APOD: A Sundial that Shows Solstice (2012 Jun 26)

Comments and questions about the APOD on the main view screen.
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APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by APOD Robot » Tue Jun 26, 2012 4:05 am

Image A Sundial that Shows Solstice

Explanation: What time is it? If the time and day are right, this sundial will tell you: SOLSTICE. Only then will the Sun be located just right for sunlight to stream through openings and spell out the term for the longest and shortest days of the year. And that happened last week and twice each year. The sundial was constructed by Jean Salins in 1980 and is situated at the Ecole Supérieure des Mines de Paris in Valbonne Sophia Antipolis of south-eastern France. On two other days of the year, watchers of this sundial might get to see it produce another word: EQUINOXE.

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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by Beyond » Tue Jun 26, 2012 5:03 am

Although i think the 'watchers' are just about purrfect, i don't really get anything out of a 4-word-a-year sundial. Except, perhaps, the word unusual.
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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by Flase » Tue Jun 26, 2012 5:54 am

Yes how well does it tell the time normally?

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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by Ann » Tue Jun 26, 2012 6:00 am

Beyond wrote:Although i think the 'watchers' are just about purrfect, i don't really get anything out of a 4-word-a-year sundial. Except, perhaps, the word unusual.
Reminds me of a Mickey-and-Goofy cartoon where Micky asked Goofy why he kept wearing his old wristwatch, which wasn't working any more. Goofy replied:

- Oh, it's not useless - once every twelve hours it gives me the perfectly right time!

(Personally I think that the "Solstice" sundial is fun. I'm glad it can do "Equinoxe" too, otherwise I would have been disappointed!)

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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by Flase » Tue Jun 26, 2012 6:38 am

I'm presuming it doesn't mark the precise moment that the plane defined by the tilt of the Earth's axis bisects the Sun?

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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by neufer » Tue Jun 26, 2012 7:50 am

Image
Flase wrote:
I'm presuming it doesn't mark the precise moment that the plane defined by the tilt of the Earth's axis bisects the Sun?
That would have been at 9 minutes past midnight
(Antipolis Standard Time).

SOLSTICE is readable for ~ 15 minutes around local noon
(for at least a month).
Last edited by neufer on Tue Jun 26, 2012 8:47 am, edited 2 times in total.
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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by nstahl » Tue Jun 26, 2012 8:19 am

It tells the time by producing a streak of light on the hour. Note the dotted lines and follow the "this sundial" link.

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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by starstruck » Tue Jun 26, 2012 8:19 am

I think it's a very good design. Have to admit, I too often find sundials quite frustrating because they seem to be hardly ever useful (or right!), but this Solstice/Equinox one has an unusual element of design and thought about it. I am still trying to get my head round how one would set about making something like this that creates recognisable letters from the shadow outlines of unrecognisable holes cut into angled layers of material. It's a clever exercise in three dimensions relating to two. It occurred to me that I'd be interested to see a time-lapse sequence of the build-up of the letters coming together, forming the word, and then breaking apart again . . only one I could find is this .avi file here, http://cadrans.solaires.free.fr/docs/EQUINOXE.avi

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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by neufer » Tue Jun 26, 2012 9:10 am

Ann wrote:
Reminds me of a Mickey-and-Goofy cartoon where Micky asked Goofy
why he kept wearing his old wristwatch, which wasn't working any more.

Goofy replied:

- Oh, it's not useless - once every twelve hours it gives me the perfectly right time!

(Personally I think that the "Solstice" sundial is fun.
I'm glad it can do "Equinoxe" too, otherwise I would have been disappointed!)
ImageImage
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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by Psnarf » Tue Jun 26, 2012 1:50 pm

Clever work of art. For many of the letters, the light passes through two planes to form the letter. That is, it is difficult to find any of the letters cut out completely on one surface. It would take me a long time to figure out an algorithm for doing that. I would kick some sand on the artist's shoes just for presenting such a connundrum.

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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by Chris Peterson » Tue Jun 26, 2012 2:21 pm

Psnarf wrote:Clever work of art. For many of the letters, the light passes through two planes to form the letter. That is, it is difficult to find any of the letters cut out completely on one surface. It would take me a long time to figure out an algorithm for doing that. I would kick some sand on the artist's shoes just for presenting such a connundrum.
I don't know what tools the artist had available. These days, doing something like this is trivial, however... no need to design an algorithm, or really, even do any math. You just need to do the artistic bit- design the structure the way you want, and then use any solid modeling CAD program (like SolidWorks or Alibre) to project the lettering you want from the ground to the Sun, and let the program subtract out the material.
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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by Moonlady » Tue Jun 26, 2012 2:45 pm

I suggest an upgrade for this Sundial:
It could show outside of solstice times "You missed it!"

