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Moonlight Reflection on the Sea

  1. Aug 13, 2011 #1
    Hello..

    I just registered to ask this question which we were discussing with my friends for a while..

    Why do you think the moonlight reflection on the sea is wider on the horizon once the moon is on the top, and the reflection goes narrower when the moon moves towards the horizon..?

    We also have seen that the reflection is longer towards us (viewing point) when the moon is closer to the horizon but it starts from far away when the moon is on the top..

    I was expecting to see the reflection in the same width with the moon's view..

    I'm really curious about the reasons behind this and i'm eager to read about reflections/moonlight if there are any sites or documents about it..

    Thank you so much..!
     
  2. jcsd
  3. Aug 13, 2011 #2

    DaveC426913

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    First thing we need to determine is: is it true under general conditions, as opposed to in this weather condition or that?

    The height and width of the reflection are highly dependent on the wave height at that time.
     
  4. Aug 13, 2011 #3

    xts

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    I think oyuncu's observation is right. I have similar impression: on still weather, if you look at Moon reflection frow high bridge or building, the reflections is smaller than if you look at it sitting in a rowing boat. Or - rather - reflection observed from the boat seems bigger than Moon itself.
    My ad hoc theory (Disclaimer: they often are quite wrong!) is that it is related to low waves, moving the moon reflection here and there, while from large distance the reflection is still.
     
    Last edited: Aug 13, 2011
  5. Aug 13, 2011 #4

    xts

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    Ad hoc explanation no.2:

    In contradiction to optics laws learnt at school, we see the Moon reflection not infinitely deep in the lake, but just as a pancake floating in the water. If we are are sitting in a boat, we see the reflection so close, that it becomes a part of our psychological proximity. Such objects appear to be bigger than distant ones of the same angular size.
     
  6. Aug 13, 2011 #5

    DaveC426913

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    Yes, this is one I was going to elaborate on. When both you and the Moon are near the lake surface, there is a very large angle of reflection (almost 180 degrees). Every little wavelet between you and the horizon has at least part of a surface that's the right angle to reflect a piece of the Moon. Every wave has a component that is horizontal (its crest and its trough).

    But if the you and/or the Moon are higher above the water surface, many of the near and far waves will fall outside a zone where a part of their surface reflect from Moon to eye.
    Only the roughest waves will have a component that is canted at a high angle.
     
  7. Aug 13, 2011 #6
    Thanks for the answers..
    I want to explain my question with two images..

    Case 1

    moonlightreflection1.png


    Case 2

    moonlightreflection2.png


    Weather conditions and the viewing spot are the same in both cases..
    Reflection in Case 2 seems somewhat meaningful to me but i don't understand Case 1..
     
  8. Aug 13, 2011 #7

    A.T.

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    The near reflection in Case 1 could be a flat water surface near the beach. In general the reflection pattern depends on the shape of the waves:

    - When the moon is high you see its reflection the surfaces that are sloped towards you. Since the waves have slopes in all directions, you see the light coming from a wide range of directions -> wide reflection

    - When the moon is low you see its reflection in the surfaces that are almost horizontal. Being horizontal, they cannot reflect the light sideways, so you don't see reflection at the sides. When you consider the shape of a wave, you will not find a surface that can reflect at large vertical angle(180°), but small horizontal angle. -> narrow reflection
     
  9. Aug 14, 2011 #8
    Do you mean i would observe exactly the same reflection in both cases if the surface was a still lake instead of a sea..?
     
  10. Aug 14, 2011 #9

    A.T.

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    No, on a perfectly still surface you would see the moon directly, like in a mirror. I just meant that the split in the reflection shown in Case 1 might be due to different wave sizes.
     
  11. Aug 14, 2011 #10

    DaveC426913

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    Yes. I suspected that too. A calm sea with slight waves would explain the far reflection. Those same slight waves rising and cresting as they enter the shallows of shore would explain the near reflection.
     
  12. Aug 14, 2011 #11

    DaveC426913

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    Hang on. There's something funny here.

    If the Moon is that height above the horizon, an ideal reflection would appear the same height below the horizon.

    In order for the water near the horizon to reflect a moon that high in the sky, you'd have to be seeing reflections off the nearside slopes of some pretty highish waves.
     
  13. Aug 14, 2011 #12

    xts

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    Yes. So the 'ideal' reflection would locate somewhere in the sand. But closer you are to the 'ideal' reflection point, smaller waves are sufficient to make 'real' reflection, and stronger the reflection is. It makes a near-shore triangle.

    There must be quite different mechanism for distant reflection. Maybe those are [I miss proper English word... falling, breaking, collapsing, splashy waves], collapsing few hundred metres off the shore, where deep water waves enter near-shore shallows?
     
    Last edited: Aug 14, 2011
  14. Aug 15, 2011 #13

    A.T.

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    The moon in the picture is roughly 3 degrees above the horizon. To reflect moonlight horizontally to the observer the water surface must be tilted 1.5 degrees. I would not call this "some pretty highish waves".
     
  15. Aug 15, 2011 #14

    xts

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    The drawing is not to the scale. This is a case of Moon high above the horizon.
    The 2nd picture was for the Moon low - and there 'low waves' explanation seems to be fine.
     
  16. Aug 15, 2011 #15

    A.T.

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    What is "high above the horizon" for you? I made a picture that shows a similar pattern at about 6° above the horizon:

    1zpkt42.jpg

    If the moon was much higher (for example 30°), the reflection would be all over the water.

    The interruption in the reflection might be due to steeper wave slopes where the waves reach shallow water. Far away the waves are high, but have less slope. And near the beach they lost their energy and are low.
     
    Last edited: Aug 15, 2011
  17. Aug 18, 2011 #16
    Thanks guys..
    Waves explanation seems logical to me now..
    My question is answered..
     
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