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

**Re: Dip: Astronomical Almanac vs. Nautical Almanac**

**From:**Brad Morris

**Date:**2013 Apr 21, 19:13 -0400

Hi Marcel

From my reading, D(H)=1.75sqrt(H,m) for the dip of the horizon.

When computing the additional refraction of a celestial object on the horizon, for rise or set, one needs an additional refraction correction of R(H)=.37sqrt(H,m).

Note the equation later on that shows A-D(H)-R(H), showing that the sign is negative.

I believe they are showing how to compute astronomical refraction for a body on the horizon (rise/set) as compared to only the terrestrial refraction, dip. If I am not mistaken, we discussed this point in a previous communication.

Best Regards

Brad

On Apr 21, 2013 5:48 PM, "Marcel Tschudin" <marcel.e.tschudin---.com> wrote:

While searching in the Internet something completely different I dropt on an excerpt of the "Explanatory Supplement to the Astronomical Almanac" containing "9.331 The Effects of Dip and Refraction". Reading its content here http://books.google.com/books?id=uJ4JhGJANb4C&pg=PA488&lpg=PA488&dq=9.331+the+effects+of+dip+and+refraction&source=bl&ots=zM_NGe6Ot8&sig=e2vFi4wJZuoDKcE9IKUwDHxlOFQ&hl=de&sa=X&ei=yjx0UaGlM8HY7AaV8oHQDg&redir_esc=y#v=onepage&q=9.331%20the%20effects%20of%20dip%20and%20refraction&f=false I notice that dip appears to differ to how I understand it is used in the Nautical Almanac. Nautical Almanac ---------------- It mentions calculating the dip consisting (to my understanding) of the geometrical angle and *including* the effects of terrestrial refraction as DIP[moa]=1.76*sqrt(H[m]) Astronomical Almanac -------------------- Here almost the same equation as in the Nautical Almanac is mentioned but for what appears to calculate only the geometrical dip: DIPgeom[moa]=1.75*sqrt(H[m]) The terrestrial refraction appears then to be considered seperately by *adding* its contribution REFdip[moa]=0.37*sqrt(H[m]) to the geometrical part and resulting in what appears to be the total (or apparent) value DIP[moa]=2.12*sqrt(H[m]) = DIPgeom + REFdip Where could the misunderstanding be for 1) Almost the identical factor for calculating dip with and without terrestrial refraction? Do the two Almanac treat the dip indeed so much different? 2) The terrestrial refraction being added whereas to my understanding it reduces the geometrical dip and should therefore be subtracted? Any ideas? MarcelView and reply to this message: http://fer3.com/arc/m2.aspx?i=123637