thanks to all that took the time to share their ideas about this. I have a pretty good idea of what course to take. I also have a lead on a what sounds like a very interesting book. Fair winds, Tom B, Mont 17 #258
On 12/4/2014 2:01 PM, Thomas Buzzi wrote: Tom I also can highly recommend the Dava Sobel book on "Longitude". She describes the basic problem that mariners had determining longitude, and then tells the story of how the British Government put up a prize for anyone who could make an accurate chronometer. The Government was pressured to do this because of all the naval and merchant ships that were lost as a result of not having an accurate way to establish longitude reliably. If you divide the earth's circumference of 360 degrees by 24 hours, your answer shows you that the earth rotates 15 degrees per hour. Wherever you may be on the surface of the earth, the sun rises, climbs to its meridian at what navigators call Local Apparent Noon (LAN) at your location, and then starts descending again. If you had an accurate time piece set to Greenwich Mean Time, and the time pieces' error rate was constant, then knowing that, and the time of your LAN it is a very simple math problem to determine how far West or East of Greenwich you are located at that specific moment If you measure the height of the sun at that point; and note the exact time of the observation, in seconds; minutes; and hours, you can now establish your longitudinal position (dependent on the accuracy of your measurements). You have to do it in that order since each second counts in the calculation, and accuracy of time of observation is critical. Seconds change rapidly, minutes only 60 seconds later.... Example: New York City is roughly at 75 degrees W Longitude from 0 degree Greenwich, England. Divide 75 degrees by 15 (the degrees in an hour) and you will see that New York's geographical position is 5 hours West of Greenwich. This goes on until you reach the 180th degree West, called the International Date Line, which is located in the Pacific Ocean. When you reach the International Date Line heading West from the USA, it is Thursday; but once you have crossed the Date Line it becomes Friday, and the beginning of a new day for the 180 degree East of Greenwich area of the world. ------------------------------- Last thought: If a salt water wave or spray hits my sextant I can just rinse it off with fresh water and dry it with a towel and it keeps working (with a drop of oil in the mechanism once in awhile) Gin also will do the job, but I don't think my rinse solutions will work for electronics.... They are nice toys and give you pretty pictures but don't trust your life or the safety of your boat to them. They are only a secondary source of information. Learn to do dead reckoning navigation the classical way, and you will always be safe. Connie
thanks to all that took the time to share their ideas about this. I have a pretty good idea of what course to take. I also have a lead on a what sounds like a very interesting book. Fair winds, Tom B, Mont 17 #258
Hi Connie, Thanks for the added info. I finally get the idea of how a chronometer works using GMT. If your onboard timepiece associates hours with 15 degrees west long then wherever you are at noon GMT is...... humm, lost it. Oh, you have to measure the height of the sun at your present position to tell you the degree long. to plot on your chart. Somehow. Too bad we don't live closer. I have always wanted to learn how to use a sextant but reading the information just never sunk in. Oh well, I can get a hand bearing compass and some local charts. Sounds like fun and a good feeling to know where I am even if the gps quits. Tom B On Thu, Dec 4, 2014 at 3:11 PM, Conbert Benneck <chbenneck@gmail.com> wrote:
On 12/4/2014 2:01 PM, Thomas Buzzi wrote:
Tom
I also can highly recommend the Dava Sobel book on "Longitude".
She describes the basic problem that mariners had determining longitude, and then tells the story of how the British Government put up a prize for anyone who could make an accurate chronometer. The Government was pressured to do this because of all the naval and merchant ships that were lost as a result of not having an accurate way to establish longitude reliably.
If you divide the earth's circumference of 360 degrees by 24 hours, your answer shows you that the earth rotates 15 degrees per hour.
Wherever you may be on the surface of the earth, the sun rises, climbs to its meridian at what navigators call Local Apparent Noon (LAN) at your location, and then starts descending again.
