# NavList:

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

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Re: A few new questions
From: Bill B
Date: 2006 May 15, 03:12 -0400

```> If I'm standing on the bow of my boat on a bay and the height of my eye is 10
> feet, how far from where I'm
> standing will I need to look out to in order to have a proper horizon?

I am not clear I fully understand the question.  The distance to the horizon
is 1.169 * square root of height of eye in feet. (Bowditch).  Dip is .97 *
square root of height of eye (in feet).

" ...how far from where I'm standing will I need to look out to in order to
have a proper horizon?"

How far the horizon is from you depends almost entirely on the height of
eye, then refraction figures in.  Let's assume height of eye is not an
option. It is what it is--you are standing where you are standing above
water level.

A better question might be,  "How far does the horizon need to be to
minimize sextant parallax errors?"

That depends on the sextant size, geometry and accuracy.  As a rule of
thumb, I would like the horizon at least 3 nm from me for an index check. 8
feet above the top of a swell (or nominal water level) would be OK for a
small craft.

I may be over explaining.  At 10 feet, the horizon is 1.169 * square root of
height-of-eye in feet= 3.7 nm (and there has been considerable discussion on
the list about the terrestrial refraction index that goes into the 1.169
figure).  Basically good to go.

Why?  Let's take a vertical measurement from center of index mirror to
center of horizon mirror.  Say it is 4 inches.  Doesn't sound like much
compared to a nautical mile.  Do the math.  Tangent (4"/ (6076.1 * 12)) =
11.31".  Almost 0.2 arc minutes.  All said and done, about three nautical
miles and any index error problems are not due to parallax.

Bill

```
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