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    Re: Moon Horizon
    From: George Huxtable
    Date: 2009 May 10, 23:59 +0100

    Thomas Kleeman asked-
    
    | since the discussions revolve around some practical issues in taking
    | sights lately, I want to bring in an puzzlement of my own:
    |
    | In several works on celnav the authors had stated the bright reflection
    | of the moon on oceans surface would obscure the visible horizon at night
    | and create a line below it, called moon horizon (at least by some of
    them).
    |
    | If I understand the arguments correctly, it must be some kind of effect
    | similar to the problems of dip and light distance?
    |
    | I thought about a function of altitude and height of eye to correct for
    | that "shortness of dip"...
    
    =======================
    
    from George-
    
    No, it has a different cause. It depends on the ripples on the surface of
    the water, and the range of angles the surface makes with the horizontal.
    That can cause moonlight to be reflected from the surface direct to the
    observer's eye, and the range of angles over which that's possible gives
    rise to a band of brightness over the sea surface. It doesn't "obscure" the
    true horizon, but because the true horizon is so faintly lit by moonlight,
    the specular reflection distracts from it.
    
    The same sort of dazzly ripple can occur with the Sun in just the same way,
    but there's enough light about to illuminate the true horizon, so it's more
    obvious that the band of reflected dazzle is a "false horizon".
    
    Does that make sense?
    
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    
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