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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by Anthony Barreiro » Tue Jun 26, 2012 4:18 pm

This sundial is really beautiful and clever. It inspires passersby to think about the relationship between the Earth and the Sun, and that's always a good thing. I love sundials.
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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by scr33d » Tue Jun 26, 2012 5:11 pm

I think there was a (tongue-in-cheek) Martin Gardner puzzle that challenge you to design a sundial so that the changing position of the sun casts a corresponding digital time shadow/projection--continuously, all the time!

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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by eltodesukane » Tue Jun 26, 2012 5:45 pm

a reference I once read about digital sundial:
Kenneth Falconer, Digital sundials, paradoxical sets, and Vitushkin's Conjecture, Mathematical Intelligencer 9 (1987) 24-27.
http://www-maths.mcs.st-andrews.ac.uk/~kenneth/
http://www.springerlink.com/content/8p231p31k7247p38/
http://resources.metapress.com/pdf-prev ... ze=largest

"In 1991, Scientific American author Ian Stewart (based on a 1987 article by K. Falconer) hypothesized the possibility of a digital sundial in an article entitled "What in heaven is a digital sundial?""
http://www.digitalsundial.com/
http://www.cthisspace.com/ftl/macaw/MR40.html

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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by Chris Peterson » Tue Jun 26, 2012 5:55 pm

scr33d wrote:I think there was a (tongue-in-cheek) Martin Gardner puzzle that challenge you to design a sundial so that the changing position of the sun casts a corresponding digital time shadow/projection--continuously, all the time!
You could actually do that with a white-light hologram. I could imagine etching the hologram onto a stainless steel surface, such that when you stood in a particular position, you'd see the digital time. You could make it show the time on an analog clock face, as well. In fact, you could have two viewing positions, one for digital and one for analog.
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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by scr33d » Tue Jun 26, 2012 6:10 pm

eltodesukane, chris,
Good links, just have time to scan the Falconer paper. It brings to mind the Banach-Tarski sphere, which is astonishing mathematics but not for the real world: the fractal-sized venetian binds would run up against the physics of diffraction.
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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by Anthony Barreiro » Tue Jun 26, 2012 6:39 pm

Here's another interesting clock, designed to run for 10,000 years, that corrects itself every day at solar noon, and uses solar energy to wind itself.
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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by RJN » Wed Jun 27, 2012 12:21 am

Chris Peterson wrote: You could actually do that with a white-light hologram.
That's an intriguing thought, Chris! I looked it up and someone has already published it though -- and patented it too. What escapes me just now, though, is how the degeneracy between two times of the year would be resolved. In other words, the Sun passes near one position on the sky twice during a year -- once before solstice (for example), and once after solstice. How would a "holodial" know which one of these two times is right?

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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by Chris Peterson » Wed Jun 27, 2012 2:34 am

RJN wrote:
Chris Peterson wrote: You could actually do that with a white-light hologram.
That's an intriguing thought, Chris! I looked it up and someone has already published it though -- and patented it too. What escapes me just now, though, is how the degeneracy between two times of the year would be resolved. In other words, the Sun passes near one position on the sky twice during a year -- once before solstice (for example), and once after solstice. How would a "holodial" know which one of these two times is right?
That's an interesting question. You're thinking of the intersection point in the figure-8 of the analemma, no? I guess one approach would be to have a different place to stand, depending on the time of year. Effective, but not very elegant. Maybe some sort of passive kinematic element in the sundial/sculpture? I'll bet I could come up with a drive mechanism that that moves based on the daily cycling of temperature (like a ratchet with a bimetallic component), or maybe a tilt mechanism that depended on the typical temperature difference between the two times of year (which might get fooled occasionally). An interesting problem... now I might not sleep well tonight!
Chris

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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by raviapte » Wed Jun 27, 2012 8:44 am

http://apod.nasa.gov/apod/ap031206.html

This sun dial is accurate to about 1 second..

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Re: APOD: A Sundial that Shows Solstice (2012 Jun 26)

Post by neufer » Wed Jun 27, 2012 12:56 pm

Image
raviapte wrote:
http://apod.nasa.gov/apod/ap031206.html

This sun dial is accurate to about 1 second..
Perhaps, but unlike most sundials it runs counterclockwise.
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