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    Re: Table A4 + elevation?
    From: Trevor Kenchington
    Date: 2003 May 3, 14:52 -0300

    Thanks George! That was obvious once you pointed it out and so provoked
    me to write out the equations.
    
    You wrote:
    
    > For all angles of altitude except very low ones, it's a VERY good
    > approximation to treat refraction of light in the atmosphere as though the
    > earth's surface were flat, as are any boundaries or contours of pressure or
    > temperature in the air above it.
    
    
    I don't think that it is necessary to assume a flat Earth. The only
    assumption that (I think) is needed is that all layers in the atmosphere
    have plane boundaries with all of the planes parallel. Given that, the
    refractive index of each intermediate layer cancels out of the
    equations, leaving only the refractive index for space (unity, as you
    noted) and that immediately around the observer's sextant.
    
    The assumption of parallel layering would, as you say, become an issue
    with light rays nearly tangential to the Earth's surface. It might
    sometimes fail when viewing objects through a weather front too, though
    cloud cover will then usually obscure astronomic bodies anyway.
    Otherwise, there are local instabilities which astronomers trouble over
    in terms of the "seeing" conditions at a particular time but the
    resulting perturbations are (always?) too small to be of navigational
    concern.
    
    > In the plane-parallel case, however, Snell's law applies, and then the
    > overall bending of light, from its original direction, as it reaches the
    > observer, is determined ONLY by the angle of incidence and the difference
    > between the refractive index as it comes into the atmosphere (which is
    > exactly 1, for free space) and the local refractive index at the level of
    > the observer. It doesn't matter a damn what happens in-between, or where it
    > happens. The index in-between may vary up or down, different layers can be
    > thick or thin, but nevertheless the total overall bending will be exactly
    > the same. Surprising but true (well, it surprised me, at first).
    
    
    It shouldn't be surprising. We are all familiar with the same phenomenon
    when looking through window glass: Objects seen through a quality glass
    pane appear to be displaced fractionally sideways by the refraction of
    light as it passes into and out of the glass but the angle at which
    those rays reach our eye is not affected by their passage through the
    window pane -- and would not be whether the pane was made of low-density
    acrylic or high-density lead crystal glass. Use the same glass to make
    the face-plate of a SCUBA diver's mask and refraction between the water
    outside and the air inside the mask has a substantial effect on vision
    but the thickness (for practical purposes) and the refractive index of
    the face-plate are of no consequence.
    
    Only if the faces of the various layers (of glass or air) are either
    curved or non-parallel is there any effect on the angle at which the
    light reaches our eyes.
    
    
    Sorry for not figuring that one out without the prompting!
    
    
    Trevor Kenchington
    
    --
    Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
    
                         Science Serving the Fisheries
                          http://home.istar.ca/~gadus
    
    
    

       
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