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Thank you, Paul, for looking up how the N. A. uses and makes reference
to these calculations.

The refraction formula is indeed the one from Bennett in a form which
provides the resutls in degrees. Bennett provided also a second
formula for further improving the result of this first formula. Does
your N.A. 1998 edition makes reference to it?

Just in case these finding should one day be documented in a paper:
How is the correct reference to this N.A. edition?

Marcel


On Mon, Apr 8, 2013 at 6:59 AM, Paul Hirose  wrote:
> ________________________________
>
> Does the Almanac still have "methods and formulae for direct
> computation"? That chapter in the 1998 edition gives the dip formula
> 0.0293° * √h, where h is meters. I tested that against the dip table
> (A2) and got perfect agreement at the extremes as well as several
> intermediate eye heights
>
> Refraction may be calculated with the formula 0.0167° / tan(H + 7.31 /
> (H + 4.4)) at standard conditions of 10C and 1010 mb, according to the
> same chapter. To correct for nonstandard conditions, multiply the result
> of the above formula by 0.28 * P / (T + 273).
>
> The refraction values in the Almanac are the total deflection of the ray
> from the observer to the top of the atmosphere. However, in the case of
> dip we are only interested in the portion of the ray from the observer
> to the point where it grazes the sea.
>
> --
> 
>
> : http://fer3.com/arc/m2.aspx?i=123432

   

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