Bill wrote:
Actually, this is an approximation. It works well when h is
small compared to R but the exact formula is:

Dist = sqrt(2Rh + h*h)

I did state h R as a condition for the validity of the formula.

Incidentally, although the formula you cite is more accurate for an
airless world, the Earth isn't airless. The recommendation from Andrew
Young is to pretend that the Earth's radius is about 15 percent larger
than it actually is. This correction probably goes away once you are
sufficiently far enough from the Earth. On the other hand, I don't
think that most people are interested in a formula for distance to the
horizon from an "altitude" of several Earth radii.

--
Brian Tung
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