If you had an accurate time piece set to Greenwich Mean Time, and the time pieces' error rate was constant, then knowing that, and the time of your LAN it is a very simple math problem to determine how far West or East of Greenwich you are located at that specific moment
If you measure the height of the sun at that point; and note the exact time of the observation, in seconds; minutes; and hours, you can now establish your longitudinal position (dependent on the accuracy of your measurements). You have to do it in that order since each second counts in the calculation, and accuracy of time of observation is critical. Seconds change rapidly, minutes only 60 seconds later....
Example: New York City is roughly at 75 degrees W Longitude from 0 degree Greenwich, England. Divide 75 degrees by 15 (the degrees in an hour) and you will see that New York's geographical position is 5 hours West of Greenwich.
This goes on until you reach the 180th degree West, called the International Date Line, which is located in the Pacific Ocean.
When you reach the International Date Line heading West from the USA, it is Thursday; but once you have crossed the Date Line it becomes Friday, and the beginning of a new day for the 180 degree East of Greenwich area of the world.
-------------------------------
Last thought: If a salt water wave or spray hits my sextant I can just rinse it off with fresh water and dry it with a towel and it keeps working (with a drop of oil in the mechanism once in awhile) Gin also will do the job, but I don't think my rinse solutions will work for electronics.... They are nice toys and give you pretty pictures but don't trust your life or the safety of your boat to them. They are only a secondary source of information.
Learn to do dead reckoning navigation the classical way, and you will always be safe.
Connie
thanks to all that took the time to share their ideas about this. I have a
pretty good idea of what course to take. I also have a lead on a what sounds like a very interesting book. Fair winds, Tom B, Mont 17 #258
On 12/4/2014 3:37 PM, Thomas Buzzi wrote: Tom I confused you with my 15 degrees per hour... Lets start sailing west from England. Your chronometer has been compared to the Royal Observatory clock at Greenwich and has been set to GMT (Greenwich Mean Time). You sail westward for several days. At this point you take a LAN observation with your sextant, and the time of the LAN observation was noted as: GMT + exactly 4 hours; 0 seconds; 0 minutes. By English Decree: Greenwich is Mean Time. The French use Paris as their 0 Base Line, so I had to be careful when using French Charts that used Paris as 0 degrees, vs, British Charts that used Greenwich while sailing down the English Channel.. Your observation tells you that you are exactly 4 hours West of Greenwich: GMT + 240 minutes (4 hours). You know that the earth turns 15 degrees per hour. If you are 4 hours West of Greenwich you are located on the 60th degree of Latitude. If your LAN measurement shows that you a few days later that you are at 73 degrees West, you are approaching the USA East Coast (NYC is at 75 degrees W). Of course it isn't quite that simple; you have to add factors such as the height of your eye above the waterline when making the observation. An observation made from the bridge of the Queen Elizabeth is different than what you are observing from the deck of a 30 foot sailboat, so corrections have to be made for this difference. But it all becomes fairly simple math as long as you can keep + and - additions correct. A sextant isn't a piece of mysterious machinery. It is just a very accurate protractor for measuring angles. Connie
Hi Connie, Thanks for the added info. I finally get the idea of how a chronometer works using GMT. If your onboard timepiece associates hours with 15 degrees west long then wherever you are at noon GMT is...... humm, lost it. Oh, you have to measure the height of the sun at your present position to tell you the degree long. to plot on your chart. Somehow. Too bad we don't live closer. I have always wanted to learn how to use a sextant but reading the information just never sunk in. Oh well, I can get a hand bearing compass and some local charts. Sounds like fun and a good feeling to know where I am even if the gps quits. Tom B
On Thu, Dec 4, 2014 at 3:11 PM, Conbert Benneck <chbenneck@gmail.com> wrote:
On 12/4/2014 2:01 PM, Thomas Buzzi wrote:
Tom
I also can highly recommend the Dava Sobel book on "Longitude".
She describes the basic problem that mariners had determining longitude, and then tells the story of how the British Government put up a prize for anyone who could make an accurate chronometer. The Government was pressured to do this because of all the naval and merchant ships that were lost as a result of not having an accurate way to establish longitude reliably.
