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
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Lake Zurich, IL
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How to tell the distance on the water?
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Posted: Sep 23, 2012 - 08:20 PM
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Posted: Sep 23, 2012 - 09:26 PM
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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Posted: Sep 24, 2012 - 06:29 AM
go to geodistance.com and you can check it out before you sail.
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Posted: Sep 24, 2012 - 07:06 AM
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. -
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Posted: Sep 24, 2012 - 09:21 AM
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.
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Damon Linkous
1992 Hobie 18
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Posted: Sep 24, 2012 - 09:31 AM
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
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Posted: Sep 24, 2012 - 10:30 AM
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
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Posted: Sep 24, 2012 - 11:22 AM
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Posted: Sep 24, 2012 - 12:24 PM
Ha, good job, knew I had it on the site somewhere.
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Damon Linkous
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Posted: Sep 24, 2012 - 12:38 PM
Hell, I can't see past my rum glasses -
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Posted: Sep 24, 2012 - 12:40 PM
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? -
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Posted: Sep 24, 2012 - 01:31 PM
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
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Posted: Sep 24, 2012 - 02:28 PM
sounds like witchcraft or voodoo to me
Whats the air speed of an unladen swallow? -
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Posted: Sep 24, 2012 - 02:38 PM
hahaha, wow!! This is impossible to read in a library!!
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Posted: Sep 24, 2012 - 05:19 PM
African or European?
Edited by klozhald on Sep 24, 2012 - 05:34 PM.
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Posted: Sep 24, 2012 - 06:08 PM
I usually turn back towards shore when the white sandy beach goes over the horizon, but not trees.
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Posted: Sep 24, 2012 - 11:12 PM
"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
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Adam Bartos
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Posted: Sep 24, 2012 - 11:20 PM
Sounds like an episode of The Big Bang Theory
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Dustin Finlinson • Magna, UT
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Posted: Sep 24, 2012 - 11:27 PM
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.
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Dustin Finlinson • Magna, UT
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Posted: Sep 25, 2012 - 01:16 AM
~~ I'll Drink To That... I'll Drink To Anything~~
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