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Homework Help: Is this statement about the orbit of the moon true?

  1. Aug 25, 2006 #1
    examine the truth of the statement

    "the moon moves in a near-circular orbit around earth. because the earth is so much more massive, it own motion is not appreciably affected by the moons presence"

    plz help
  2. jcsd
  3. Aug 26, 2006 #2


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    I just depends on how one defines "appreciably". As the Earth and moon are gravitationaly bound to one another in what could be called a single system (Earth-Moon System), they each orbit around the center of gravity for that system. But because the difference between the two objects in that system is so great, the system looks very much like a stationary Earth with the Moon going in a circle around it.

    The difference between the two masses means that the center of gravity for the total system is deep inside the Earth, about 1/4 of the way down to the core. So, while the Earth goes around that center of gravity in tiny circles that are somewhat smaller than the radius of the planet, the Moon swings around in a circle with a radius of about a quater-million miles.

    If one were to model the system with a model small enough to see the whole thing at once, the movement of the Earth might not even be visible with the naked eye.
  4. Aug 26, 2006 #3


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    Last edited: Aug 26, 2006
  5. Aug 26, 2006 #4
    so how does the moon effects the earth

    i know it effects it by tides but are there any others and how does it make earth have two tides every day

    i am very confused
  6. Aug 26, 2006 #5


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    The moon pulls hardest on the side of Earth closest to it. It pulls medium on the middle of the Earth, and it pulls the least on the side of the Earth farthest from it. This gravity gradient through the Earth's sphere pulls the Earth into an oval with the long axis orientated towards the Moon. But since the Earth spins, it is not exactly towards the Moon. It is slightly ahead of the Moon. This causes a torque that slows down the Earth's rotation and pulls the Moon into a higher orbit.
  7. Aug 26, 2006 #6
    Thanks tony873004 that makes so much sense know
  8. Aug 26, 2006 #7
    can u help me with this

    imagine that the universe is static and not expanding. predict the observations that would be made in regards to:

    a)red shift in galaxy spectra

    b)temputer of cosmic backround radiation
  9. Aug 26, 2006 #8


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    a) You might expect to see a random distribution of red/blue shifts. In an expanding universe, only galaxies that are very close, such as M31 will exhibit a blue shift (coming towards us). But in a static universe, even distant galaxies may exhibit blue shifts.

    b) tough question. There may not even be one. I'd want to know the new theory that replaced the big bang and expansion to guess at this one.
  10. Aug 26, 2006 #9
    ok thanks last one
    it is observed that Mars is somewhat brighter when in opposition (ie. sun- earth - mars in that order) that at other times. How can this be explained using
    a) The Ptolemaic model and
    b) The Copernican model of the solar system.
    (Assume that Ptolomy's construction describes the actual motion of the planet
  11. Aug 26, 2006 #10


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    Don't take my word on the expanding universe question. Those are just my guesses. It's a hypothetical question so many people may have many different answers.

    "last one"... Hmmm.... I'm starting to get the feeling I'm doing your homework for you....
    In both models Mars is closest to Earth at opposition. Google "Kepler's Pretzel" for the Ptolemaic model.
  12. Aug 26, 2006 #11
    k thanks alot for ur help
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