Ok, I see what you mean--I misunderstood because it's a kind of dial that hadn't occurred to me. It sounded good, but then I realized that latitude is built into such a dial, so it's only usable at a certain latitude. I've sometimes used a compass or solar direction-finding when hiking in my own county, but mostly when in an unfamiliar town. When you make a sun-compass, you don't necessarily know where you'll need it. That's something that I like about the Universal Analemmatic Dial as a suncompass. ...and the fact that constructing it need consist only of printing out the markings, which, by themselves, can tell you the Sun's azimuth at any latitude, date and time.
Though AW determines azimuth primarily from time, it's still true that, as you said, an altitude mis-estimate could have more effect near mid-day, because the higher the Sun is, the more greatly the altitude affects the difference between h and Az. So, what you said about that was true. I was just looking more at the error due to substituting h and Az for their sines, and assuming that the altitude estimate is good. Michael Ossipoff On Thu, Oct 25, 2018 at 1:19 AM Steve Lelievre < steve.lelievre.can...@gmail.com> wrote: > Michael, > > > > > On 2018-10-24 8:25 p.m., Michael Ossipoff wrote: > > A Shephard’s Dial wouldn’t help as a sun-compass. It just gives time if > you know the date, or date if you know the time. > > By writing "a Shepard's Dial marked out as a solar compass" I meant that > one for which the lines drawn on the cylinder are the azimuth corresponding > to altitude instead of the usual option of the hour corresponding to > altitude. So, yes, a sun compass. > > > Sure, an Altitude-Dial is at its least accurate near noon, but this AW > method, and the TA that it’s based on, are different. The error is 0 at > noon, if you’re using the right EoT and longitude. The altitude (ideally > along with the declination) adjusts h, to get the azimuth from south. > > . > > The error is max sometime during mid-afternoon because, because it’s 0 at > noon, and because, when the sun is low near sunset, h is multiplied by a > only a factor, closer to 1, because cos dec * sec Alt is closer to 1 then. > > . > > AW’s error comes from the fact that it substitutes h and Azimuth for their > sines. When the factor by which sin h is multiplied is closer to 1, the > error from that substitution is smaller. > > . > > So AW has its greatest error around mid-afternoon, between noon when it’s > 0, and near sunset when it’s error is low due to that multiplicative factor > being closer to 1. > > OK, I see what you're saying now. I was coming at it just by imagining how > hard it must be to get an accurate altitude measurement - perhaps a few > degrees out. My thinking was that around noon the azimuth changes a lot > from a small change in altitude so any measurement error would be > multiplied considerably, whereas later or earlier in the day the same small > change in altitude would correspond to a smaller change of azimuth. > > Cheers, > > Steve > > > >
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