The common horizontal bifilar sundial has as shadow casters an east-west and a north-south wire. The north-south wire is higher than the east-west one.
The time is read where the shadows of the wires cross.
If the heights of the two wires exhibit a special ratio, the hour line spacing becomes equiangular at 15 degrees.
In that case, longitude and equation-of-time adjustments are possible by rotating the surface of hour lines.
Here, therefore, a universal time readout is possible.
By fixing the east-west wire to two movable supports, one can adjust the sundial for latitude.
The angle which these supports make with the horizontal equals the latitude.
This makes the sundial universal with respect to its location as well.
See also: this Internet site, under
– Calculate and construct
– Bifilar sundials/basic constructions
– Basic procedure part 1
Fer de Vries
Sundial by Sergio Garcia Doret, Brazil.
Last Modified 03/07/2016 08:20:18
English translation: RH