Do you know how to tell the distance from boat you are sailing on to the horizon or shore with out GPS or other devises? I heard ones that with good visibility the horizon is in about 7 miles. Is that right?

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Adam Bartos
Nacra 5.0
SolCat 18 (sold)
Lake Zurich, IL
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Depends on how far above sea level your eyes are.
If there are mountains on shore you can see them from 30 miles out.
If you are looking for your bright orange floating VHF radio from your catamaran you can see about 200 yards in calm water.

Wikipedia:
Ignoring the effect of atmospheric refraction, distance to the horizon from an observer close to the Earth's surface is about d≈3.57√h

where d is in kilometres and h is height above ground level in metres.
Examples:
For an observer standing on the ground with h = 1.70 metres (5 ft 7 in) (average eye-level height), the horizon is at a distance of 4.7 kilometres (2.9 mi).
For an observer standing on the ground with h = 2 metres (6 ft 7 in), the horizon is at a distance of 5 kilometres (3.1 mi).

Clear as mud?

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go to geodistance.com and you can check it out before you sail.
marc

Quote

For an observer standing on the ground with h = 1.70 metres (5 ft 7 in) (average eye-level height), the horizon is at a distance of 4.7 kilometres (2.9 mi).
For an observer standing on the ground with h = 2 metres (6 ft 7 in), the horizon is at a distance of 5 kilometres (3.1 mi).

So using this logic, you can't see past the horizon?

how is it at sea level we can see islands, bridges, and buildings well past 3 miles?



Edited by MN3 on Sep 24, 2012 - 07:08 AM.

MN3

So using this logic, you can't see past the horizon?

how is it at sea level we can see islands, bridges, and buildings well past 3 miles?

You can't see the horizon past the horizon.

The formula was simply for a straight-line distance to the "horizon" from eyes a certain height above the ground. The horizon is the ground/water separation of the earth and sky. Buildings and bridges stick up above the horizon and are of course visible from much further. Just like if,instead of being 6' 7 and seeing 3.1 miles I was 67 feet tall I could see much further, works on either end.

Some people might refer to a "true horizon" as above vs a "visible horizon" which would be what would you would see if the true horizon was obscured by a solid line of trees or buildings. That visible horizon might be way further than the the true horizon.

I've always found bridges and sets of tall condos very misleading on the water as far as judging distance, they look so "close" but can really be very far.

--
Damon Linkous
1992 Hobie 18
Memphis, TN

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I realize most of you aren't still in school like I am, so let me give you a little mathematical recap.
Pythagorean theorem a formula used to find the length of any side in a right triangle when you already know 2 sides.
a^2 + b^2 = c^2

a and b are legs while c is the hypotenuse.

A in this horizon case becomes a constant. 20,925,524.9 ft.

now C is going to be the hypotenuse where we stand so C = A+X. X being the height of the vantage point. once we have 2 of the three we solve for B the distance you can see.

EXAMPLE:
find distance man whose eyes are six feet above surface can view to horizon.

A= 20,925,524.9
C = A + 6

B^2 = A^2 - C^2

B = 15, 846.3 ft or 3.001 miles

--
Nacra 6.0 NA
Ogden Dunes, IN
--

Love math nerds, the original beachcats mail list back in the 1990's was heavily composed of .edu and NASA email addresses and the inanely complicated theoretical mathematics discussions that ensued were hilarious.

I wish I could find a copy of the whitepaper done back then titled "The Butt Cheek Differential".



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Damon Linkous
1992 Hobie 18
Memphis, TN

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Butt Cheek Differential
Found it on google

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-Zach
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PurdueZach

Butt Cheek Differential
Found it on google

Ha, good job, knew I had it on the site somewhere.

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Damon Linkous
1992 Hobie 18
Memphis, TN

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Quote

You can't see the horizon past the horizon.

Hell, I can't see past my rum glasses

Quote

The horizon is the ground/water separation of the earth and sky. Buildings and bridges stick up above the horizon and are of course visible from much further.

so i can only see 3 miles out in the gulf, unless there is a tanker boat, then my vision more than doubles and i can see 8?

