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Frank, you gave me� very clear, down-to-the-Earth� answers.� I thank you very much!



----- Original Message ----
From: "frankreed@HistoricalAtlas.com" 
To: NavList@fer3.com
Sent: Tuesday, January 13, 2009 6:07:38 PM
Subject: [NavList 7042] Re: Table 8, Bowditch


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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