--Prof. Mike Edmunds: Dear Dr Smith, Thank you for your interesting message. Although the remnants of the Antikythera Mechanism were found on a ship, we are certain it was not a navigational instrument. All of the functions we have so far discovered were for astronomical or for calendar purposes. I have some sailing experience and (modest) navigation qualifications - and I can confirm that none of the function would be of use in day-to-day navigation of a boat. --WDS comment: I'm dubious. Does this guy really navigate using chronometer, sextant, and spherical trigonometry? I doubt anybody has done that for decades. And one's initial reaction on seeing a chronometer (which is just a wind-up clock with high-quality innards) might well have been "no use for day to day navigation of boat." This judgment would be totally wrong. The only way to realize its use is to investigate mathematical theory, not by just looking at it and saying "I've sailed on boats." And by the way, the whole phrase "day to day" employed by Edmunds is misleading. More accurate would be "week to week." To get an idea of reality, it helps to consider the famous sea-voyage by Shackleton with 5 others in 1914 in open rowboat 800 miles. Did they do "day to day" navigation? No they did not, because it was impossible. They set off 24 April. The log of this voyage reveals that they were able to obtain successful sun sights on 26 April, 29 April, 4 May. On 8 May they spotted land (their destination). Landed 10 May. (They then did some further local sailing next few days.) Thus the entire voyage lasted 17 days during which they only accomplished 3 sun-sights. Similarly, the plan that was being investigated by the British for navigation based on the moon (this was before the success of the chronometer) would have required accurate measurement of angles from moon to horizon, stars, sun. Again, such measurements would probably have been impossible to accomplish every day. For example in bad seas or cloudy. I repeat, this whole scheme was under development by the British admiralty as a possible navigation method until it got superseded by the unexpected success of the chronometer which allowed simpler method. --Edmunds: Its possible use for determining longitude at eclipse times has been suggested before, but this is totally impractical for navigation because eclipses occur so rarely. --WDS: I agree eclipses rare, and I agree eclipses basically no use, but you do not need eclipses to determine longitude. And if you were relying on eclipses, you would be being an idiot. My whole point was not to do that and not to be an idiot. For example, did the British admiralty intend to rely on eclipses? Of course not. Edmunds appears not to have read my letter and to be highly unfamiliar with the whole idea behind this kind of navigation. --Edmunds: It has been argued that such devices might assist in setting longitude at, say, opposite ends of the Mediterranean sea in a campaign lasting several years, as may have been attempted by the Chinese in other waters in the 15th century - but again this is not day-to-day navigation. Anyway, all they would have needed is a written compilation of eclipse months! --WDS: Did the Chinese even know that the Earth was round? The Greeks were well aware of it by 5th century BC. Wikipedia http://en.wikipedia.org/wiki/Spherical_Earth has "In the 17th century, the idea of a spherical earth, now considerably advanced by Western Astronomy, ultimately spread to Ming China, when Jesuit missionaries, who held high positions as astronomers at the imperial court, successfully challenged the Chinese belief that the earth was flat and square." citing http://www.uni-tuebingen.de/uni/ans/eastm/back/cs11/cs11-4-martzloff.pdf Jean-Claude Martzloff: "Space and Time in Chinese Texts of Astronomy and of Mathematical Astronomy in the Seventeenth and Eighteenth Centuries", Chinese Science 11 (1993-94): 66-92 (69). So, I'm not terribly impressed by this Edmunds quote, because (a) eclipses are irrelevant hence everything Edmunds says about eclipses is irrelevant, and (b) also his whole side-remark about the Chinese (which also is pretty irrelevant) seems somewhere between dubious and refuted anyhow. --Edmunds: We know from a study of ancient literature sources - see e.g. http://pos.sissa.it/archive/conferences/170/019/Antikythera%20&%20SKA_019.pd... - that the device was certainly not unique, indeed we have a fair number of references to such astronomical devices over the period approx 250 BC to 500 AD, and no mention whatever of a similar navigational device. The subtle mechanical details of the Antikythera device would be impossible to design without considerable previous experience, and it must come from a tradition of the making of such geared mechanisms. By far the most likely explanation for its presence on the (trading) boat is that it was being transported, possibly as part of a large consignment of goods, or simply being delivered on its own. --WDS: I'm happy to agree it was on that boat merely for transport. In fact, I had speculated the device was experimental and stolen by the Romans. So we are in agreement. So therefore this disclosure by Edmunds has no logical bearing at all on my entire hypothesis. Which he would have realized had he actually read my letter. And the URL Edmunds gave does not seem convincing to me of anything much. First of all, before Galileo invented the pendulum, it was almost utterly pointless to build a mechanical clock in the sense accuracy would have been totally pathetic. A mechanical clock making an error of say, 30 minutes per day, which probably would have been about best possible before the year 1200, would have been for navigational purposes off by 500 miles per day even if portable (which they weren't). Any clock anyone ever made pre-Galileo and especially pre-1200 AD would have been merely as a joke, toy, or mystical device intended to impress somebody with mysticism, not actually be useful. I do not see the point of it. The first pendulum clock was built by Chr.Huygens in 1656. Second, the "sphaerae" referred to in this manuscript may not have been a mechanical device with moving parts at all, at least for all the evidence I see given in the manuscript! (They mention a Cyclopaedia 1810 article, but what the heck did they know in 1810?) --Edmunds: With Best Wishes, Prof Mike Edmunds --WDS: to make a genuine assessment of whether a device like the Antikythera could have been intended for navigation (or could have been intended as experimental with possible future development into a workable nav. device) the main question that needs to be answered is an accuracy question. There is angle-measurement accuracy for observations of moon etc. And there is predictive accuracy via the device. All previous assessments of predictive accuracy for the device that I have seen, were wrong-headed and hence largely irrelevant, since they concerned multiyear predictions. In fact, if it were used for navigation, the device would be reset to truth at start of voyage, then the voyage lasts (say) 3 weeks. So what we need to assess is the accuracy during this 3 week span assuming perfect accuracy at start. As the observers of the Antikythera device have already noted, the device was built to make it easy to take apart and reset. The question is 2-fold (a) what accuracy could this device itself have achieved, and (b) what about a similar device but with optimized parameters?