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If the sun were to approach the earth

  1. Feb 7, 2009 #1
    ... as the sun was traveleing at a closer distance towards the earth , would earth bound objects start to fly away from the earth surface and head towards the sun's surface , or would everything on earth surface fly towards the sun's surface as the same time the earth itself flew towards the sun's surface?
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  3. Feb 7, 2009 #2


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    Hi pentazoid! :smile:
    The second one …

    except that the surface of the ocean would get very slightly further away from the ocean floor (it's an inverse-cube ratio, I think) towards the centres of the hemispheres of the Earth facing and opposite to the Sun. :smile:
  4. Feb 7, 2009 #3
    Assuming unrealistically, that everything on the earth's surface doesn't vaporize, I find it hard to imagine that everything on the earth's surface and the earth itself will simultaneosly gravitiate itself towards the surface of the sun. It seems more of a realistic scenario to me that as the sun travels towards the earth, objects on the earth's surface gradually start to pull away from the earth surface and start to gravitate toward the sun's surface.. Its not like the earth bound objects are attched to the surface. Well we are attached to the earth surface but thats because there not an object around that is larger than the earth surface.
  5. Feb 7, 2009 #4


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    Any force pulling objects off the surface of the Earth would be due to the difference between the Sun gravity pulling on the center of the Earth and at the Earth's surface. This difference is called tidal force. For this tidal force to be strong enough to actually be stronger than the force of gravity holding objects to the surface of the Earth, you have to be close enough to be within the what is called the "Roche Limit". The Roche limit any two bodies depends on their masses and densities.

    Because the Sun is much less dense than the Earth, it turns out the Roche limit for the Earth would be below the surface of the Sun. IOW, the Earth would hit the Sun before the Sun's tidal force could ever pull an object off the surface of the Earth.
  6. Feb 8, 2009 #5


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    That's the trouble with imagining. Imagining is based on common sense, and common senese is based on things we've experienced before in the course of our natural lives. And I'm faaairly certain you haven't experienced the tidal effects of a close approach of the Earth and Sun. :smile:

    Janus' explanation is brilliantly concise.

    I'll add just one other aspect to it:

    Any force that would pull an object off the Earth's surface is just as much going to act on the Earth as on the object. i.e. if the sun's pull could suck the object into it, then it's going to suck the Earth into it. We on the Earth won't notice any change, the Earth - and everything on it - will fall normally toward the sun.
    Last edited: Feb 8, 2009
  7. Feb 8, 2009 #6


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    Seconded! :approve:

    Janus, how about a PF Library entry on "Roche limit", or "tidal force"? :wink:
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