On 12 Oct, 10:46, "Roger Smith" wrote:
"Joe Egginton" wrote in message

...I It had methinks, at the distance of 20 miles what's difference between
the true and astronomical horizon?

--
Joe Egginton
Wolverhampton
175m asl

Joe, not sure what your question is in this context, but a rough-and-ready
figure I use is that at 1000 feet altitude the horizon is 40 miles away. *It
is essentially a square root relationship, so that to see a point on the
surface 20 miles away you need to be 250 feet up.

In a navigator's table the reductions allow for sighting to be reduced
from ship's bridges. I think the horizon observed at sea level is 3
miles. At least, IIRC, that is the computed limb observation time.