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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?

Marcel

   

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