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From: Jasen Betts on 9 Mar 2010 04:00 On 2010-03-08, rickman <gnuarm(a)gmail.com> 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 solstice is not a day it is an instant, and it does not happen same the date and wall time every year. most years have no days equidistant from the solstice. therefore noon (or any other hour) on most days will duplicate the same elevation of the sun above the horzon. --- news://freenews.netfront.net/ - complaints: news(a)netfront.net ---
From: Jasen Betts on 9 Mar 2010 04:04 On 2010-03-08, D Yuniskis <not.going.to.be(a)seen.com> wrote: > 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? Given lattitude and longitude (or equivalent) and sufficiently good instruments, and the right data and skills, yes. --- news://freenews.netfront.net/ - complaints: news(a)netfront.net ---
From: D Yuniskis on 9 Mar 2010 12:20 Jasen Betts wrote: >> 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? > > Given lattitude and longitude (or equivalent) and sufficiently good > instruments, and the right data and skills, yes. So, a *device* that watched these things could deduce date/time (?) How much more would it have to do to deduce location (or, at least, latitude)? Probably just watch for a longer period of time?
From: rickman on 9 Mar 2010 15:30 On Mar 9, 12:20 pm, D Yuniskis <not.going.to...(a)seen.com> wrote: > Jasen Betts wrote: > >> 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? > > > Given lattitude and longitude (or equivalent) and sufficiently good > > instruments, and the right data and skills, yes. > > So, a *device* that watched these things could deduce date/time (?) > How much more would it have to do to deduce location (or, at least, > latitude)? Probably just watch for a longer period of time? I can't say this for certain, but I believe the combination of length of day and elevation of the sun at the zenith is a unique combination for each day of the year and latitude. So I think you can get your latitude the same day. But I'm not sure you don't have the same two day ambiguity. Otherwise I think the combination is unique. Even the North-South issue can be resolved because of the eccentricity of the Earth's orbit making things a little different in the two hemispheres. But you may also be foiled beyond the artic/anartic circles where the sun never sets. Then you only get one parameter, the elevation at the zenith. But you might be able to make up for that by measuring the time between the sun at due east and due west... other than at the poles where there is no east or west... ;^) Rick
From: Walter Banks on 9 Mar 2010 16:13
rickman wrote: > I can't say this for certain, but I believe the combination of length > of day and elevation of the sun at the zenith is a unique combination > for each day of the year and latitude. So I think you can get your > latitude the same day. It is close but not exact. The earth's orbit is not an exact number of days. For the calendar's purpose we accumulate errors and adjust the calendar. These adjustments are every 4 years and sometimes on the century. There are other errors that have an impact on the observations depending on the required accuracy. I saw a sundial on a beach near Kobe Japan that had elaborate error correcting instructions that was probably good to a second after ten minutes of calculations. There were a lot of factors involved it accounted for earths orbital period > But I'm not sure you don't have the same two > day ambiguity. Otherwise I think the combination is unique. Even the > North-South issue can be resolved because of the eccentricity of the > Earth's orbit making things a little different in the two > hemispheres. But you may also be foiled beyond the artic/anartic > circles where the sun never sets. Then you only get one parameter, > the elevation at the zenith. But you might be able to make up for > that by measuring the time between the sun at due east and due west... > other than at the poles where there is no east or west... ;^) Above the arctic circle the sun 24 hour path is tilted but there are other factors that are significant. For a couple weeks around June 21 the sun never sets as far as 80 miles or so south of the arctic circle. Most of this is due to the optic effects of the atmosphere. Even above the arctic circle actual and observed position of the sun has significant differences. Regards, w.. -- Walter Banks Byte Craft Limited http://www.bytecraft.com |