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    Re: Table 8, Bowditch
    From: Frank Reed
    Date: 2009 Jan 13, 18:07 -0800

    Ark, you asked:
    "Just wants to make it clear for myself :>: to calculate a distance to the 
    visible horizon I need to use statute miles, not nautical.  Is this a correct 
    statement?"
    
    No, I was just talking about a mistake that people sometimes make that causes 
    confusion because it seems to yield the table values without any refraction 
    correction. But not to worry: everything is nautical miles in navigation 
    calculations.
    
    And:
    "Also, just curious: radio waves and light wave are electromagnetic waves 
    which are differ in frequency.  Therefore, it means the refraction-wise high 
    frequency signal is refracted more.  Is it logarithmic or just a linear 
    correlation for refraction rate between low and high frequency waves?"
    
    The variation of refraction with frequency is called a "dipersion relation." 
    There are no simple rules for such things. In fact red light, which has a 
    longer wavelength, bends less than blue light which is opposite the apparent 
    rule for radio versus visible light.
    
    You also asked:
    "you also mentioned a temperature gradient in the low atmosphere as a factor 
    which may effect visible horizon's distance calc by 15-20%  Is it included in 
    the Table 8?.  If not, how to apply this correction factor to the 
    calculation?  Another table exist, I presume?"
    
    First, I said this wrong. It's maybe 10-20% (in many cases) of the correction 
    factor which is itself a number around 0.15 so the actual variation in the 
    distance to the horizon is much smaller: a couple of percent under common 
    conditions. But under somewhat more extreme meteorological conditions, it can 
    be much greater. In fact, if the temperature changes at just the right rate, 
    light rays will travel parallel to the Earth's surface, and, therefore, the 
    horizon is technically at infinite distance (visually it just vanishes in a 
    distant "fog"), and you could expect to see vessels and other objects dozens 
    of times farther away than normal. So how do we correct for this? The 
    suprising answer is that we can't. First of all, we can't measure the actual 
    temperature profile of the air between us and the horizon. And equally 
    important, this is just one type of variation in the refraction. In the real 
    world, there can be much more complex variations in atmospheric conditions 
    resulting in mirages and so on. But even if we can't correct for it, there is 
    still a point to all of this. When you look at these tables for distance to 
    horizon, dip, and dip short, etc., you need to bear in mind that they apply 
    to mean conditions. In practice, dip and distance to the horizon are somewhat 
    variable.
    
    -FER
    
    
    
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