Richard Harrison, KB5WZI, wrote:

See the 19th edition of the "ARRL Antenna Book" pages 23.5 and 23.6.
Eqn. 3:
Dmiles = 1.415 sq rt Hfeet
This can be rewritten:
D = sq rt 2H

Wasn`t it Albert Einstein who wrote something
like: "Don`t make things any more complicated
than necessary?"

I think it was:

"Everything should be made as simple as
possible, but not simpler". -- Albert Einstein

But... whatever. I think Herr Doktor Einstein would
appove of the derivation from first principles found
on:

http://www.wolfram.demon.co.uk/rp_horizon_distance.html

K7JEB
Glendale, AZ