# NavList:

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

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Re: Table 8, Bowditch
From: Ark Shvetsky
Date: 2009 Jan 12, 23:34 -0800

```Thank you, Frank!

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?

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?

Ark

----- Original Message ----
From: "frankreed@HistoricalAtlas.com"
To: NavList@fer3.com
Sent: Monday, January 12, 2009 11:19:43 PM
Subject: [NavList 7031] Re: Table 8, Bowditch

"I'm trying to understand why for calculating the approximate radar range at
which any given target will return an echo-it is suggested that Bowditch
Table 8 can be used with the distances increased by 4-5%% to allow for the
slight curvature of radar waves over the geometric horizon but that provision
is apparently not used for the visible horizon calc?"

This table already incorporates the refraction for visible light. The
refraction for radio waves is greater. So you can use the visible light table
by applying that 4-5% correction, if I am remembering correctly.

All of the behavior of light with respect to terrestrial refraction for dip,
dip short, etc., but NOT astronomical refraction, can be calculated by
pretending that the radius of the Earth has increased (one can show using
some physics that this is actually rigorously correct under certain
simplifying assumptions about the atmosphere). For visible light it's
effectively about 15-20% bigger under normal atmospheric conditions and
that's already built into the tables.

By the way, if you've worked out the distance to the horizon by geometry, and
it appears to match nearly enough the values in the tables, beware that it's
a spot where it's easy to get nautical miles and statute miles working
against you beause, just by chance, the 15% difference in those types of
miles happens to be about the same as the 15% correction for refraction of
light. Do the whole calculation in nautical miles, and you will find that the
table values do not match a purely geometric calculation (for the actual size
of the Earth, that is).

Note that these phenomena depend on the density gradient of the air in the
lower atmosphere which itself depends primarily on the temperature gradient.
Depending on the weather, these things can change by 5-10% easily and
sometimes quite a bit more.

-FER

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