MN3

Quote

The horizon is the ground/water separation of the earth and sky. Buildings and bridges stick up above the horizon and are of course visible from much further.

so i can only see 3 miles out in the gulf, unless there is a tanker boat, then my vision more than doubles and i can see 8?

Yes, you can see the tanker 8 miles away, but you can't see the water the tanker is floating in.

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Damon Linkous
1992 Hobie 18
Memphis, TN

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sounds like witchcraft or voodoo to me

Whats the air speed of an unladen swallow?

PurdueZach

Butt Cheek Differential
Found it on google

hahaha, wow!! This is impossible to read in a library!!

--
Proud owner of a Mystère 6.0
Ottawa, Ontario, Canada
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MN3

Whats the air speed of an unladen swallow?

African or European?



Edited by klozhald on Sep 24, 2012 - 05:34 PM.

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Sheet In!
Bob
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Prindle 18-2 #244 "Wakizashi"
Prindle 16 #3690 "Pegasus" Sold (sigh)
AZ Multihull Fleet 42 member
(Way) Past Commodore of Prindle Fleet 14
Arizona, USA
--

I usually turn back towards shore when the white sandy beach goes over the horizon, but not trees.

--
Nacra 6.0 NA
Ogden Dunes, IN
--

"Depends on how far above sea level your eyes are.
d≈3.57√h
where d is in kilometres and h is height above ground level in metres."

The only thing missing from that formula is the wave hight and number of beers. Often in my case: wave hight = number of beers lol

--
Adam Bartos
Nacra 5.0
SolCat 18 (sold)
Lake Zurich, IL
--

Quote

I wish I could find a copy of the whitepaper done back then titled "The Butt Cheek Differential".

Sounds like an episode of The Big Bang Theory

--
Dustin Finlinson • Magna, UT
Member: Utah Sailing Association
1982 Prindle 18
1986 Hobie 17
1982 Prindle 16
1980 Prindle 16(mostly)
1976 Prindle 16(mostly)
Check out "Prindle Sailors" on Facebook.
--

MN3

Quote

You can't see the horizon past the horizon.

Hell, I can't see past my rum glasses

Yes I guess judging the distance to an object on the water would have a direct relation to how long you have been sailing and therefore depend on how many drinks you have had. This would also depend on how much wind there was as there is direct relationship to the number of drinks had to how much wind there is. Where the distance is relatively equal to the inverse of the wind times the number of drinks divided by pi.

--
Dustin Finlinson • Magna, UT
Member: Utah Sailing Association
1982 Prindle 18
1986 Hobie 17
1982 Prindle 16
1980 Prindle 16(mostly)
1976 Prindle 16(mostly)
Check out "Prindle Sailors" on Facebook.
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~~ I'll Drink To That... I'll Drink To Anything~~

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~ Vietnam Vet 69-71~ 17 Hobie w/big jib, ~18 Hobie mag,~DN Ice sailor,
and other toys.......
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Bernhards Bay. ~~~~~~
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"Ah, but if the man that was doing the waking buys a beer for the fellow taht was sleeping whilst said fellow listens to a plan, then the fellow that doing the waking is ...
Or...
"Curse you for breathing you slack jawed idiot!"

A friend once asked me; "And just how far can you see through those beer googles?"
My reply; "All the way across the bar to that beutiful woman sitting on that barstool!"

Quote

Yes I guess judging the distance to an object on the water would have a direct relation to how long you have been sailing and therefore depend on how many drinks you have had. This would also depend on how much wind there was as there is direct relationship to the number of drinks had to how much wind there is. Where the distance is relatively equal to the inverse of the wind times the number of drinks divided by pi.

mmm pie

Quote

African

African.

PS i have done more searches on this exact topic than probably any other (and i have been a webmaster for over a dozen years)



Edited by MN3 on Sep 25, 2012 - 11:12 AM.

Quarath

MN3

Quote

You can't see the horizon past the horizon.

Hell, I can't see past my rum glasses

Yes I guess judging the distance to an object on the water would have a direct relation to how long you have been sailing and therefore depend on how many drinks you have had. This would also depend on how much wind there was as there is direct relationship to the number of drinks had to how much wind there is. Where the distance is relatively equal to the inverse of the wind times the number of drinks divided by pi.

