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Variations in the moon's orbit

  1. Sep 14, 2011 #1
    Why does the moon's orbit move from low in the sky to higher in the sky, then back? I have tried accessing information on precession, etc. with no understandable answer. Wikipedia was no help.

    It's a minor issue, but one that has left me scratching for a long time.
     
  2. jcsd
  3. Sep 14, 2011 #2

    DaveC426913

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    What? Explain. Over what period do you think this happens? Monthly?
     
  4. Sep 14, 2011 #3

    Integral

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    Do you something similar to what the sun does through the year?
     
  5. Sep 16, 2011 #4
    The moon's orbit plane is not aligned with the earth's tilted, and wobbling axis. This is also why solar eclipes can't be observed in the same location on Earth every month.
     
  6. Sep 22, 2011 #5
    Precession is not something you can notice just looking at the sky (unless you live for thousands of years). What I think you are talking about is just the variation in height of the moon's daily path across the sky?

    The sun does the same thing --- because the earth's axis is tilted about 23.5 degrees from vertical, the noon sun is on the celestial equator (a baseline in the sky that runs from due east on your horizon, to a height of (90 - your latitude) at due south, to due west on your horizon) on the spring and fall equinoxes; it is 23.5 degrees north of that (higher in the sky) the first day of summer, and 23.5 degrees south of that (lower in the sky) the first day of winter. (All of that assumes you are in the northern hemisphere.) The sun's path in the sky is called the ecliptic.

    The moon's orbit is close to the plane of the ecliptic, so it follows approximately the same path in the sky. But because it goes through the cycle in a month instead of a year, the difference from day to day is much more noticeable.

    Also, its orbit is close, but not exactly on the ecliptic. It's tilted about 5 degrees, so its extremes of high and low in the sky are greater than those of the sun (it can be as much as 28.5 degrees north or south of the celestial equator), again making it more noticeable.

    Hope this helps.
     
  7. Sep 22, 2011 #6

    DaveC426913

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    The OP's question is far too vague for a meaningful answer. We need to know what he means when he observes it moving 'from low in the sky to higher in the sky, then back'.
     
  8. Sep 22, 2011 #7
    What else could he mean, other than the question I attempted to answer? He is clearly talking about something noticeable to a very casual observer, and he clearly is too intelligent to be talking about simple rising and setting, so what else is there?
     
    Last edited: Sep 22, 2011
  9. Sep 22, 2011 #8
    --The moon orbits the earth. It takes ~27.3 days (sidereal month)
    --This orbit has an inclination, resulting in the moon going between +23.4 degrees and -23.4 degrees throughout the sidereal month.

    Check these images out:
    http://www.google.ca/imgres?q=earth+moon+sun&um=1&hl=en&biw=1440&bih=733&tbm=isch&tbnid=GiBzvlyq3rXqgM:&imgrefurl=http://scienceblogs.com/startswithabang/2009/07/the_best_eclipse_of_the_centur.php&docid=RrlAv9t7iFlUJM&w=694&h=473&ei=qoV7TqTZLYeu8gOEqaQR&zoom=1
    Picture 1

    http://www.google.ca/imgres?q=earth+orbit+tilt&um=1&hl=en&sa=N&tbm=isch&tbnid=dYSh6j074cQJkM:&imgrefurl=http://www.earthgauge.net/kids-archives/december2010_archives&docid=IMIvevmhNpXBjM&w=529&h=327&ei=jIZ7TrzWBc_dsgaJhPDdDw&zoom=1&biw=1440&bih=733&iact=rc&dur=141&page=1&tbnh=100&tbnw=161&start=0&ndsp=28&ved=1t:429,r:2,s:0&tx=40&ty=32
    Picture 2

    What you have to understand is that the inclinations (angles WRT to the ecliptic) of these orbits do not change in their orientation. The "celestial direction" of the moon's perigee is always the same. Just as the Earth's axial tilt always points the NP to the same general celestial direction. So as these begin to differ, the moon takes different positions in the sky. Depending on where it is in it's orbit and where the Earth is in ITS orbit. Obviously also where you are on the Earth.
     
    Last edited: Sep 22, 2011
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