ahhh I see that i have confused again... I have been refering to only the rotation and NOT the orbit. Howver your point about the direction of orbit always being in the east is very interesting to me.... do any known planets in the solar system rotate the opposite direction in that is the direction of rotation linked somehow with the direction of orbit.

    I am thinking inertia and cold mass......vs direction
  14. Oct 18, 2003 #13


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    If this effect exits at all, and that is not clear to me, the magnetutude is very small. We well understand the gravitational interaction of the sun and the earth, all you need do is pick up any decent physics text to gain an understanding.

    Ground temperature variations are studied and well understood, they lag by about 1 season. That is the maximum ground temp occurs at the start of fall, the minumum at that start of spring. Consider day length as the critical factor. At the begining of fall the day lengths are equal so any point on earth will spend as much time in the dark as the light. As long as a point on the surface of the earth is able to see the sun, it increases in temprature, as long as in unable to see the sun it decreases in temperature, so through the fall, as day length shortens and nights become longer the surface of the earth loses more energy from the sun then it gains the ground temperature falls.

    Due to the thermal mass of the surface of the earth such temperature variations are seasonal rather then daily. Further consider that most of the surface of the earth is water, convection currents will have more effect on water temperature then time of day, so once again your imaginary forces do not exist.

    I hope you realize that you are attempting to recreate, on your own, the works of some of the best minds produced by mankind over last 500yrs. Does it not make more sense to use the accumlated knowledge then to grope blindly, assuming that no one has gone down the path already?
  15. Oct 18, 2003 #14


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    scott_sieger sez:
    I think there is a philosophical issue here. When it is pointed out that even if the sunny side of the earth is less dense than the night side, this results in no gravitational torque to accelerate the earth's spin, then scott_sieger points out that gravitational theory is "incomplete" and therefore open to criticism. Therein lies the problem.

    You cannot criticize a well-established theory in physics on the strength of some speculations. You can only knock it down based on experimental evidence. Physicists will tell you time and again that it is a waste of time to do such tests on well-established theories; they take on faith the truth of a huge body of experimental evidence. If they had to re-test say F=ma or the gravitational Gauss' Law every time they needed it, physics would never progress.

    People from outside the field do not take the existing body of knowledge on faith, and so controversy erupts between "closed-minded" practitioners of physics and non-physicists who come up with "What if ..." this or that?
  16. Oct 18, 2003 #15
    hi guys,

    hey thanks for your input. I do appreciate it.

    And i might add I do agree with you
  17. Oct 18, 2003 #16


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    Again, the direction the planet goes is NOT always in the east. If it is daybreak where you are standing, the planet is actually travelling 'up' in your reference frame. East is the direction the sun is, and the Earth travels more or less perpendicular to the sun.

    Planets, no. All follow the right hand rule: If your right thumb points up, then your fingers curl in the direction of motion. Some small moons and asteroids do go the other way - most likely a result of eons old collisions.
  18. Oct 18, 2003 #17


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    Doesn't Venus rotate the other way?
  19. Oct 18, 2003 #18


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    I'll be. You are correct, Hurkyl.

    That's my thing I learned today.
  20. Oct 19, 2003 #19


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    And Uranus is essentially lying on its side.

    In any case, scott_sieger, your idea of there being an effect where the earth is gaining and losing mass due to temperature fluctuations from day to night simply does not exist*.

    *Relativistic effects of temperature would be infinitessimally small.
  21. Oct 19, 2003 #20


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    And Uranus is actually tipped more than 90°, so it is considered to have a retrograde rotation.

    Pluto also has a retrograde rotation.
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