brian whatcott wrote:

On 7/26/2010 1:12 PM, Terje Mathisen wrote:

I believe that h/2R term is significantly smaller than the normal
atmospheric effects, i.e. today I can see features on the other side of
the Oslo fjord that I know should be below the horizon. :-)


You betcha! It's usual to take the notional radius of the Earth as 20%
bigger than the real radius .. to account for refraction.

Ah, so that's why the fiddle factor in the "standard" rule of thumb
is 1.2 instead of 1.0 (or 1.064)! Or is it because the rule is for
non-nautical miles?

(which of course itself varies because of
the oblateness of the spheroid)

But only by a negligible amount (by comparison to the refraction effect),
the equatorial radius exceeding the polar radius by only about one third
of a percent.

One counterintuitive aspect worth mentioning is that while of course the
polar radius is smaller than the equatorial radius, the "radius" we must
use for horizon distance purposes is biggest at the poles and smallest
at the equator. That's when the horizon is N or S of the observer. An
equatorial observer needs to use a different radius when looking E or W
than when looking N or S.