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George H wrote:
"Bennet has provided a   formula which is an empirical attempt to fit that
averaged data. At large  angles of altitude, it becomes proportion to the
tan of the zenith angle, as  Snell's law requires. Near the horizon, where
refraction rises sharply, the  divergence from Snell's law shows up in
correction terms which turn out to be  remarkably simple."

Of course, Snell's Law always applies. It's just that  a simple "slab" model
of the atmosphere doesn't work near the  horizon.

And:
"In some publications, such as the Nautical Almanac, it  appears that
Bennett's
formula itself is used as the basis for the refraction  tables"

I don't think there's any reason to suppose that Bennett's  formula is used
in the Nautical Almanac. The tables predate the formula.

And:
"(though the constants have recently been tinkered-with a bit to  improve the
fit to
recent Pulkova data)"

I know that someone (possibly  you --?) previously speculated that the
changes in the almanac refraction tables  were based on "recent Pulkova data" but
has that been confirmed?

-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars

   

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