NavList:

As you noted the discrepency in the horizon formula constant between the 2.07 and 2.12 , I just did a back calculation of the numbers and it brought me right back to a question I stumbled upon long long ago in basic navigation studies when I did my masters license.

I had seen the horizon formula constant (english units) quoted in different places as 1.14 & 1.17.    Interestingly enough, that matches the metric 2.07 & 2.12 issue close enough for me to guess that numerical value 2.07 shared a common origin with the 1.14 number.

I may well still have the refrence in my sea chest of books that stated the constant as 1.14 (can't remember who). I checked multiple sources at the time and finally settled on the 1.17 as most correct by majority vote (not the safest methodology).

In my defense i did try to use trigonometry to prove which was more correct, but attempted it solely with planar trig calcs in a faulty enough muddle back then to prove nothing to my satisfaction. 

It does set me to thinking....What is the origin of the difference?  Maybe the oblate spheroid dilemma and averaging locations?   Purely a mistake requoted?   I am not expecting super precison out of the whole horizon thing based on the inputs......but I do have to say, I like my constants...to be...well...CONSTANT.  (Oh Heck....or at least predictably variable) .

Any thoughts on why the second pair of  numbers does exist? 

I note the other constant frequently quoted several different ways is the radar horizon constant I see quoted as 1.22, 1.23 & 1.24.