Diptych
From Free net encyclopedia
Current revision
Image:Diptych Areobindus Louvre OA9525.jpgA diptych is any object with two flat plates attached at a hinge. Devices of this form were quite popular in the ancient world, types existing for recording notes and for measuring time and direction. The term is also used figuratively for a thematically-linked sequence of two books.
Note: This article discusses diptyches in the first sense. For paintings arranged in such a way, see polyptych.
Traditional diptychs are boxwood, with stamped hour lines and lacquered or varnished finishes. Some were also ivory (superior because it is easiest to read and less prone to wear than wood), or metal (sturdy, harder to read but less expensive than ivory).
One form of diptych was like a shallow box. It had two wooden leaves with hollows on the inside edges, filled with wax, and space for a small wooden scriber. This permitted one to take waterproof notes in the wax without wasting money on paper. The wax could be smoothed and reused. It was probably excellent for shopping lists or other reminders.
The other form was a portable sundial. A face was on the inside of each leaf. One leaf formed a vertical sundial, the other a horizontal sundial. The shadow caster, or gnomon was a string between them, and calibrated how far open they should go (the angle is critical).
A sundial can be adjusted to any latitude by tilting it so its gnomon is parallel to the Earth's axis of rotation. However, the longitude is critical for an accurate local solar time, and is corrected by leveling the diptych on its axis from east to west.
If the hinge of the diptych is level with the ground (classically measured with a rolling marble in a slot), and both dials show the same time, the dials will show the apparent solar time, the hinge faces north (in the northern hemisphere), and the gnomon is parallel with the axis of rotation of the Earth.
Image:Sundial pocket watch.JPGAchieving all these functions is almost a lost art. A north-indicating diptych is possible only if the two sundials do not have the same complementary sun angle. The best real diptychs never consisted of two mirror-imaged 45 degree sundials; usually they were adjusted so that at the owner's latitude, the bottom leaf was level not just east-to-west, but north-to-south. That is, if the gnomon is not parallel to the earth's rotational axis, then since the two faces have different trigonometric projections, they will show different times. For example, if the gnomon deviates from the correct elevation angle at 9am or 3pm, each degree of error in the gnomon's elevation creates a difference of four minutes (one degree of angle) in the time readings of the two faces. However, at 6am, 6pm and noon, a deviation in the gnomon's elevation angle produces no change in times. Near noon, if the gnomon deviates from pointing north and south, the times of the two faces will deviate. At 6am and 6pm, deviations from north and south have no effect. Holding a diptych so that it's gnomon-string is at the correct angle is often finicky, especially near sunrise, sunset and noon, so many later diptychs had magnetic compasses and plumb-bobs to help, but these were luxuries, not necessities.
Some diptychs also had rough calendars, in the form of pelikinons calibrated to a nodus in the form of a bead or knot on the string. These are accurate to about a week: Good enough to time planting of crops, but not as accurate as a well-kept calendar.
Some diptychs had compass roses (to measure bearings to geographic features) and latitude measurement bobs. Some authorities believe that large versions (a meter or more in width) were used for maritime navigation before magnetic compasses were well-known. Diptychs may thereby have come to acquire an air of magic in the ancient popular mind.
Of course, all these functions could be combined in one pocket-sized artifact. Diptychs that combined writing and timekeeping often have a slot on one leaf to hold the gnomon. The gnomon can be detached from that end so the diptych can be opened completely for writing. On these the gnomon often has two knots, one for timekeeping and the other to latch the diptych shut and protect the wax. The "decorative" bead often seen on the end of extra-long gnomon cords may have been rolled in a slot, or dangled as a plumb-bob to determine if the diptych's hinge was level, or to measure latitudes.
It could be a very convenient thing to keep in one's pocket even in the current era, particularly in an area with few well-developed roads. Once a template is made for a current latitude, construction from nearly any available sturdy materials would be trivial.
External links
- Diptych The Catholic Encyclopedia, Volume V, Robert Appleton Company, Online Edition.
- Diptych sundials, National Maritime Museum.
cs:Diptych de:Diptychon es:Díptico fr:Diptyque it:Dittico pl:Dyptyk (sztuka sakralna)