Dan— .
Here are the maximum errors, in degrees, of the three solar direction-finding methods I’ve been discussing, for Romania. . I used lat 45 as a typical latitude there. Of course a person is most likely to need a direction-finding method in a district that they don’t live in. . For the Watch method, I assumed that that method is used at the edge of a standard-size time-zone, and that EoT and longitude aren’t taken into account. . For the Altitude-Watch method, I assumed that EoT and longitude are taken into account, and that solar declination is known or well-estimated. …and that solar altitude is accurately estimated. . Shadow-Tip: 33.25 . Watch-Method: 37 . Altitude-Watch Method: 21 . (I didn’t analytically-maximize—I just looked at various whole and half-hours in the afternoon at the summer-solstice, to find which of those hours and half-hours gave the method the most error.) . Michael Ossipoff On Mon, Oct 22, 2018 at 12:20 PM Dan-George Uza <[email protected]> wrote: > Dear John and others, > > Thank you for your insights. Although I haven't been able to track the > analysis I was looking for eventually I did find something similar. Google > "On the method of direction finding by Sun and Watch by Norman Pye". The > author makes an analysis for true azimuths and watch hour angles, dealing > with values projected onto the horizontal plane. The directional error is > due to the Sun moving in a different plane from the horizontal. From the > table I attach below it seems that the watch method works best in winter > because then the Sun stays close to the horizon and doesn't have a great > spread in azimuth. > > Hope this helps, > > Dan > > > On Mon, Oct 22, 2018 at 1:45 AM <[email protected]> wrote: > >> Hi Dan, >> >> Sorry for the delay in replying. >> >> From a PRACTICAL point-of-view, as we all know, analogue watches >> replaced sundials, digital watches replaced analogue watches, and smart >> phones have replaced watches. Digital compasses replaced analogue >> compasses, and now smart phones have replaced compasses. I confess to >> having a mobile phone, and no longer wearing a watch. Also when I go >> bushwalking, now I carry a GPS with real-time tracking on appropriate-scale >> topographic maps. I still have a digital compass, but it was pretty fiddly >> to use, so it now sits somewhere at home. I only use a magnetic compass >> when doing serious field work, and I need to know the orientation of some >> feature I am measuring. >> >> But what has practicality to do with anything related to sundials???? >> >> Several years ago I was also intrigued about the accuracy of using a >> watch as a compass, and I decided to investigate it the empirical way. So I >> made up a little “tool” and every weekend when I went bushwalking, I would >> set it up and compare compass north with watch north at regular intervals. >> >> I ended up with quite a few measurements before life got in the way of >> plans, and the project petered out. I still have the results but I have >> never analysed them. In part because even then it was obvious to me that >> the question could be investigated using standard equations. But my feeling >> at the time was that the whole watch / north method was getting close to an >> urban myth. I had collected several variations on instructions, but I seem >> to have lost them in one of my several moves. But I do remember that >> depending on which you used, the error could be 30o or more. None of the >> methods said anything about the difference between true and magnetic north, >> but that may be irrelevant anyway unless you are somewhere like Antarctica >> where the difference can be 70o. More important would be DST which could be >> a major trap for the unwary. >> >> At one stage the watch / north method was called the “Boy Scout”method. >> During my travels on public transport when kids are going to school, I >> haven’t seen too many wearing analogue watches. Or watches of any form. >> They are completely welded to their smart phones, so it’s pretty obvious >> that the method has gone the way of sundials. >> >> When you find the analysis of the watch / north idea, can you post it on >> the List please? >> >> Cheers, John >> >> John Pickard >> [email protected] >> >> >> *From:* Dan-George Uza <[email protected]> >> *Sent:* Saturday, September 29, 2018 4:57 AM >> *To:* Sundial List <[email protected]> >> *Subject:* Accuracy of wristwatch as compass >> >> Hello! >> >> I'm sure you know the method of pointing the analogue wristwach hour hand >> towards the Sun and then bisecting the angle to 12 o'clock in order to find >> south (or north, if you live down in the south). Actually I guess what you >> should be doing is bisect the angle to your noon time and not necessarily >> 12 o'clock, but anyway. A few years ago I read an interesting seasonal >> accuracy analysis of this method. I also vaguely remember the demonstration >> involved Vitruvius' analemma and I'm pretty sure it was all in a book. >> Thing is - I can't remember where! Can you help? >> >> Dan Uza >> >> ------------------------------ >> --------------------------------------------------- >> https://lists.uni-koeln.de/mailman/listinfo/sundial >> >> --------------------------------------------------- >> https://lists.uni-koeln.de/mailman/listinfo/sundial >> >> --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > >
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