If you divide the earth's circumference of 360 degrees by 24 hours, your answer shows you that the earth rotates 15 degrees per hour.
Wherever you may be on the surface of the earth, the sun rises, climbs to its meridian at what navigators call Local Apparent Noon (LAN) at your location, and then starts descending again.
If you had an accurate time piece set to Greenwich Mean Time, and the time pieces' error rate was constant, then knowing that, and the time of your LAN it is a very simple math problem to determine how far West or East of Greenwich you are located at that specific moment
If you measure the height of the sun at that point; and note the exact time of the observation, in seconds; minutes; and hours, you can now establish your longitudinal position (dependent on the accuracy of your measurements). You have to do it in that order since each second counts in the calculation, and accuracy of time of observation is critical. Seconds change rapidly, minutes only 60 seconds later....
Example: New York City is roughly at 75 degrees W Longitude from 0 degree Greenwich, England. Divide 75 degrees by 15 (the degrees in an hour) and you will see that New York's geographical position is 5 hours West of Greenwich.
This goes on until you reach the 180th degree West, called the International Date Line, which is located in the Pacific Ocean.
When you reach the International Date Line heading West from the USA, it is Thursday; but once you have crossed the Date Line it becomes Friday, and the beginning of a new day for the 180 degree East of Greenwich area of the world.
-------------------------------
Last thought: If a salt water wave or spray hits my sextant I can just rinse it off with fresh water and dry it with a towel and it keeps working (with a drop of oil in the mechanism once in awhile) Gin also will do the job, but I don't think my rinse solutions will work for electronics.... They are nice toys and give you pretty pictures but don't trust your life or the safety of your boat to them. They are only a secondary source of information.
Learn to do dead reckoning navigation the classical way, and you will always be safe.
Connie
thanks to all that took the time to share their ideas about this. I have a
pretty good idea of what course to take. I also have a lead on a what sounds like a very interesting book. Fair winds, Tom B, Mont 17 #258
Tom, Connie is giving some interesting information. But you don't have to calculate longitude with a clock and sextant for purposes of using a chart as long as you are within sight of land. The point is you can do really good navigation manually with a chart, compass, and binoculars to take bearings without GPS. Sailing blue water out of sight of land over long crossings is another matter...I agree that having manual systems that can't fail are really useful. That is why for example my life jacket is not the auto inflating kind. I just don't want to depend on technology when my life depends on it. Similarly I truly try to sail "engineless" so that I become a better sailor. I have learned for example how to sail backwards by pushing out the boom, and other maneuvers that might keep me safe. Fun, too! On Thu, Dec 4, 2014 at 2:17 PM, Conbert Benneck <chbenneck@gmail.com> wrote:
On 12/4/2014 3:37 PM, Thomas Buzzi wrote:
Tom
I confused you with my 15 degrees per hour...
Lets start sailing west from England.
Your chronometer has been compared to the Royal Observatory clock at Greenwich and has been set to GMT (Greenwich Mean Time).
You sail westward for several days. At this point you take a LAN observation with your sextant, and the time of the LAN observation was noted as: GMT + exactly 4 hours; 0 seconds; 0 minutes.
By English Decree: Greenwich is Mean Time. The French use Paris as their 0 Base Line, so I had to be careful when using French Charts that used Paris as 0 degrees, vs, British Charts that used Greenwich while sailing down the English Channel..
Your observation tells you that you are exactly 4 hours West of Greenwich: GMT + 240 minutes (4 hours).
You know that the earth turns 15 degrees per hour. If you are 4 hours West of Greenwich you are located on the 60th degree of Latitude. If your LAN measurement shows that you a few days later that you are at 73 degrees West, you are approaching the USA East Coast (NYC is at 75 degrees W).
Of course it isn't quite that simple; you have to add factors such as the height of your eye above the waterline when making the observation. An observation made from the bridge of the Queen Elizabeth is different than what you are observing from the deck of a 30 foot sailboat, so corrections have to be made for this difference.
