>> The clock would then only have to remain within spec for 24 hour Not really. The only useful clock is a VERY accurate one. You cannot stay linked with Greenwich by using an inaccurate clock and "correcting" it for local time. For one thing, when you are actually moving east west, local time "noon to noon" is no longer exactly 24 hours. In fact, it is the discrepancy that you are trying to detect to see how far east west you have gone. If you are not moving, then correcting your clock every 24 hours does you no good. Part of the engineering problem was creating a clock that remained accurate under the varying conditions of temperature, humidity and motion that are inherent on a sailing vessel. >> On Jul 30, 2010, at 9:20 AM, Craig Jungers wrote: >> The clock would then only have to remain within spec for 24 hours AND >> MEASURE LOCAL TIME and not for months or years to maintain >> synchronism with >> Greenwich. Presumably there were enough stars visible in both the >> southern >> and northern hemispheres so that Cook in Australia could use the >> tables >> developed at Greenwich TO TRY OUT THE LUNAR METHOD........... >> >> Thinking this over, the ancient Pacific navigators could have used >> very > much the same system using a sand-clock calibrated for 24 hours. > Accuracy > would not have been so great but then they would at least have had > some > measure of local time. What does local time have to do with figuring out longitude? You need a clock accurate enough to detect that if you sail 100 miles west the day is longer than if you travel 100 miles east. That is not a sand clock. And they did not have sand clocks anyway. They had no glass. You try and make a even vaguely accurate sand clock out of the materials that they had and sell it to your tribe as something that was useful. It is implausible that the natives were aware that the day was shorter when they sailed east than when they sailed west. This is awfully sophisticated stuff and would require a lot of formal experimentation -- the earth is round, it rotates on its axis etc. What would even impel them to want to make an accurate sand clock? They were not trying to run a train system. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On Sun, Aug 8, 2010 at 5:48 PM, Robert Livingston <bearboat2_at_comcast.net>wrote: > > > What does local time have to do with figuring out longitude? You need a > clock accurate enough to detect that if you sail 100 miles west the day is > longer than if you travel 100 miles east. That is not a sand clock. > Your line of longitude is exactly under the sun at its zenith measured from Greenwich. Your line of latitude is calculated from the elevation of the sun at zenith. You can get an approximation of your longitude if you take a noon sight. The trouble being that the sun "hangs" at zenith for a period of time that makes this calculation only an approximation. And you'd need the time. So measuring your longitude from Greenwich does need accurate time. But you can get a bit more sloppy if you are only measuring a couple hundred miles from one location to another. You wouldn't need any real concept of "longitude" either... just a concept of the earth being round and some reference points. Local noon plus or minus the elapsed time would give you a rough approximation of how far east or west you traveled. It wouldn't be very accurate... and it would have to be referenced to a departure point - but it could give you some idea of when you might expect to see your destination over a relatively short passage. For instance.... if they had a way of measuring 24 hours then if the sun were at zenith exactly 24 hours after they started their "clock" then they would not have moved much in longitude. They almost certainly didn't understand degrees... but they could understand concepts such as "the sun begins to descend 1 hour after the "clock" reaches zero" would be, perhaps, the next group of atolls west. And they did not have sand clocks anyway. They had no glass. You try and > make a even vaguely accurate sand clock out of the materials that they had > and sell it to your tribe as something that was useful. Ok... how about a sand coconut? I'm merely offering up suggestions that seem to me to be more plausible than reading the pattern of the waves or somehow "just knowing" where they were. They could clearly move around in primitive craft over an area of thousands of square miles. Running down the latitude really only works well for going downwind. Upwind, where you need to tack, you'd also need to keep track of time somehow. Otherwise you risk missing the island because you extended one leg of the tack too far. Although it probably works ok for continents. :P Craig Jungers Moses Lake, WA www.nwkayaking.net *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
PeterO wrote ........The clock would then only have to remain within spec for 24 hours and measure and not for months or years to maintain synchronism with Greenwich. Presumably there were enough stars visible in both the southern and northern hemispheres so that Cook in Australia could use the tables developed at Greenwich to try out the lunar method........... Robert wrote .......Not really. The only useful clock is a VERY accurate one. You cannot stay linked with Greenwich by using an inaccurate clock and "correcting" it for local time. For one thing, when you are actually moving east west, local time "noon to noon" is no longer exactly 24 hours. In fact, it is the discrepancy that you are trying to detect to see how far east west you have gone............... G'Day Robert I was trying to explain that there is no need at all to use a clock to stay linked to Greenwich or any other longitudinal reference point when navigating by the stars, moons and planets. The lunar method implies this. The navigator would need a record or memory of their relative locations at particular days of the year and times of the night. Days of the year and times of the night can be measured in many ways without the need for sophisticated manufacturing and star maps were a commonplace. Some articles describing the techniques of Polynesian navigation. http://www.penn.museum/sites/navigation/intro.html http://www.teara.govt.nz/en/canoe-navigation/2 All the best, PeterO *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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