On open sea how far away is the visible horizon?

[rowenj]

Or from the coast at ground level how far away is the horizon when looking out to sea?

[excelsior-1]

i think i remember reading once, a long time ago - it is about 19 miles
but i may be a little out.

[gsr600]

I heard 18 miles - so somewhere near I pressume - unless you're in the crows nest

[bluebirdino]

if i remember correctly , if your six feet tall its about two miles (but i could be wrong !)

[bibblebub]

take distance of eyes above sea-level, multiply by 1.6, and take square root to get distance in miles

[bibblebub]

should have said that distance of eyes above sea-level is in feet

[HowardKennitby]

http://en.wikipedia.org/wiki/Horizon

5' 7" = 2.9 miles

[ChuckFickens]

There isn't an exact answer.

It depends in the tide at the time (yes there are tides in the middle of the ocean)

[pdq1]

There seems to be a variety of answers. I was once told it was 7 miles.

[Graham-W]

With d in miles and h in feet,

d=approx 1.22*sqrt{h}

Examples, assuming no refraction:

For an observer on the ground with eye level at h = 5 ft 7 in (5.583 ft), the horizon is at a distance of 2.9 miles (4.7 km).

[mibn2cweus]

http://www.ringbell.co.uk/info/hdist.htm

[Charity]

This was on QI - but I can't remember the answer.
I think it was a lot less than expected.

[jomifl]

3 ABers agreeing (Howard, Graham and Mibs), it must be the right answer and it is!

[DaisyNonna]

With or without the fog?

[jomifl]

Daisy, the horizon is still there with or without the fog and even at night. That is why the water doesn't run over the edge. :-)

[Hopkirk]

If there is a swell, the distance to the horizon must keep changing dramatically.

[wildwood]

^^ I think the assumption is that one stands at the water edge.

It can never be calculated accurately because of the moon's gravition, it bulges the sea at high tide and flattens it at low tide.

[HowardKennitby]

Also the earth is not spherical which will have an effect as well.