I was exploring some unrelated things (as usual), and I found an interesting
shadow clock that was easy to model

https://www.dezeen.com/2015/10/23/breaded-escalope-touch-shadow-clock-vienna-design-week-2015/

And then I was noticing how the sun came into my apartment, and thinking back on
a solar energy model I made a few years ago, and Ingo's sunpos.inc file, and I
recalled that an ordinary watch can be used as a makeshift compass.

Orient the hour hand to point at the sun (accurately done with a blade of grass
to cast a shadow across the hour hand) and then divide the angle between the
hour hand and the 12:00 mark.  That's roughly South.

I wanted to see how it worked, and get an idea of how far off it would be.

So naturally I modeled it in POV-Ray   :)

Seems to work ok, and be off by +/- 30 degrees - dead on at noon.

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Attachments:
Download 'watchsouth.avi.dat' (2101 KB)

"Bald Eagle" <cre### [at] netscapenet> wrote:
> I was exploring some unrelated things (as usual), and I found an interesting
> shadow clock that was easy to model
>
>
https://www.dezeen.com/2015/10/23/breaded-escalope-touch-shadow-clock-vienna-design-week-2015/
>
> And then I was noticing how the sun came into my apartment, and thinking back on
> a solar energy model I made a few years ago, and Ingo's sunpos.inc file, and I
> recalled that an ordinary watch can be used as a makeshift compass.
>
> Orient the hour hand to point at the sun (accurately done with a blade of grass
> to cast a shadow across the hour hand) and then divide the angle between the
> hour hand and the 12:00 mark.  That's roughly South.
>
> I wanted to see how it worked, and get an idea of how far off it would be.
>
> So naturally I modeled it in POV-Ray   :)
>
> Seems to work ok, and be off by +/- 30 degrees - dead on at noon.

Interesting animation. Not sure why or how you did the sine wave type thing but
since 1 hour equals 15 degrees longitude that would cause some inaccuracy if not
near the middle of a time zone. And of course Daylight Savings Time, if in areas
using that.

As usual, I was making your description more complicated than it is. Shadow
across hour hand... then I realized I was making matters worse by imagining the
minute hand to be the hour hand! Found a wikiHow page telling about it but only
made matters worse before I realized the drawings weren't exactly right, then it
finally made sense.
http://www.wikihow.com/Use-an-Analog-Watch-as-a-Compass

I like that wall shadow clock. Someone made a comment about it not showing the
hour and minute differently, would have to guess. Not too much of a problem if
you know the general time of day already but overlapping would cause trouble.
Solution might be to use 2 different color lights.

Bob

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"omniverse" <omn### [at] charternet> wrote:

> Interesting animation. Not sure why or how you did the sine wave type thing but
> since 1 hour equals 15 degrees longitude that would cause some inaccuracy if not
> near the middle of a time zone. And of course Daylight Savings Time, if in areas
> using that.

Well, South is a 180 degree rotation from my North / 12:00 starting point, and I
obviously have to calcualte the position of the hands, the rotation of the
watch, and the rotation of the arrow.
So I make those calculations, and find the difference between that angle and 180
degrees, and plot it vs time.


> I like that wall shadow clock. Someone made a comment about it not showing the
> hour and minute differently, would have to guess.

I use 2 different angles for hour and minute, to make them unequal lengths, use
a stepped cone atop a cylinder to further accentuate the difference, and I also
tried the different colored lights - as well as negative color values  :)

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Good watching [sic] here:

https://www.youtube.com/watch?v=IJhgZBn-LHg

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"Bald Eagle" <cre### [at] netscapenet> wrote:
> Good watching [sic] here:
>
> https://www.youtube.com/watch?v=IJhgZBn-LHg

Yeah, thanks for that. The manakin sitting on Earth watching the Sun is my
favorite part. A fun "watch" indeed.

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Le 13/12/2016 à 14:02, Bald Eagle a écrit :
> I was exploring some unrelated things (as usual), and I found an interesting
> shadow clock that was easy to model
> 
>
https://www.dezeen.com/2015/10/23/breaded-escalope-touch-shadow-clock-vienna-design-week-2015/
> 
> And then I was noticing how the sun came into my apartment, and thinking back on
> a solar energy model I made a few years ago, and Ingo's sunpos.inc file, and I
> recalled that an ordinary watch can be used as a makeshift compass.
> 
> Orient the hour hand to point at the sun (accurately done with a blade of grass
> to cast a shadow across the hour hand) and then divide the angle between the
> hour hand and the 12:00 mark.  That's roughly South.
> 
> I wanted to see how it worked, and get an idea of how far off it would be.
> 
> So naturally I modeled it in POV-Ray   :)
> 
> Seems to work ok, and be off by +/- 30 degrees - dead on at noon.
> 

1. The hour should be corrected to be solar (ditch away summer time &
other delta: for instance at Paris, in winter, the hour is aligned with
Berlin, and about 1 hour wrong. It becomes 2 hours wrong in summer time)
(Paris is about 3° East, and does not justify a delta of 1 hour from
Greenwich : one hour is 15° of longitude)

2. It works only in North hemisphere (the further above the tropic, the
better (tropic is about latitude 28° ))
Between the north tropic and south tropic, you would have period of the
year with problems.
South of the south tropic, the clock is not turning in the right sense
to follow the sun (which is North at noon)

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