But it all becomes fairly simple math as long as you can keep + and - additions correct.
A sextant isn't a piece of mysterious machinery. It is just a very accurate protractor for measuring angles.
Connie
Hi Connie,
Thanks for the added info. I finally get the idea of how a chronometer works using GMT. If your onboard timepiece associates hours with 15 degrees west long then wherever you are at noon GMT is...... humm, lost it. Oh, you have to measure the height of the sun at your present position to tell you the degree long. to plot on your chart. Somehow. Too bad we don't live closer. I have always wanted to learn how to use a sextant but reading the information just never sunk in. Oh well, I can get a hand bearing compass and some local charts. Sounds like fun and a good feeling to know where I am even if the gps quits. Tom B
On Thu, Dec 4, 2014 at 3:11 PM, Conbert Benneck <chbenneck@gmail.com> wrote:
On 12/4/2014 2:01 PM, Thomas Buzzi wrote:
Tom
I also can highly recommend the Dava Sobel book on "Longitude".
She describes the basic problem that mariners had determining longitude, and then tells the story of how the British Government put up a prize for anyone who could make an accurate chronometer. The Government was pressured to do this because of all the naval and merchant ships that were lost as a result of not having an accurate way to establish longitude reliably.
If you divide the earth's circumference of 360 degrees by 24 hours, your answer shows you that the earth rotates 15 degrees per hour.
Wherever you may be on the surface of the earth, the sun rises, climbs to its meridian at what navigators call Local Apparent Noon (LAN) at your location, and then starts descending again.
If you had an accurate time piece set to Greenwich Mean Time, and the time pieces' error rate was constant, then knowing that, and the time of your LAN it is a very simple math problem to determine how far West or East of Greenwich you are located at that specific moment
If you measure the height of the sun at that point; and note the exact time of the observation, in seconds; minutes; and hours, you can now establish your longitudinal position (dependent on the accuracy of your measurements). You have to do it in that order since each second counts in the calculation, and accuracy of time of observation is critical. Seconds change rapidly, minutes only 60 seconds later....
Example: New York City is roughly at 75 degrees W Longitude from 0 degree Greenwich, England. Divide 75 degrees by 15 (the degrees in an hour) and you will see that New York's geographical position is 5 hours West of Greenwich.
This goes on until you reach the 180th degree West, called the International Date Line, which is located in the Pacific Ocean.
When you reach the International Date Line heading West from the USA, it is Thursday; but once you have crossed the Date Line it becomes Friday, and the beginning of a new day for the 180 degree East of Greenwich area of the world.
-------------------------------
Last thought: If a salt water wave or spray hits my sextant I can just rinse it off with fresh water and dry it with a towel and it keeps working (with a drop of oil in the mechanism once in awhile) Gin also will do the job, but I don't think my rinse solutions will work for electronics.... They are nice toys and give you pretty pictures but don't trust your life or the safety of your boat to them. They are only a secondary source of information.
Learn to do dead reckoning navigation the classical way, and you will always be safe.
