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From: D Yuniskis on 8 Mar 2010 14:30 rickman wrote: > On Mar 6, 1:43 am, Jasen Betts <ja...(a)xnet.co.nz> wrote: >> On 2010-02-27, D Yuniskis <not.going.to...(a)seen.com> wrote: >> >>> Hi Joe, >>> Joe Pfeiffer wrote: >>>> The clock/calendar I hope to build over the next year or so will be >>>> solar. The shadow of a post uniquely determines both date and time, if >>>> you look at both angle and length.... >>> Hmmm... is that (really) true? >> yes. >> >>> Or, don't you end up with >>> *two* date,times for each angle,length? >> that may happen for some dates of some years :) > > Actually, it happens for every day of every year other than the > solstices. The two solstices (actually a day or two on either side > depending of the season) has the lowest or highest path across the > sky, so no other day will have quite in that same path. But every > time of every other day (excluding a few seconds at the start and end > of the day when one day has sunshine and the other does not) will > match a time of two days, between spring to fall and one between fall > to spring. The path of the sun may not be the same on those two days, > but each point will map to two different times and days. So (thinking in terms of a *truly* unique hack), if you *watched* the motion over the course of a particular day (e.g., 'yesterday'), could you *uniquely* determine that day?
From: Joe Pfeiffer on 8 Mar 2010 16:29 rickman <gnuarm(a)gmail.com> writes: > On Mar 6, 1:43 am, Jasen Betts <ja...(a)xnet.co.nz> wrote: >> On 2010-02-27, D Yuniskis <not.going.to...(a)seen.com> wrote: >> >> > Hi Joe, >> >> > Joe Pfeiffer wrote: >> >> The clock/calendar I hope to build over the next year or so will be >> >> solar. The shadow of a post uniquely determines both date and time, if >> >> you look at both angle and length.... >> >> > Hmmm... is that (really) true? >> >> yes. >> >> > Or, don't you end up with >> > *two* date,times for each angle,length? >> >> that may happen for some dates of some years :) > > Actually, it happens for every day of every year other than the > solstices. The two solstices (actually a day or two on either side > depending of the season) has the lowest or highest path across the > sky, so no other day will have quite in that same path. But every > time of every other day (excluding a few seconds at the start and end > of the day when one day has sunshine and the other does not) will > match a time of two days, between spring to fall and one between fall > to spring. The path of the sun may not be the same on those two days, > but each point will map to two different times and days. In terms of the project I've got in mind, people are really over-thinking this..... I'll be very surprised if the shadow ends up distinguishing the day with better than a couple-of-days precision anyway. That said, two days won't follow *exactly* the same path: a fall day's shadow is going to be between two spring days' shadows, and so forth. -- As we enjoy great advantages from the inventions of others, we should be glad of an opportunity to serve others by any invention of ours; and this we should do freely and generously. (Benjamin Franklin)
From: Joe Pfeiffer on 8 Mar 2010 16:36 Arrrgh -- I misread what you were saying, so my response didn't make much sense. But, I still don't expect a point to necessarily correspond to two date/time pairs. Time is continuous, but days aren't. There will be a gap between any two days' shadow tracks (probably smaller than the fuzziness of the shadow caused by diffraction but there all the same). Unless two tracks exactly overlay for some meaningful part of the tracks, a given track can only intersect other days' tracks at a finite number of points points. -- As we enjoy great advantages from the inventions of others, we should be glad of an opportunity to serve others by any invention of ours; and this we should do freely and generously. (Benjamin Franklin)
From: -jg on 8 Mar 2010 17:20 On Feb 27, 6:47 pm, Joe Pfeiffer <pfeif...(a)cs.nmsu.edu> wrote: > The clock/calendar I hope to build over the next year or so will be > solar. The shadow of a post uniquely determines both date and time, if > you look at both angle and length.... Sounds like a cool idea. Would need this sort of correction : http://www.swanstrom.net/sundial/gnomon.htm which means it could take a couple of days to 'train', in order to be certain. For the best precision, you'd probably do a simple sliding-data- match, where maybe the last 7? days of readings, are moved along to a point of least-errors. (and maybe a cloudy-day default, where it just increments the day ? I don't see why you think you can't get 'correct day' precision out of this ? Pushing the actual-time precision is likely to be more challenging ? -jg
From: Paul Keinanen on 8 Mar 2010 20:36
On Mon, 8 Mar 2010 14:20:46 -0800 (PST), -jg <jim.granville(a)gmail.com> wrote: >On Feb 27, 6:47�pm, Joe Pfeiffer <pfeif...(a)cs.nmsu.edu> wrote: >> The clock/calendar I hope to build over the next year or so will be >> solar. �The shadow of a post uniquely determines both date and time, if >> you look at both angle and length.... > >Sounds like a cool idea. >Would need this sort of correction : >http://www.swanstrom.net/sundial/gnomon.htm > >which means it could take a couple of days to 'train', >in order to be certain. > > For the best precision, you'd probably do a simple sliding-data- >match, where maybe the last 7? days of readings, are moved along to a >point of least-errors. >(and maybe a cloudy-day default, where it just increments the day ? > > I don't see why you think you can't get 'correct day' >precision out of this ? Possibly in March or September, but absolutely not in July or December, just look at the graph. A 7 day period would be sufficient to figure out, if it is March or September by checking the direction of the solar movement. > Pushing the actual-time precision is likely to be >more challenging ? That is trivial. The apparent solar diameter is 30 arc minutes and since the sun moves 15 degrees each hour, it only takes 2 minutes to move its own diameter, thus the actual noon can be determined with much better precision. In order to get the solar mean time noon, you also need to know the approximate date to apply the equation of time. |