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Moon's orbit.

  1. May 11, 2006 #1
    I wanted to know if moon's orbit is always in the same route and that it never change? if no why??

  2. jcsd
  3. May 11, 2006 #2
    No it isn't it chages. It's why we don't have a solar elcipse and a lunar elcipse every month.
  4. May 12, 2006 #3


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    No, The reason we don't have eclipses every month is that the Moon's orbit is tilted to the ecliptic.

    But to answer the original question, the Moon's orbit does vary over time. It increases in size by about 4cm a year, it precesses, it is stretched and squeezed by tidal froces from the Sun, etc.
  5. May 12, 2006 #4
    Is the only change in orbit 4 cm per year, as you pointed out we don't have an eclipse each mont, doesn't this mean that the changes are so often??? I would be glad if someone would be able to provide me with the orbital details. Thankyou.
  6. May 12, 2006 #5


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    It depends on what you mean by "changes". ALL of the properties of the Moon's orbit that Janus mentioned except the increase in size are cyclical and therefore can be said to be constant from a certain point of view. Heck, even the tilt of the earth's axis and the shape of the earth's orbit affect when/where we get solar and/or lunar eclipses and they don't have anything to do with the shape and orientation of the moon's orbit.

    About a third of the way down on this site are the various periodic properties of the moon's orbit (which doesn't include all the earth's motion's affects on the moon's position in the sky): http://en.wikipedia.org/wiki/Moon

    But if your question is just about whether the moon will be exactly the same place in the sky the next time it hits full as it was the last time it hit full, then no, it will not.
    Last edited: May 12, 2006
  7. May 23, 2006 #6


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    'Changes' in the moon's orbit over your lifetime are essentially negligible, but as Janus and Russ pointed out, there are many aspect that change if you are looking for precise calculations or long-term views.

    If the moon orbited the Earth in the same plane as the Earth-Sun orbit, then there would be a solar eclipse (and a lunar eclipse) every month. But the moon's orbit is skewed by 5 degress (if I recall correctly) from that plane, so the proper lining-up of the sun-earth-moon (horizontally and vertically with respect to that plane) for an eclipse is much rarer.
  8. May 27, 2006 #7
    Bye-Bye Moon...the orbit of the moon does change. Much earlier in history long the moon was closure to the Earth, but is slowly expaning in its orbit from Earth--some day there will no longer be a Moon--Question is, would the Sun in its dying throws consume the Earth before the Moon slips away.
  9. May 27, 2006 #8


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    Here's a link to an animate GIF showing the Moon's orbit over an 18 year period, the "Saros cycle". This may help illustrate why eclipses can only happen every 6 months.


    It illustrates the precession of the Moon's orbital nodes. Notice the Moon's perihelion and aphelion also trade places at a rate of about twice per Saros cycle.

    Most of the motion you see here is caused by the Sun's strong influence on the Moon at its present distance. The Moon orbits the Earth at a distance of almost 1/3 of the Earth's Hill Sphere radius. 1/3 of this radius is approximately the boundry between where things can orbit in prograde orbits and still keep a relatively round stable orbit.
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