Connie
thanks to all that took the time to share their ideas about this. I have a
pretty good idea of what course to take. I also have a lead on a what sounds like a very interesting book. Fair winds, Tom B, Mont 17 #258
I purchased my M17 from a gent who set off from San Francisco towards Hawaii. His ONLY means of navigation was one hand held GPS unit. Not sure if he had any paper charts, but he did not have a sextant, so if the GPS gave out, was lost overboard or otherwise put out of commission, not sure how he was going to find Hawaii. That is a big place to wander around looking for such a tiny spec of land. Lucky for him he turned back after 4 or 5 days, so it never became an issue. Anyway, yet another solution to one GPS and a sextant is several cheap GPS units. Two or three of them combined are less expensive than a sextant, but do the same thing. Leave the spares and fresh batteries safely stashed in the box until you need them. BTW, I use an older Garmin GPS chart plotter with depth finder built in. So in addition to knowing where I am, I know how much water is beneath me. Mine also has the nifty feature of tide tables built in. On tidal waters, that is more handy than you might think. But like the others, I still don't venture far on unfamiliar waters without paper charts. I pay as much attention to them as the GPS. On Dec 4, 2014, at 3:37 PM, Thomas Buzzi wrote:
Hi Connie, Thanks for the added info. I finally get the idea of how a chronometer works using GMT. If your onboard timepiece associates hours with 15 degrees west long then wherever you are at noon GMT is...... humm, lost it. Oh, you have to measure the height of the sun at your present position to tell you the degree long. to plot on your chart. Somehow. Too bad we don't live closer. I have always wanted to learn how to use a sextant but reading the information just never sunk in. Oh well, I can get a hand bearing compass and some local charts. Sounds like fun and a good feeling to know where I am even if the gps quits. Tom B
On Thu, Dec 4, 2014 at 3:11 PM, Conbert Benneck <chbenneck@gmail.com> wrote:
On 12/4/2014 2:01 PM, Thomas Buzzi wrote:
Tom
I also can highly recommend the Dava Sobel book on "Longitude".
She describes the basic problem that mariners had determining longitude, and then tells the story of how the British Government put up a prize for anyone who could make an accurate chronometer. The Government was pressured to do this because of all the naval and merchant ships that were lost as a result of not having an accurate way to establish longitude reliably.
If you divide the earth's circumference of 360 degrees by 24 hours, your answer shows you that the earth rotates 15 degrees per hour.
Wherever you may be on the surface of the earth, the sun rises, climbs to its meridian at what navigators call Local Apparent Noon (LAN) at your location, and then starts descending again.
If you had an accurate time piece set to Greenwich Mean Time, and the time pieces' error rate was constant, then knowing that, and the time of your LAN it is a very simple math problem to determine how far West or East of Greenwich you are located at that specific moment
If you measure the height of the sun at that point; and note the exact time of the observation, in seconds; minutes; and hours, you can now establish your longitudinal position (dependent on the accuracy of your measurements). You have to do it in that order since each second counts in the calculation, and accuracy of time of observation is critical. Seconds change rapidly, minutes only 60 seconds later....
Example: New York City is roughly at 75 degrees W Longitude from 0 degree Greenwich, England. Divide 75 degrees by 15 (the degrees in an hour) and you will see that New York's geographical position is 5 hours West of Greenwich.
This goes on until you reach the 180th degree West, called the International Date Line, which is located in the Pacific Ocean.
When you reach the International Date Line heading West from the USA, it is Thursday; but once you have crossed the Date Line it becomes Friday, and the beginning of a new day for the 180 degree East of Greenwich area of the world.
-------------------------------
Last thought: If a salt water wave or spray hits my sextant I can just rinse it off with fresh water and dry it with a towel and it keeps working (with a drop of oil in the mechanism once in awhile) Gin also will do the job, but I don't think my rinse solutions will work for electronics.... They are nice toys and give you pretty pictures but don't trust your life or the safety of your boat to them. They are only a secondary source of information.
Learn to do dead reckoning navigation the classical way, and you will always be safe.
Connie
thanks to all that took the time to share their ideas about this. I have a
pretty good idea of what course to take. I also have a lead on a what sounds like a very interesting book. Fair winds, Tom B, Mont 17 #258
Hi Howard, If... I was going to cross an ocean I do believe I would not only have a few spares of gps equipment I would also become a celestial navigator, having spent my life thinking that if a belt is good, suspenders can't hurt. Thanks for your thoughts. Tom B On Thu, Dec 4, 2014 at 4:47 PM, Howard Audsley <haudsley@tranquility.net> wrote:
I purchased my M17 from a gent who set off from San Francisco towards Hawaii. His ONLY means of navigation was one hand held GPS unit. Not sure if he had any paper charts, but he did not have a sextant, so if the GPS gave out, was lost overboard or otherwise put out of commission, not sure how he was going to find Hawaii. That is a big place to wander around looking for such a tiny spec of land. Lucky for him he turned back after 4 or 5 days, so it never became an issue.
