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Old 08-05-2015, 02:39 PM   #1
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Thumbs up Understanding Nautical Measurements

At the Helm: Understanding nautical measurements - Valley Morning Star : Boating
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Old 08-05-2015, 03:06 PM   #2
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The astronomers, mathematicians and scientists who developed longitude and latitude had divided the map of the earth into 360 degrees and then divided that by 24 hours which gave them 24 segments of 15 degrees each. Each degree was then divided by 60 minutes and each minute by 60 seconds. It was then that they calculated a nautical mile, one that follows the Earth’s contour that was 6,076 feet, 1/60th of a degree. So, if a ship traveled at 1 nautical mile per hour that was termed as a “knot.”

1/60th of a degree of latitude, anywhere, and 1/60th of a degree of longitude at the equator is 6076 feet. But that formula does not work for longitude away from the equator. But, multiplying 6076 by the cosine of the corresponding latitude does give the correct distance for 1/60th of a degree of longitude at any latitude.

Note that at the equator, the longitude is 0, the cosine of 0 is 1 so no conversion is necessary, and at the poles, the lattitude is +/- 90, the cosine of which is 0, 0*6076 is still zero, which is the distance between degrees of longitude at the poles.
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Old 08-05-2015, 03:07 PM   #3
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Pretty darn interesting!
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Old 08-05-2015, 03:55 PM   #4
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Thanks for the explanation MYTraveler. I've wondered that myself and now I know!
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Old 08-05-2015, 04:11 PM   #5
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I didn't know, and I didn't know I didn't know that.
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Old 08-05-2015, 04:26 PM   #6
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sorry folks but that article is so full of misinformation I thought it was a gag or some kind of a test. (1.2 imperial gallons, 4.somthing liters to a US gallon?) Just saying be cautious how you take what you read on the web.
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Old 08-05-2015, 04:54 PM   #7
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I just skipped down to the nautical section as that was the most pertinent part of the article and found that to be a bit misleading as well.

To accurately plot a position using a sextant you need three celestial bodies preferably 60 degrees apart as well as an accurate chronometer. This wasn't possible thousands of years ago since the chronometer wasn't invented until the early 1700's.

The chronometer's accurate measurement of units of time on a circular face enabled Mariners to translate measures of arc into nautical measures of length where 1 minute of arc equals 1 nautical mile at the earth's equator. This was further divisible by 60 into seconds yielding a linear measurement of approximately 101'. Not sure who or when someone figured out how to use the chronometer as a compass by directing the hour hand towards the sun and then bisecting its angle relative to noon hour to find north or south but also a handy trick from back in the day.


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Old 08-05-2015, 05:05 PM   #8
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Originally Posted by boatdriver47 View Post
sorry folks but that article is so full of misinformation I thought it was a gag or some kind of a test. (1.2 imperial gallons, 4.somthing liters to a US gallon?) Just saying be cautious how you take what you read on the web.
"As an example, it takes 1.2 Imperial gallons, approximately 4.5 liters, to equal one U.S. gallon."

I agree. It appears the writer mixed up his Imperial gallons with his US gallons. 1 Imp Gal = 1.2 US gal=4.5 L.
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Old 08-05-2015, 05:43 PM   #9
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Opps didn't mean to post crap.......


Maybe someone with more knowledge could send the writer and/or publication a note of their mistakes..
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Old 08-05-2015, 05:47 PM   #10
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An Imperial gallon is larger than a US gallon. A liter is bigger than a quart. There are fewer than four liters in a US gallon.
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Old 08-05-2015, 06:10 PM   #11
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To accurately plot a position using a sextant you need three celestial bodies preferably 60 degrees apart as well as an accurate chronometer.
That is not technically correct. A running fix prepared by the observation of a single celestial object at intervals of, say, a few hours is can produce entirely usable fixes. It can actually be equivalent to the two-star sight technique, although movement of the observer between observations must be taken into account. This method, when used with the Sun is sometimes called a Sun-Run-Sun fix.

