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Conjuction of planets to Earth, it affects our weight?

  1. Jan 21, 2013 #1
    Today an interesting question came to my mind...
    I have heard that when is a conjunction of planets to Earth, it affects our weight.
    If the conjunction occurred at larger planets to Earth - Jupiter, Saturn, Uranus, Neptune (in one line with Earth) - how it affects our weight?
    Surely we will use third Newton's law, but beyond that I do not know what to do. Have you got any ideas?

    Sorry for my English and thank you very much.
  2. jcsd
  3. Jan 21, 2013 #2


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    The affect that ANY other celestial body has on our weight on Earth is absolutely miniscule. Even the Moon, whose gravity affects us most barely causes any change.

    My rough calculations put Jupiter's acceleration on us at about 2.3x10-7 m/s2. For comparison the Earth's gravity accelerates us at 9.8 m/s2.

    Even during conjunction when the planets "line up" with each other, nothing special happens other than their forces add up. But they were already doing this beforehand anyways, just to a slightly lesser amount. Even adding up every outer planet the acceleration would still be less than 3x10-7 m/s2 or so. This is an extremely small amount.
  4. Jan 22, 2013 #3
    It is clear to me that it will not be much difference, but I want to know how to calculate.
  5. Jan 22, 2013 #4


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    I use this calculator: http://www.wsanford.com/~wsanford/calculators/gravity-calculator.html

    Put in the mass of one object, say Jupiter, in the first line, and the mass of the 2nd object, say yourself, in the 2nd line. Then put in the distance and hit calculate.

    A1 is the acceleration OF THE 1ST OBJECT, caused by the gravity of your 2nd object. It's how much Jupiter accelerates thanks to your gravity.

    A2 is the acceleration of the 2nd object, you, caused by the gravity of Jupiter.
  6. Jan 22, 2013 #5
    Not only the effect is very small; the attraction acts on planet Earth as well as on you. So the net effect is null.
  7. Jan 22, 2013 #6


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    That is an important point - you don't feel the attraction itself, you only feel a different acceleration for you and for earth, if your distance to the planet is different. As that difference is extremely small relative to the distance of other planets, the influence on your weight reduces by some additional orders of magnitude. I calculated that here, multiply it with your mass to get the weight difference.
    Any visible grain of sand will have a greater influence on your weight than any planet.
  8. Jan 23, 2013 #7
    And even if you want to take into account those tidal effects, the Sun's influence here is much greater than all of the planets' – though still very small.
  9. Jan 23, 2013 #8


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    Lets consider the sun and moon's effect on your weight, both of which are much closer than Jupiter - and the sun is hugely more massive than Jupiter. And the answer is absolutely negligible - maybe 10-15 milligrams. Changes in barometric pressure, by comparison, can cause you weight to fluctuate by several grams.
  10. Jan 26, 2013 #9
    And one more question...

    We have line Sun - Earth - Jupiter - Saturn - Uranus - Neptune in conjuction. I calculated the value of accelerations the objects for the Earth (how acceleration acting on Earth). Sun vector is the opposite of the others vectors. The resulting acceleration is difference Sun acceleration and others planets acceleration.Value of the resultant acceleration at the Earth is difference of gravity acceleration and my previous calculation.

    It is good idea?
  11. Jan 26, 2013 #10


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    Yes. That's a method of measuring the perturbations in the Earth's orbit. Of course, that's only measuring the perturbation at some given instant, so it's of limited value. You'd have to do your calculations over and over using different locations of each of the planet to get a very precise trace of the Earth's orbit around the Sun (but it would still follow a basic two-body trajectory with small perturbations simply because the Sun is so big compared to the planets).

    On a more practical level, one could measure the net gravitational acceleration of some object between the Earth and Sun, figure out how fast an object would have to be traveling in order to stay in that spot, and then you could place a satellite in that location. (SOHO or ACE, for example.)

    Check out LaGrange points.
  12. Jan 27, 2013 #11
    When the confines of the major part of the gravity of the Earth is left the effects of the Sun, Moon and other planets will be more important.
    Last edited: Jan 27, 2013
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