Anyway, yet another solution to one GPS and a sextant is several cheap GPS units. Two or three of them combined are less expensive than a sextant, but do the same thing. Leave the spares and fresh batteries safely stashed in the box until you need them.
BTW, I use an older Garmin GPS chart plotter with depth finder built in. So in addition to knowing where I am, I know how much water is beneath me. Mine also has the nifty feature of tide tables built in. On tidal waters, that is more handy than you might think.
But like the others, I still don't venture far on unfamiliar waters without paper charts. I pay as much attention to them as the GPS.
On Dec 4, 2014, at 3:37 PM, Thomas Buzzi wrote:
Hi Connie, Thanks for the added info. I finally get the idea of how a chronometer works using GMT. If your onboard timepiece associates hours with 15 degrees west long then wherever you are at noon GMT is...... humm, lost it. Oh, you have to measure the height of the sun at your present position to tell you the degree long. to plot on your chart. Somehow. Too bad we don't live closer. I have always wanted to learn how to use a sextant but reading the information just never sunk in. Oh well, I can get a hand bearing compass and some local charts. Sounds like fun and a good feeling to know where I am even if the gps quits. Tom B
On Thu, Dec 4, 2014 at 3:11 PM, Conbert Benneck <chbenneck@gmail.com> wrote:
On 12/4/2014 2:01 PM, Thomas Buzzi wrote:
Tom
I also can highly recommend the Dava Sobel book on "Longitude".
She describes the basic problem that mariners had determining longitude, and then tells the story of how the British Government put up a prize for anyone who could make an accurate chronometer. The Government was pressured to do this because of all the naval and merchant ships that were lost as a result of not having an accurate way to establish longitude reliably.
If you divide the earth's circumference of 360 degrees by 24 hours, your answer shows you that the earth rotates 15 degrees per hour.
Wherever you may be on the surface of the earth, the sun rises, climbs to its meridian at what navigators call Local Apparent Noon (LAN) at your location, and then starts descending again.
If you had an accurate time piece set to Greenwich Mean Time, and the time pieces' error rate was constant, then knowing that, and the time of your LAN it is a very simple math problem to determine how far West or East of Greenwich you are located at that specific moment
If you measure the height of the sun at that point; and note the exact time of the observation, in seconds; minutes; and hours, you can now establish your longitudinal position (dependent on the accuracy of your measurements). You have to do it in that order since each second counts in the calculation, and accuracy of time of observation is critical. Seconds change rapidly, minutes only 60 seconds later....
Example: New York City is roughly at 75 degrees W Longitude from 0 degree Greenwich, England. Divide 75 degrees by 15 (the degrees in an hour) and you will see that New York's geographical position is 5 hours West of Greenwich.
This goes on until you reach the 180th degree West, called the International Date Line, which is located in the Pacific Ocean.
When you reach the International Date Line heading West from the USA, it is Thursday; but once you have crossed the Date Line it becomes Friday, and the beginning of a new day for the 180 degree East of Greenwich area of the world.
-------------------------------
Last thought: If a salt water wave or spray hits my sextant I can just rinse it off with fresh water and dry it with a towel and it keeps working (with a drop of oil in the mechanism once in awhile) Gin also will do the job, but I don't think my rinse solutions will work for electronics.... They are nice toys and give you pretty pictures but don't trust your life or the safety of your boat to them. They are only a secondary source of information.
Learn to do dead reckoning navigation the classical way, and you will always be safe.
Connie
thanks to all that took the time to share their ideas about this. I have a
pretty good idea of what course to take. I also have a lead on a what sounds like a very interesting book. Fair winds, Tom B, Mont 17 #258
participants (4)
-
Conbert Benneck -
Daniel Rich -
Howard Audsley -
Thomas Buzzi