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).

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.