# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**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 --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---