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Q & A: Orbiting the earth and the sun at the same time

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Most recent answer: 12/15/2013
Q:
According to Wikipedia, Hill Sphere is : the volume of space around an object where the gravity of that object dominates over the gravity of a more massive but distant object around which the first object orbits. True as this may be, it just mathematically supports a phenomenon that has been observed but it does not give reason or logic as to why does this happen in the first place. I mean why should the gravity of a less massive object dominate the gravity of a more massive one? I wasn't aware of the Hill Sphere until recently when I was trying to visualize the orbits of different celestial bodies. The Hill Sphere comes closest to explaining why the moon orbits the Earth, more than it orbits the Sun and why the Earth orbits the sun, more than it orbits the center of our galaxy. By this logic all celestial bodies within the Gravitational pull of the center of our galaxy should directly be orbiting the center. My argument is that if the Hill sphere of the Sun is as large as the solar system itself, any object within this sphere should be orbiting the sun. Why was the moon caught into the earth's gravitational pull in the first place when it had a much stronger pull from the sun? The answer to this would also eventually clarify why the earth orbits around the sun and not the center of the milky way.
- Saurabh Khare (age NA)
India
A:

Dear Saurabh
I presume you have already looked at which explains quite a bit of your question.   The main point is that the moon orbits both the earth and the sun as exemplified by the red circle that shows the moon orbiting the earth.   The blue circle shows the earth-moon system orbiting the sun.  Beyond the orange circle an object would not orbit the earth, only the sun.

As far as the solar system orbiting the center of our  galaxy the answer is yes.   There is a definite rotation of the constituents of our own galaxy about its center.   Studies have been made of the rotational directions of many other galaxies, their average comes to nearly zero showing that in our observable universe there is no net rotation.  

 

LeeH


(published on 12/15/2013)

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