diptych

A picture consisting of two panels, hinged like the pages of a book. Small portable devotional pictures and altarpieces sometimes took this form in the late Middle Ages.

A diptych is any object with two flat plates attached at a hinge.

Note: This article discusses diptyches in the first sense.

Traditional diptychs are boxwood, with stamped hour lines and lacquered or varnished finishes.

One form of diptych was like a shallow box.

The other form was a portable sundial.

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. 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; 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. Holding a diptych so that its 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.

Some diptychs had compass roses (to measure bearings to geographic features) and latitude measurement bobs. Diptychs may thereby have come to acquire an air of magic in the ancient popular mind. 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.