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Gravitational effect of sun on earth

  1. Oct 9, 2004 #1
    Does anyone know or is it possible to know how much percent of earths motion is due to the effect of suns gravity? Im thinking very close to 100%, but not exactly 100%. If it were 100%, the earths orbit would be a perfect circle; not to mention the orbits or the other planets. So what causes the orbits to be eliptical? Elipcies have 2 focul points. The suns one, whats the other?
  2. jcsd
  3. Oct 9, 2004 #2
    Yes, the Sun in responsible for nearly 100% of the Earths motion...although I'm not sure how you'd go about figuring exactly what percentage. Remember, Earth feels the gravitational force from every single object in the universe. Also, the center of the Sun is not exactly on one of the focal points of the ellipse. Rather, the planets of the solar system and the Sun rotate about a common center of mass, which I think is somewhat close to the surface or the Sun, with Jupiter causing most of the effect. As for the other focus, there's nothing there. Look into Kepler's Laws of planetary motion if you're interested. Momentum of the planets are conserved, so when a planet is closer to the Sun it travels faster, or you could say it "sweeps out" equal areas in equal time intervals, so nothing has to be there (at the focus).
    Hope that explains somethings.
  4. Oct 9, 2004 #3

    This is true. Do you think, for arguments sake, the sum of all the outer planets (including the astroid belt) with a center of mass i'd say somewhere near Jupiter's orbit could have a gravitional effect on earths orbital motion say 5% that of suns gravitational effect? Im thinking that all the planitary orbits are eliptical (proportional elipces) with the exception of pluto, which interchanges orbits with neptune for a few decades. A cause for this being that there are no outer planets past pluto to counter suns (and all the inner planets) gravitational force. I know that at any point in time all the planets can be at any point on their orbit. But when our solar system was born, the reason the orbits became the way they are, and planets the distances they are from the sun was due to the "push and pull" effect of gravity between the inner and outer planets and the sun with respect to each planet? (You could say when the solar system was being born, in place of planets, there were dense clouds of matter/gas/hydrogen. Wheather it be gas or planet, the mass would be nearly the same as today, so same difference.)
  5. Oct 9, 2004 #4


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    If the Sun were to suddenly dissappear, the Earth wouldn't stop. It would continue to move with the same speed, and in the same direction as it was moving when the Sun vanished. So the Sun can not be responsible for 100% of the Earth's motion. The Earth's inertia is responsible for a great deal of this motion. And the Sun is responsible for pulling what would be a straight line of travel into an ellipse.

    Over long periods of time, the Earth's eccentricity changes. It goes from almost a perfect circle to about twice what it is now. I forget how long it takes to do this. But Jupiter and Venus are primarily responsible. Jupiter is the reason that Mars has such a high eccentricity. Mars' eccentricity also periodically changes from nearly-circular to even greater than it is now. Being closer to Jupiter, its maximum eccentricity is much greater than Earth's.
  6. Oct 9, 2004 #5
    The earth would continue in orbit of the now non-existent sun for several minutes. Then it would experience big time tidal forces for a split second. I'm too lazy to do the calculations, but they should be catastrophic in nature.
  7. Oct 9, 2004 #6


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    You're right. It would take about 8 minutes for the Earth to stop feeling the gravity since the speed of gravity is the speed of light. And the instant-off on the Sun's tidal force would be somewhat of a rubberband being released kind of effect. Even if it wasn't catastrophic, we'd be only a few days away from a major ice age :surprised
  8. Oct 10, 2004 #7
    1) Elliptical orbits are what you get from two bodies interacting with an inverse square law. This has nothing to do with other bodies in the system.

    2) Gravity does not cause motion, it causes acceleration.
  9. Oct 11, 2004 #8


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    Earth's motion relative to what? The sun? The center of the Milky Way? The Virgo Cluster of galaxies? The speed around the sun (67,000 mph) is small compared to the speed toward the Great Attractor (about 2.2 million mph). Presumably, we're just talking about the motion relative to the sun.

    In that case, in addition to the above, don't forget about the Moon's influence. It gives the Earth a nice little wobble in its elliptical orbit around the sun.
  10. Nov 17, 2004 #9


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    1] There is no 'other' force pulling the Earth's orbit into an ellipse - certainly not the other planets. Ellipses are an inherent part of orbits; if there were no other planets in the system, Earth would still follow an elliptical orbit.

    2] The ellipse swept out by the Earth has nothing to do with two foci method for constructing an ellipse. (I have often wondered about the relationship.)

    Note that comets use the same math. They follow ellipses that are very, very elongated, going well outside our solar system. Thier 'second focus' would be only a few million miles from their aphelion - way at the outer edge of their orbit.

