Sundial Park Genk
10. Large shadow plane sundial
Not many visitors will recognize a sundial in this quarter circle of seating elements. It must be one, though; why else would this be a Sundial Park?
In fact it is an example of a new sundial type, the shadow plane sundial. A very promising development, as it allows an endless variety of unusual, intriguing and surprising sundials. A challenge to artists, craftsmen and sundial buffs alike!
The sundial consists of seven triangular bluestone prisms. Parallel to each of the two shorter horizontal edges a copper strip is visible in the ground, labeled by an hour number in red. When the shadow of a particular edge just hits the corresponding strip, the hour strikes. At that moment, the sun just passes the imaginary plane through the edge and the strip, hence the name shadow plane sundial.
Each hour has its own shadow plane. The south-west edges serve the morning hours, from 6 to 12 hr local time. The south-east edges are for the afternoon hours, from 13 to 18 hr.
The blocks are 40 cm high (1.3 ft); excellent for tired strollers.
The sundial has been designed by Patric Oyen, Board member of the Flemish Sundial Society.
How to read the dial
The information panel lets the visitor guess how to read the time. An example may help.
The sun moves from east to west across the sky; the shadows move the other way. The 11 hr strip in the picture is catching sun already, so it is past 11 o'clock. The 12 hr strip is still shaded; it is not yet noon. The distance the shadow has moved past the 11 hr line is about half the distance the shadow has to move yet towards the 12 hr line. Hence, it is about 11:20 hr (local time). Simple, uh? Not really, but it is fun!
In the afternoon the strips are on the other sides of the blocks, so that the shadows go the other way round: at 14:30 hr the 14 hr line is already shaded and the 15 hr line still sun-lit.
The 12 and 13 hr lines are far apart, so that one has to march back and forth to see if it's time for lunch already. A bit of exercise doesn't harm, of course, but I'll give away a little trick. In fact, the side of the 12 hr block is itself in the 12 hr shadow plane. Note that the 12 hr line lies directly against the block. The same is true for the eastern side of the eastern-most block, although the copper line is omitted there. You can just use that side and the 13 hr line at the next block.
The shadow plane principle
Let's have a look at an armillary sphere such as nr. 1. When the sun shines, it causes a thin sheet of shadow, a 'shadow plane', behind the pole-style. That plane rotates at 15° per hour, as the sun moves across the sky. Each full hour one hour point on the hour band lies in this plane. The orientation of this plane is independent of the altitude of the sun, hence of the date; it only depends on the local time. Together, 12 full hour planes exist. One hour plane serves 6 and 18 hr, another 7 and 19 hr, etc.
Now imagine that these 12 hour planes are uncoupled from the pole-style and moved, without changing their orientation. Wherever the planes end up, at each hour the sun will juist lie in the corresponding hour plane.
How can one check in which hour plane the sun is just lying? You know that any plane is uniquely defined by (a) three points, (b) a point and a line, (c) two intersecting lines, or (d) two parallel lines. So the shadow of a line may hit a point, as in the armillary sphere (b), or it may hit another, parallel line, as in this dial (d), etc.
This is the principle of the shadow plane sundial. It will be clear that the freedom in arranging the hour planes opens up a vast area of creativity and engineering for new sundial designs. Great times are coming!
More on shadow plane sundials can be found in these articles by Fer de Vries, Mac Oglesby and Bill Maddux.
Sundials by Patric Oyen
Patric Oyen constructed yet another shadow-plane block sundial, after a design of Marinus Hagen (Netherlands). It can be seen in the Belgian Sundial Village of Rupelmonde. Here the blocks are rectangular, and their arrangement is mixed up a bit. The base plate is 35 cm square (a bit over 1 ft). This design is closely related to the dial in Genk, but quite another result! The same street corner has Oyen's bifilar dial. His polar bifilar dial can be found elsewhere in Rupelmonde, flanked by an intriguing monofilar dial, which has straight hour and date lines (picture below).
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