Agree, when drinking beer you must piii...

--
Adam Bartos
Nacra 5.0
SolCat 18 (sold)
Lake Zurich, IL
--

40 years ago, we used a technique learned in Cadets to judge distance by what was called "visibility of common objects",(it was derived from old school military spotting tactics, & a search there might yield results). That is, the distance a commonly known object could be discerned & identified as such.
Most common objects were cars, transport trucks, houses & people. IIRC a standing adult can be discerned at 3 miles with the naked eye.
When discussing how far a horizon can be seen, the equations ignore three things, clarity of atmosphere, sharpness of light, & refraction.
Obviously anything that reduces atmospheric transparency, think haze, mist, smoke, reduces the distance at which the earth & sky become one, & severely impedes judgement of distance. This, combined with flat light is what killed JFK Jr. The visible horizon was not that far, but lighting conditions made it impossible to discern. His lack of instrument flying skills sealed the coffin.
Clarity of atmosphere cuts both ways. Many tend to think the clearer the better, but this is not always the case. You can see further the clearer the air, but that does not mean you can judge distance better, especially in landscapes with few objects, or landscapes where all objects are the same. Human eyes are very poor at distance estimation if they have few objects of known size in simultaneous view. This is easily demonstrated by looking at a distant single light, on a clear dark night, or a single floating object on a calm sea, you simply have no real accurate idea of how far away it is. I think we have all looked at what we thought was a distant object, sitting on glass calm water, only to put up the binoculars & realize it was in fact a fairly close bird.
Relatively flat snow/sand covered terrain, or even prairie results in the same optical illusions. This deficiency of the eye has been known for centuries, hence the development of rules of thumb for seafarers & military spotters. Change the atmospheric conditions, & that distant marker you accurately judged yesterday becomes the reef you wreck on today.
There is a well known phenomenon in aviation, known as the black hole approach. In my experience it resulted from tiny (pop 300-500) villages situated in the high Arctic Archipelago. The air was exceedingly clear, there was no Air Traffic Control or tower, no radar, only a small speck of lights perched in blackness. Many times it was impossible to differentiate where terra firma met sky, even though visibility was 100+ miles. At night you would swear the lights,(or were you confusing village lights with a newly risen star?) were only 10-20 miles out, & be tempted to begin a descent, when in actual fact they were 80+ miles distant. The fatality reports are full of such occurrences.
Refraction is another bugbear. Did you know that the setting sun is an optical illusion? It actually sets several minutes before you see it go under. Light rays are refracted, or bent, when passing through different densities. this is what makes corrective lenses work. The rule is, when passing from a less dense to a more dense medium, they are bent towards the norm. You are looking at a refracted image of the sun, the actual disc passed below the horizon minutes ago. This is more pronounced in a marine environment, hence the different definition of civil twilight, & nautical twilight.
In a nutshell, the eye, unless trained, is a poor judge of distance, when the distance becomes further than we can throw a ball. Binocular vision is what enables us to "know how far" an object is, via triangulation The further the object is, the less the degree of triangulation.
The wider your eyes are apart, the better you are at it, though past 100 yds the advantage becomes negligible. Damon, being a big guy, probably has a big head, with eyes set further apart than average. He could be expected to be a better judge of distance than average.
Try playing catch, or shoot baskets with one eye closed, monocular vision, it becomes very difficult to judge how hard to throw the ball.

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Hobie 18 Magnum
Dart 15
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Damon, being a big guy, probably has a big head, with eyes set further apart than average.

i got tickled when i read this!...good one ed!

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bill harris
hattiesburg, mississippi
prindle 16- "BLUE RIBBON"
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I just ask the guy sailing with me.

Take your average speed, and multiply Time = Distance

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Daniel Evans
N I20 Miami FL
N I20, N 6.0NA, H 20, H 16 Chesapeake bay
I have a hard time passing up a good deal on a big cat.
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Personally, I get nervous when I lose sight of my cooler!