How hard is to build a processor?

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?

Reply to
D Yuniskis
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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.

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Reply to
Joe Pfeiffer

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.

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Reply to
Joe Pfeiffer

Sounds like a cool idea. Would need this sort of correction :

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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

Reply to
-jg

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.

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.

Reply to
Paul Keinanen

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.

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Reply to
Jasen Betts

Given lattitude and longitude (or equivalent) and sufficiently good instruments, and the right data and skills, yes.

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Reply to
Jasen Betts

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?

Reply to
D Yuniskis

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

Reply to
rickman

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

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

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Reply to
Walter Banks

The fish-hook here is in the careful wording of "and sufficiently good instruments, and the right data and skills, yes"

So a smarter question, could be what is practical ?

- and using what measurement systems ?

I found this revealing page, which has real datapoints, and a practical location (ie less than ideal)

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Most revealing are the nice dots-on-the-door

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and the red arcs, are snapshots of the actual path, ~4wks - note they include a dot on alternating sides of the analemma, as the 12 arcs interlace.

This site below shows the analemma actually moves yr-yr, so that's more data to track ;)

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-jg

Reply to
-jg

I always thought a cool hack would be a motorized sundial. (i.e., the motorization being a cleverly hidden aspect) E.g., with nice, evenly spaced markings -- and a motor to rotate the whole assembly such that the shadow fell "where it should" (on this nicely marked indicator).

It;s the sort of thing that would elicit comment *only* from someone who *knew* it was "quite impossible" to work as it *suggests* it works...

(obviously, I like things that mess with people's heads :> )

Reply to
D Yuniskis

By observing when the sum passes the meridian, one cloud free week in the spring and one in the autumn should give a quite good resolution for the latitude, provided that some internal time reference is capable of measuring the number of days between the measuring periods with at least +/-12 hour accuracy. During one week long period the sun moves south and on he other it moves north.

Of course, there is the north/south hemisphere ambiguity, but with additional sensors to the left and right of the meridian line should help solve this ambiguity. After all, in order to detect meridian passing you would have to align the device towards true north.

A camera with at least 150 degree field of view pointing directly upwards towards zenith, should be able to detect the orientation, latitude, date and local solar time within a year of observations.

Reply to
Paul Keinanen

The hardware for this is in the current issue of Circuit Cellar. Different programming needed....

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As we enjoy great advantages from the inventions of others, we should
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Reply to
Joe Pfeiffer

6 months would be sufficient, probably shorter periods too.

the earths's axial wobble, and orbital precession, are probably going to make it impossible do it in less than a week. OTOH if you can see the stars and planets at night that would help a lot...

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Reply to
Jasen Betts

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