On 4 Sep 2004 at 17:45, PeterO wrote: > Three questions if you have time. i) What is GPSD compared with GPS Differential GPS (DGPS) uses a special second receiver to pick up a radio signal from a ground station (usually obtained by paying a fee to the operator of the ground station). This signal transmits information that indicates accurate position information (IIRC, the info is more precise satellite positions). It can be used to produce a position estimate with much higher accuracy. > ii) Is it possible to determine longitude without a time reference > (Greenwich) using celestial navigation. Determining longitude depends on two things - finding your local time and knowing Greenwich time. The difference in these two times, taking into account that the Earth rotates once every 23h56m (24h sidereal time), allows you to determine longitude. Local time can be determined from observing time of transit of the sun (hence the importance in the days of sailing ships of measuring noon), the moon, planets or stars. With a sextant, you can determine this indirectly without resort to actually observing a transit. (Transit is when a celestial body passes your meridian or is at it's highest point in the sky.) Regardless of how the maths are done, the process is equivalent to the above. Hence, the clock is critical. > iii) With your measurements on > the trimaran is the sextant gimballed in any way to help with > stability or is that a ludicrous concept? If you swing a sextant side to side, the apparent angle changes and the minimum angle is the correct one. The quality of the navigator is indicated by their ability to get the angle correct regardless of how the ship is moving. At one time, there were experiments with gimballed chairs to steady the observer. They were largely futile. This was in the days of the lunar distance method. The latter was a process of measuring the angular distance between the moon and several stars to detemine the moon's true position. Once this is known, the time at Greenwich can be determined. It was an unweildy method and was largly ignored once marine chronometers were perfected. Lunar distance tables were still published up to the first world war, however (aah, naval tradition!). What made the method unpopular was not the measurement, but the many hours of computation required to produce the time. It was the lunar distance method that gave us the sextant. The octant was sufficient for measuring the altitude of stars, but too narrow in range to measure the lunar distances. Hence the larger sextant was developed. There are many excellent books that cover the history of this stuff - notably Dava Sobel's "Longitude". It was also made into a superb made-for-TV movie starring Jeremy Irons and Michael Gambon. Check it out of the video store! (In the film, Irons plays the role of Rupert T. Gould, an amateur chronometer enthusiast who rebuilt the original Harrison chronometers and got them working. Gould's book "The Marine Chronometer" is out of print (both editions) and is quite valuable. I spend untold numbers of hours in used bookstores when traveling to other cities in the hopes of finding a copy - it's on Amie's list of things to look for at garage sales). Another book I like is David Landes' "Revolution in Time", but it focusses almost exclusively on the development of the chronometer rather than the other aspects of navigation. The root of modern astronomy and much of physical science arises from the problem of determining longitude. All of the significant observatories were set up to determine the star positions with great accuracy. All the great scientists were involved and many theories and discoveries in physical science resulted. Galileo proposed using the moons of Jupiter as an accurate timepiece (they were used to establish longitude on land for many years). Robert Hooke's experiments on springs (to power timepieces) resulted in the foundations of the theory of elasticity. Newton invented the Calculus of Variations to explain the characteristics of a perfect pendulum to govern a clock. The concept of open scientific publication was also debatably enhanced as a result of the constant communication between seventeenth and eighteenth century scientists trying to solve the problem - all this in spite of frequent wars. Economists can point to the collapse of the guild system and the development of the free market in Britain as a reason why the technology of navigation blossomed in Britain and was stifled in the rest of Europe. Fascinating stuff! But I digress... Mike *************************************************************************** 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/ ***************************************************************************Received on Sat Sep 04 2004 - 10:20:28 PDT
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