basic procedure part 1

This page describes the basics of a bifilar sundial.
The description is limited to the horizontal implementation as first described by Hugo Michnik in 1923.

Draw the necessary hour lines as if on an equatorial sundial.
All hour lines are fifteen degrees apart.
Of course, half-hour lines could be drawn every 7.5 degrees, of even quarters of an hour, or minutes.
All this is easily done with compass and ruler, and no special constructions of calculations are needed.

Hour lines spaced at 15º intervals

The north-south wire should be over the north-south line (which is also the substyle) at a height g1 that may be chosen freely. The height of the east-west wire should then be g2 = g1. SIN(latitude).
If, for example, g1 = 20 mm and the sundial is intended for a latitude of 50 degrees, then g2 = 20 . SIN(50) = 15.32 mm.
But where should the east-west wire be?
It should be over a point that is on the north-south line, a distance y = g1 . COS(latitude) from centre C.
In our example, this would be y = 20 . COS(50) = 12.86 mm.

Position of the wires

In the Northern Hemisphere, the hour numbers run clockwise, in the Southern Hemisphere anti-clockwise.
If the hour lines and numerals are on a plate that can rotate about C, then any correction (for longitude or Equation of Time) may be applied by such rotation.
This is a great advantage of any sundial with equiangular hour lines.

The final result for a horizontal bifilar sundial on the Southern Hemisphere

Draw twenty-four hour lines, all spaced fifteen degrees. (Not shown in the figure)
Place a wire over the north-south line, at height g1.
Place a similar wire over the east-west line through C, but make the supports rotatable about the east-west line.
Adjust the angle between the supports and the horizontal plane to the desired latitude.
This will automatically place the east-west wire at a height g2 = g1 . SIN(latitude) and distance y = g1 . COS(latitude) from the hour line centre C.
Swinging the wire northward produces a sundial for the Northern Hemisphere; swinging it southward produces one for the Southern Hemisphere.

Universal bifilar sundial

Many other lines may be applied to a bifilar sundial, but that is outside the scope of this basic article. These lines are subject of a more extensive procedure. See basic procedure, part 2.

The bifilar sundial is not suitable for low latitudes.
As shown by the universal instrument, the east-west wire will be almost in the dial plane, and the dial would have no practical use.

Likewise, the bifilar dial is not attractive for use on either Pole.
Both wires will be at the same height, and might just as well be replaced with a single gnomon.