At 10:59 AM 9/9/96 PDT, you wrote: >Are there references to the use of sundials in navigation? >In the days before magnetic variation was mapped, they must have been useful >(with the limitations of stability that a dial on board a ship would have.) >No doubt travellers on land used them, are there special types that >developed for this purpose? >>Also, does anyone know how accurately these dials are at orienting to >>north? They seem to have the advantage that they don't have to be >>adjusted for magnetic declination and that they aren't affected by >>other magnetic fields. (And of course the obvious disadvantage that >>they must have sun in order to work.) So, they seem like they would >>be a great companion to a compass. >\\\\\\\\\\\\\\\\\\\\//////////////////// > Peter Abrahams [EMAIL PROTECTED] >the history of the telescope, the microscope, > and the prism binocular > Hi Peter
1. Sundials as such were seldom, if at all, used in sea navigation even before the appearance of magnetic compass which occurred in late Medieval times. To tell you the truth methods of navigation used by Vikings in 8-th and 10-th century are still a mystery, the only thing known for sure is that they navigated freely in the North Atlantic 500 years before Columbus and most probably they did not use magnetic devices. Sun and stars were of course heavily used for determination of direction and latitude. Probably Vikings understood the motion of the Sun (and other bodies) so well that they could use it for determination of direction at any time. They had no doubts about sperical shape of the Earth but being preoccupied with other things did not contribute much to cosmology nor theology. 2. Even after common usage of magnetic compass the existence of variation did not show at once. Compass roses were not very precise at the time and variation in Europe is well below 10 degrees. We know that Columbus and his navigators were surprised by the phenomenon once it became really evident in Mid-Atlantic. Simply compass North was far off from the Polar Star. They did not arrive at the explanation of the phenomenon. This happened much later. 3. On moving ship using the equal altitudes method or waiting for culmination is rather problematic. However when moving on land it makes the difference. The equal altitudes method can be very accurate if you can spend a day or two in one place. 4. Bill Maddux - our fellow Sundial List Member - has worked out extremely accurate and still "home" method of using the Sun and equal altitudes for determination of local meridian, true noon, true solar time e.t.c.. His method relies on shadow marking and measurements but is really very accurate. I have the description of the method but do not dare to publish it without his permission because it was in private correspondence. I think that Bill's method may yield few minutes of arc of accuracy (or better) when determining the direction. Such accuracy is much too much for practical navigation but may be welcome in back-yard astronomy. 5. Today the best companion to compass is a good chart with marked variation and its annual changes which of course was not the case in the past. Allowing for variation (and other corrections) is no big deal once you know them of course which again was not the case in the past. Few hundred years ago navigation methods and instruments were kept secret. This is no more but many other things are secret instead. Regards - Slawek Grzechnik
