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Moon's Inclination To Earth's Equator

  1. Jan 22, 2009 #1
    I'm trying to find a graph showing the cycle of the Moon's orbital inclination relative to the Earth's equatorial plane. A google search has revealed that the Moon's inclination is between 18.29° and 28.58° to the Earth's equator. But this is where the information ends. Where is the graph showing the orbital cycle of inclination relative to the equator? Wikipedia seems to side-step the issue entirely.

    I live at the very southwest tip of England (St. Ives) and often I can look north into the sky and see the Moon. But I'm at 50°N. How does this reconcile with the given maximum inclination of 28.58° to the equator?
     
    Last edited: Jan 22, 2009
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  3. Jan 22, 2009 #2
    I believe the maximum inclination would be attained every 18.5996 years, the nodal cycle. At this time the ascending node would be at the vernal point.
    One could mark a "lunar arctic circle" that would be 66.56° - 5.14° = 61.42°N. Here the moon would be due north on the horizon at the time of the maximum inclination.
    I was just reading about the ancient stone circles of England. By their arrangement, it seems that this northern moon was significant to behold and auspicious to these ancient people's now-forgotten religion.
     
  4. Jan 22, 2009 #3

    D H

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    You are not observing the Moon from the center of the Earth, which is the origin for the equatorial reference system. You are observing from a point on the surface of the Earth. The angles that describe the location of some object in the horizontal coordinate system are azimuth and elevation rather than right ascension and declination.

    Think of it this way. At 50°N, on June 21 you will see the Sun rise in the northeast and set in the northwest. The path of the full Moon in the winter sky follows similar path to as does the Sun in the summer sky. The full Moon a couple of weeks ago occurred a couple of weeks after winter solstice, so you would have seen the Moon rise in the northeast and set in the northwest.
     
  5. Jan 22, 2009 #4
    Thanks Helios, that was very helpful. My next question is: how often is the Moon at syzygy, (i.e. a full or new Moon), at the same time as on the "lunar arctic circle"?

    Or should I say "what is the duration of the Moon's orbit above 50°N, part of an 18 year cycle?"
     
    Last edited: Jan 23, 2009
  6. Jan 23, 2009 #5

    tony873004

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    Here's a web page I made about the Saros cycle. It includes an animation that shows how the moon's orbit changes over a complete 18 year cycle:
    http://www.orbitsimulator.com/gravity/articles/saros.html
     
  7. Jan 24, 2009 #6
    Very impressive tony. The simulation helps alot. What is the cycle of lunar eccentricity though? Is it 18 years as well?
     
  8. Jan 24, 2009 #7

    Janus

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    Do you mean the period of the Lunar perigee precession? If so, it is just under 9 years.
     
  9. Jan 24, 2009 #8

    D H

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    The Saros cycle (6585.33 days or 18.03 years) is more-or-less a numerological coincidence. 223 synodic months, 242 draconic months, and 239 anomalistic months represent nearly the same time span (6585.32, 6585.36, and 6585.54 days). The Moon's apsidal precession (see post #7) period is 8.8504 years and is caused primary by solar tidal forces. The Moon's nodal precession period is 6793.5 days or 18.6 years and is caused primary by Earth's oblateness. Note that neither precession fits neatly into the mystical Saros cycle.
     
  10. Jan 24, 2009 #9

    tony873004

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    I think it's also cause by the sun. Both the 18-year nodal precession and the 9-year perigee precession are in the animation I posted in the above post, yet the simulation that created the animation used a point-mass for Earth, rather than an oblate sphereoid.
     
  11. Jan 27, 2009 #10
    I'm interested in the 'mystical' element. Is it remotely possible that the orbit of the Moon was different in the recent past? Is it possibe that it's orbit could have been altered by a near-miss with an object of comparable size for example?
     
  12. Jan 27, 2009 #11

    D H

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    Due to tidal interactions, angular momentum due to the Earth's rotation about its axis is being transfered to the Moon's orbit around the Earth. The Moon was a lot closer to the Earth a long time ago.

    Possible, yes. Likely, no. Needed to explain things? Absolutely not.
     
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