Talk:Diptych

I have heard of the diptych. I believe that the info given in the article is not accurate. Specifically, I have read that no matter which way you hold a diptych, both dials will show the same time.

Some people apparently believe that the diptych doesn't measure all these things. One little bit of errata placed over in sundial said that both sides of a diptych measured the same time no matter the angle it was held! The only times this is true are noon, noon, sunrise and sunset. At 9am and 3pm, each degree of error in the gnomon's latitude creates a difference of four minutes. The trick is that the dyptych's hinge must be level, which is easy to measure with a bubble-level (or more anciently, a rolling marble).

No, no, no! If I have it level, but rotate it horizontally, both dials will show the same time, but it will be the wrong time!

I'm a bit confused. The pictures I've seen (The clearest one is the "diptych dial" at http://www.sundials.co.uk/projects.htm ) show straight lines at various angles radiating from the string attachment point in each of the 2 plates of the diptych. If I look at the 3pm lines, both of them (the one on the vertical plate and the other one on the horizontal plate) intersect the hinge at exactly the same point. If I rotate the diptych around until the string casts a shadow on that point, then the string casts a shadow along the entire 3pm line on the vertical dial and the 3pm line on the horizontal dial. I find it impossible to create a "difference" in time between the two dials, no matter which direction I point it while keeping the horizontal plate level. (I also find it impossible to create a "difference" in time between the 2 dials, no matter what angle (in effect, latitude) I tilt it while keeping the hinge running east-west, or any combination of rotation+tilt). In other words, I don't see how a diptych all by itself can give latitude or point out true north. What am I missing ? --DavidCary 08:04, 19 Jan 2005 (UTC)

The article mentions the angle is critical" -- I don't understand. The horizontal dial is always horizontal, the vertical dial is always at right angles to it, vertical, right? Or is the critical angle the angle of the gnomon?

A quick google on "diptych sundial" gave me these pages with pretty pictures of diptych sundials:

• http://www.metmuseum.org/toah/hd/kuns/hob_03.21.38.htm
• http://www.mathsyear2000.org/museum/floor2/gallery4/gal3p1.html
• http://www.liverpoolmuseums.org.uk/nof/sun/san6.html
• http://www.stedmunds.co.uk/lifestyle/manor/port-sun.html

I can clearly make out the "Greek axe" (pelekinon) at the top of most of these diptych. So there *is* something extra ... some sort of point (nodus shadow) to trace out that "Greek axe", not a line (gnomon shadow) to circle around the string attachment point. But I can't quite figure out what casts a shadow on that "Greek axe".

Determining direction...

It would seem to me that we can determine which direction north is only if we know the correct time. If we know which way north is, we can determine the time... but if either of those variables isn't pinned down we won't have a clue. Direction and Time are two sides of the same problem, and if one isn't defined we'll never figure out the other.

That would probably explain the compass on later versions of the diptych. I can't imagine adding one to a device that is already complete without it.

That having been said, I suggest we get rid of the comment about finding north, or at least ammend it to say "if the current time is known, by aligning the shadow to the current time, North can be found".

My guess is a traveller using a diptych would have to mark their heading against Polaris at night, orient themselves at dawn and sunset and pick a landmark, or ask locals which direction north was in order to use the time-keeping functionality of their diptych. That having been said, I intend to build one for myself and attempt to use it for a while.

I believe that it would have been general knowledge of how to find North as this may have been more important in earlier times when these diptyches were in use. For example in the 'build a sundial' workshop I run for young people I always point out:

Buildings: In Christian countries graves were dug on an East – West line with the Headstone west, this rule is not followed so much now, so look for older graves. Churches were also built on the same lines, with the alter at the East, and the main entrance West facing. This is but one specific example in the sheet of information they are given, there are four ways of finding North taken contemporary publications. Edmund Patrick ( ^confer _work) 19:09, 21 September 2007 (UTC)

Merge?

It seems to me that this page should either be only about folding thingies in general, or needs a separate section for the sundial. In either case, there ought to be as little redundancy as possible between sundial and here. --Belg4mit 00:09, 11 March 2007 (UTC)

Factual accuracy

I removed substantial text in the sundial portion which was patently false or misleading. A diptych consisting of a horizontal and vertical sundial will read the same time regardless of orientation, and I gave credible citations which say so. There do exist diptychs which are self aligning with the use of an analemmatic dial on the horizontal surface. These would still require knowledge of the date to adjust the gnomon on the analemmatic dial. The original image was not even a diptych in the sense of the sundial portion of the article, it was merely a portable horizontal sundial with a table of latitudes at the top. Regarding the text in the unsigned comments above: the dial at www.metmuseum.org would have had little gnomons placed in the empty holes above and in front of the dials to indicate date on the pelekinons. None of the links show-self aligning diptych dials.--Ron E (talk) 02:12, 20 January 2008 (UTC)

Diptych as an art term

Diptych is a standard term when describing artwork and it means a work made up of two equal parts. I see no reference or discussion of the word diptych in that context here. I think that needs to be mentioned near the top. Yellow Swans (talk) 19:22, 31 August 2009 (UTC)

added some more, though I would not agree at all that it is a standard term in modern art, despite some artists using it. Johnbod (talk)

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