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B What part of the Moon am I looking at?

  1. Feb 8, 2017 #1
    I'm standing at a location near the equator and see a full moon in the sky overhead. From my vantage point, the top point of the Moon is (approximately) at the Lunar equator and the Lunar poles are 90 degrees away at the sides. Is this correct?
     
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  3. Feb 8, 2017 #2

    phyzguy

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    It depends how you define, "the top point of the moon". Is the moon directly overhead (i.e. at the zenith)? If so, which point is the "top"? If it is not at the zenith, where in the sky is it?
     
  4. Feb 8, 2017 #3
    A bit above the horizon. The top meaning the point on the Moon furthest up in the sky.
     
  5. Feb 8, 2017 #4

    phyzguy

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    I think it's much more complicated than you say. It depends where on the ecliptic the moon is, since this will determine the angle between the ecliptic and the horizon. Also, the axis of the moon's orbit is tipped 5 degrees relative to the ecliptic plane, and the Earth's axis of rotation is tipped 23 degrees relative to the ecliptic plane. Why don't you try sketching it out when the moon is on the celestial equator, at it's highest point above the equator, and at its lowest point below the equator?
     
  6. Feb 11, 2017 #5

    sophiecentaur

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    You can never see both poles at the same time. There is a significant shortfall when the object you are looking at is as large and near as the Moon.
    A bit of trigonometry would tell you just how much is invisible.
     
  7. Feb 11, 2017 #6

    Janus

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    The moon's polar axis is also tilted 6.687 degrees to its orbital plane, which gives the moon a ~1.54 degree axial tilt to the ecliptic.
     
  8. Feb 12, 2017 #7

    mfb

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    That effect is smaller than 1% for a perfect sphere: You can see more than 49% of that idealized sphere. Mountains etc. lead to additional surface area you cannot see, but it also leads to points beyond the idealized horizon that are visible.
     
  9. Feb 13, 2017 #8

    sophiecentaur

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    Because of libration, we see much more than half of the Moon's surface so that 1% is not very relevant.
    The effect is much more relevant for satellite observations (only around 2200km for the ISS), where the distance is a lot smaller. The effect of reducing the distance to the horizon is even worse when you are in a small boat at sea, when it reduces to zero in the limit.(Except when viewing high objects, of course)
     
  10. Feb 15, 2017 #9
    Still having trouble grasping this concept unfortunately. When I see a half-moon in the sky divided diagonally (the dividing mark between the light and dark halves is not straight up and down), does that mean the lunar poles are approximately at the two ends where dark and light converge?
     
  11. Feb 15, 2017 #10

    sophiecentaur

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    Have a look at the diagram here. That shows the angles involved. The sun lies in the plane of the ecliptic and none of the angles of the Earth's rotation, the Moon's rotation or the Moon's orbit are zero. Also, your view from the Earth will be affected by your latitude. A simple model of the system will not show you exactly what you will see.
    The lunar axis will appear to be (approximately) horizontal when viewed from the Equator and (approximately) vertical when viewed from the poles. The position of the full Moon in the sky will vary in azimuth as the year progresses. In winter, the Sun appears low in the sky but the full moon (being the opposite side of Earth) will appear high but not by the same amount every year. The diagram in the link shows this situation. The opposite applies in Summer. But the Moon has its own tilt and so you will see more or less of each pole, depending where the lunar month phase is related to the Solar year (everything is out of sync).
    The Moon has its own version of Summer and Winter because of the tilt of the axis to the ecliptic.
    Do some waving of the hands around that diagram and see what happens as the bodies orbit and rotate.
    Alternatively, get hold of a (free) copy of Stellarium and, after a few hours of practice (unfortunately for you) you can simulate all of it. Red Shift (not free) will show you these things happening from other points in space, once you have learned to drive it.
     
  12. Feb 15, 2017 #11
    Thanks. This fully answers my question. I will have a look at the diagram anyway.
     
  13. Feb 15, 2017 #12

    sophiecentaur

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    Google Images has pages and pages of pictures of what you want. Search terms like tilt of the moon's orbit etc. Very entertaining.
     
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