A greater number of celestial objects will normally enhance the accuracy of the sight and minimize the ambiguity of a two-star sight where the respective circles of position intersect at two positions, and must be further resolved in the sight reduction.
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Old 08-05-2015, 07:08 PM   #12
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"dates back thousands of years when sailors used a sextant..."
He probably needs an history lesson also. Thousands of years would put it in the B.C. eras when in fact in was developed in or around the 1600's if my memory serves me correctly. I think Isaac Newton had something to do with it.
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Old 08-05-2015, 07:11 PM   #13
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I also use the imperial fathom, cable, nautical mile rule. 100 fathoms = one cable, 10 cables = 1 nautical mile which is close less about 75'.
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Old 08-05-2015, 07:13 PM   #14
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I agree with LarryM Good navigators start their day with morning stars giving multiple position lines, These would be followed by running fix's single sun position lines Then a noon sight which would give Lat and long (2 position lines). Followed by running fix's through the afternoon, giving single position lines. Till evening star's giving multiple position lines. All these lines are transferred hourly along the dr course to reveal a extremely accurate position constantly. Astro-nav is great fun and occupies people on long ocean passages. All this is of course at the behest of the weather!
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Old 08-05-2015, 07:27 PM   #15
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"dates back thousands of years when sailors used a sextant..."
He probably needs an history lesson also. Thousands of years would put it in the B.C. eras when in fact in was developed in or around the 1600's if my memory serves me correctly. I think Isaac Newton had something to do with it.
The Chinese Navigators measured the altitude of Polaris or the Southern Cross above the horizon with an instrument called a qianxingban. The qianxingban was a board consisting of twelve pieces of square wood; the board would be aligned with the horizon, and navigators used the lengths of their arms to calculate the position of the stars. Another, simpler instrument used for this purpose was the liangtianchi, a vertical ruler.

Then Backstaffs, octants were all used in the 1500'sbefore Hadley invented the "Modern sextant" in 1731.
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Old 08-06-2015, 12:36 AM   #16
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My wife had me watch an early segment of the show Vikings where Ragnar demonstrated how to navigate with something like a sundial and - this was the cool part - a sunstone (https://en.m.wikipedia.org/wiki/Sunstone_(medieval)) when the sky was overcast.
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Old 08-06-2015, 12:44 AM   #17
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Dava Sobel Wrote a book on the history of the search to solve the longitude problem. It was made into a film "Longitude" with Jeremy Irons Michael Gambon chronicling the life of John Harrison the inventor of the 1st chronometer. Captivating movie, very factual and based on Sobel's book. Well worth a watch one evening..
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Old 08-06-2015, 02:30 AM   #18
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Haven't seen the movie, but the Sobel's book is very worthwhile.
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Old 08-06-2015, 06:13 AM   #19
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"To accurately plot a position using a sextant you need three celestial bodies preferably 60 degrees apart as well as an accurate chronometer. This wasn't possible thousands of years ago since the chronometer wasn't invented until the early 1700's."

Not if you are good at navigation,and have the tables

Read Slocum to find out how the Moon is used AS a clock for other celestial nav.

Still in the books as a lunar.

A sun line (noon fix) will give north or south and was used before clocks .One then sailed east or west till hitting land.

The sweetest is the early Polynesian system.

A coconut with holes taught the navigator star positions for each island..

As travel was annual the stars were in about the right position every year.

When one particular star for an island just broke rising on the horizon (gave time) the navigator would lay down flat on the deck and see which direction the locating star that was on top of the island lay.

The ZN for the sub point of the observed body was Zero.

In other words the star was directly over the island.

When they got close releasing a bird would either have the bird come back (no island yet) or fly off to land.

Second radar was to toss a pig over the side , that would smell the land and swim toward it.

Must have been a few centuries on the learning curve to create the system.

Celestial Navigation (various papers on)

www.ccas.ws/celestialpapers.html


Various Papers on Celestial Navigation by Date from NAVIGATION, The Journal of The Institute of Navigation courtesy CCAS ... LUNAR DISTANCE METHOD IN THE 19th CENTURY: ..... POLYNESIAN NAVIGATION (NOTES AND COMMENT).
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Old 08-06-2015, 09:17 AM   #20
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This has somehow morphed from nautical measurements into navigation. My original point was that the article sighted missed the most important measuring tool of all. The chronometer. It wasn't possible to accurately know your position on the globe without an accurate measure of time though you could still navigate around it safely enough.

Even after the chronometer's invention sailors continued to use deduced reckoning (Ded reckoning) plotting running fixes by advancing sun lines and celestial fixes. This not only required an accurate deck log of time speed distance heading wind direction currents etc.. But a steady helmsman else Mother Nature might move you somewhere you don't want or know. Shooting lunar distances as slocum did is tedious in its calculations and takes more time than taking 3 or 5 shots provided of course nature gives you that chance. There were numerous days when slocum didn't get any sights at all and the winds had shifted whilst he was asleep and his boat sailed a different course unbeknownst to him at the time. Obviously there is more to navigation than a simple fix, but since the fix is where it starts and ends it tends to become the focal point. So when you don't know where you are, or where you have been exactly, and you need to figure it out immediately, what do you need to plot that proverbial cocked hat in order to fix your position? Yep the iPhone.


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