    Rogue bodies can also come from interstellar space. they follow a hyperbolic path, though again, it's the same math as an ellipse - it's just that this hypothetical 'second focus' would be father away than infinity.
  11. Dec 14, 2004 #10


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    To get an order of magnitude estimate, the easiest thing to do is to look at relative mass and distance:

    In units of Earth masses and minimum distance from Earth Astronomical Units (which overstates the average planetary effects by understating average distance, most significantly for Venus and Mars), rounded to nearest integer and highly rounded calculations:

    Sun: Mass=332,950, Distance=1
    Moon: Mass=0.01, Distance=0.002 (i.e. roughly 1/200th as much g as sun)
    Jupiter: Mass=318, Distance=4 (i.e. roughly 1/16,000th as much g as Sun)
    Venus: Mass=0.8, Distance=0.25 (i.e roughly 1/28,000th as much g as sun)
    Saturn: Mass=95, Distance=8 (i.e. roughly 1/192,000 as much g as sun)
    Mars: Mass=.1, Distance=1/3 (i.e roughly 1/300,000th as much g as sun)
    Uranus: Mass=15, Distance=17 (i.e. roughly 1/540,000 as much g as sun)
    Mercury: Mass=0.06, Distance=0.5 (i.e. roughly 1/1,200,000 as much g as sun)
    Neptune: Mass=17, Distance=30 (i.e. roughly 1/2,000,000 as much g as sun)
    Pluto: Mass=0.002, Distance=30 (i.e. roughly 1/16,000,000,000 as much g as sun)

    Since the planetary masses are in differing directions which cancel each other out, the planetary contributions are significantly less than simply adding up the contribution of each planet would imply.

    Thus, all 9 planets in the solar system combined (yes, I know there is dispute about Pluto's status) have less than 1/10,000th the combined gravitational effect on Earth as the sun. The Moon is more than 50 times as significant in its gravitational impact on Earth as all the other planets combined. The Sun accounts for more than 99.8% of the gravitational impact experienced by Earth.

    For the planets, one can get a feel for how many miles of mean orbital radius are a result of that planet, by crudely dividing the percentage of solar gravitational impact by the mean radius from the sun (about 91 million miles):

    Pluto 200-300 feet
    Neptune 45 miles
    Mercury 76 miles
    Uranus 168 miles
    Mars 303 miles
    Saturn 780 miles
    Venus 3250 miles
    Jupiter 5700 miles

    (The moon, of course, wobbles the orbit but consistently changes its direction over the course of the lunar orbit).

    The orbit predicted by a three body system consisting of the Sun, Earth and Moon, is exceedingly similar to its actual orbit. And, you would get three significant digit accuracy simply by modeling a Sun-Earth system. Once you get to levels of detail beyond a five body system of Sun-Earth-Moon-Jupiter-Venus, you are approaching the point were general relativity corrections might be significant relative to the effect of adding precision through the addition of new planets into the calculations.

    There are, of course, other objects in the solar system and universe. But, the solar system impact from other objects is negligible compared to that of the planets due to their small size, great distances and even distribution about the sun and Earth (most extra-planetary objects are beyond Mars orbit from the Sun). And, the extra-solar system objects in our universe pretty much, are so distant that they have virtually no impact on the relative motion of the Earth and the Sun.

    You would have an elipse even in a pure two body system as explained above. This follows from Kepler's law which in turn follows from Newton's law and some basic Newtonian mechanics.
    Last edited: Dec 14, 2004
  12. Dec 21, 2004 #11
    Yet, the gravity of the other planets show distinctively.

    The Earth's spinning axis is making a precession movement. But there are two precession rhythms. The period of precession of the equinoxes is roughly 26,000 years but the Croll/Milankovitch precession cycle of the aphelion and the perihelion is about 19 - 23,000 years. Theoretically they should be the same. Why the difference?

    Apparantly, mostly that very weak gravity of Jupiter, that pulls the direction of the elliptical axes of Earth orbit opposite to the direction of the precession, speeding up the cycle of the aphelion / perihelion.
  13. Oct 12, 2005 #12
    I think the core is the cause of orbital motion. A larger iron core generates faster orbital motion. Otherwise, why does mercury contain an iron core twice as large?
    Last edited: Oct 12, 2005
  14. Oct 12, 2005 #13


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    Wow, 10 months later and...
    What term in Newton's law says anything about iron?

    The root cause of orbital motion is latent rotational kinetic (and further back, unbalanced gravitational potential) energy in the early formation of the solar system.
  15. Oct 12, 2005 #14
    Thanks. I read about the nebular hypothesis. Accordingly the solar system formed from a solar nebula which collapsed under its gravitational field, forming a disk.
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