Example of a bifilar sundial
Source: Die Sonnenuhr, René R.J. Rohr
Hugo Michnik of Germany invented the bifilar sundial in its horizontal form in 1923.
His idea was the first really new sundial type since centuries.
Instead of a pole style or other single gnomon, two crossing wires are used.
One wire runs north-south, the other east-west.
They are parallel to the dial plate but have differing heights.
At a certain height ratio, all hour lines come out equally spaced, fifteen degrees apart, like on an equatorial sundial.
The sundial is read where the shadows of the wires cross.
A procedure for this simplest form is described here.
A bifilar sundial could also be called a two-wire dial or a cross-wire dial, but instead of wires, the edges of planes could be used.
For a long time after its invention, not much was published about the bifilar sundial. About 1980 however, this dial began to attract interest again, and it was developed further. One example, the use of curved wires, is shown below.
English translation: RH