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Alex,

You wrote:

> Everyone knows that measuring altitudes close to 90 is
> hard, however I have not seen the precise explanation of this
> in the books on navigation that I read.
>
> As I think this is of more relevance to
> practical navigators than my previous message,
> I include a quantitative explanation.
>
> The source of error is that you cannot determine precisely
> the point on the horizon which is exactly "below" the body.


I think you have that wrong. The point on the horizon that is exactly
below the body is the point that the body is brought down to when the
sextant frame is exactly vertical. The problem is ensuring that verticality.

With a body of moderate altitude, one can swing the arc and fairly
easily see when the body reaches the lowest point, which is when the
sextant is vertical. As the altitude increases towards 90 degrees, the
apparent radius of the swung arc increases and the point along it
corresponding to the vertical becomes harder to determine.


Trevor Kenchington




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Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
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Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555

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