Article of the month December
Bi-gnomon sundial
Sundial for clock time
An idea of Hendrik Hollander
The sundial below is based on two articles* published in 2005.
This dial incorporates, in a special way, both longitude correction and the equation of time.
Thus, it shows clock time directly.
There are two slanting gnomons, intersecting in a point over the sundial face.
One is used during the half year of increasing daylight (approximately 21 Dec to 21 June) and the other during the half year of decreasing daylight (approximately 21 June to 21 Dec).
There are date lines in the pattern. The figure shows them according to the zodiacal signs, so that just seven lines are needed; more can be added, of course.
To read the sundial, one must know the date.
The time is read at the intersection of the current date line and the shadow of the gnomon currently in use.
When DST is in force, one hour should be added. Alternatively, the hour lines could have a double numbering.
The photo shows a preliminary paper model of this marvelous sundial.
A more practical implementation of this sundial would use a single gnomon, pivoting around the intersection of the two described earlier.
One would place the gnomon in its correct position at the beginning of summer and of winter.
Actually, the gnomon should be slightly longer, projecting past the intersection.
The paper model does not show this yet.
In this example, the following dimensions apply:
Let the distance between the foot points (marked with +) be 2 units; then the gnomon intersection is 0.5 unit above the sundial face, and 0.5 unit north of the connecting line between the foot points.