NavList:

Bill,

The difference between the line of sight distance and the earth surface distance from the bridge of a ship is very small and can be ignored. Refraction is what needs to be allowed for thus the multiplier in the Bowditch formula.

Compared are the distances in nautical miles (NM) to the horizon from 100 ft above sea level:

Tangent line of sight ( right triangle trig ).................... 10.641153 NM
Along curvature of Earth ( mean radius 3440.1 NM ) ... 10.641119 NM
Bowditch formula adjusted for refraction...................... 11.694970 NM

For your Navy MK V bubble octant it will be necessary to figure separate index errors for the bubble and horizon prism before dip can be measured.

Greg Rudzinski

[NavList] Re: Bowditch tables 12 and 13
From: Bill B
Date: 15 Aug 2012 22:38
On 8/15/2012 9:07 AM, Jeremy C wrote:
> I don't know where the .06 factor comes from as geometric horizon would
> seem to me to be a straight line tangent to the curve of the earth.

Thanks Jeremy, Greg and Lu.

In the bigger picture I am trying to lay the foundation for devising a
method of using the practice bubble, or a sextant like the Navy Mark V
that can see the horizon, as crude dip meter (from a stable platform.)

I see the first step as calibrating the bubble error from a natural
horizon. More on that later under a separate subject heading.

Bill B

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