Sundial Park Genk

4. Polyhedron sundial

South and east side, and base plate

A powerful educational example: all basic types of pole-style dials integrated into one object! The four walls carry vertical dials, the 'roof' has a polar and an equatorial dial, and the base plate is provided with a horizontal dial. They read Central European Time (CET), or if relevant, Summertime (= East European Time, EET). Finally, a graph of the Equation of Time is provided. Very nice!
The bottom part is a cube of 1.0 m edge (over 3 ft). Unfortunately, the dark granite does not show of very well in pictures; a good excuse for a physical visit...

North and west side, after rain

The relationship between the vertical south dial and the horizontal dial is visualized by the shared pole-style. The equatorial dial also has a bronze style, and the four others have a bronze plate serving as gnomon. The gnomons of the east, west and polar dials have a V-shaped notch, for indicating the date. No date lines have been cut, except for the equinox line. Instead, the hour lines extend to the imaginary curves for the solstices: very elegant!

The table lists the details of the seven dials.
CET = Central European Time, EET = Eastern European Time, LT = Local Time.

Face Sundial type Range and hour type
south side vertical south 7 - 18 hr CET
east side vertical east 4 - 11 hr CET
north side vertical north 6-9 and 19-21 hr EET
west side vertical west 14 - 22 hr CET
top, south slope polar 9 - 18 hr EET
top, north slope equatorial 4 - 20 hr LT
base plate horizontal 6 - 20 hr CET

The sundial has been designed by Willy Ory, Board member and webmaster of the Flemish Sundial Society, after a concept from Ignace Naudts (Belgium), and executed by the sculptor Anja Roemer (Netherlands).
Ignace Naudts (1949-1992) was also the man who convinced Johan Gijsenbergs, then director of the nearby Euro-planetarium, to pursue the creation of a sundial park.

My comments:

Hour types

In the time system of 24 equal hours per day, three types of hours should be distinguished:

  1. Local time, also called local apparent time or solar time. When the sun is due south, it is 12:00 hr local time. Here LT is used with the equatorial dial on the north slope on top.
  2. Local time for the longitude corresponding to the local time zone (in Belgium, 15° E). The hour lines are shifted by the longitude correction (for Genk 9.5°, corresponding to 38 minutes of time). This type of hours is used here with the south, east, west and horizontal dials. An extra hour may be added for summertime (daylight saving time), as used here with the vertical north and the polar dial.
  3. Standard or civil time (in Belgium, Central European Time), which is the mean time for 15° E longitude. Or one hour extra in summer.
    Sundials reading this type of hours have figure-8 shaped hour lines, or a mechanism to provide the date-dependent correction for the Equation of Time (EoT).
For the first and last types, the abbreviations LT and CET are used. Unfortunately, no common term for type 2 exists. In the Bulletin of the Dutch Sundial Society, CET* is sometimes used, to be pronounced as 'CET without EoT correction'.

Equatorial dial

This is the simplest type of pole-style dial, with regard to both its principle and its construction. A rod perpendicular to the dial face, which is oriented parallel to the equatorial plane. Hour lines evenly spaced at 15° intervals. Date lines, if present, would be circles.

The dial face is only illuminated during the summer half-year. A complete equatorial dial would require a second dial face at the bottom side. Reading the time there might require som gymnastics. This can be avoided by moving the bottom face up, as Marten Hugenholtz (Netherlands) did in a design for the NAM headquarters in Assen (Netherlands).

Another dial type that is also called 'equatorial' is the armillary sphere. This type should better be called 'equatorial disk dial'; see this page. In my view, these two types are not closely related. In a sensible classification of pole-style sundials, the first sub-division might be:

Vertical south dial and horizontal dial

The common pole-style of the vertical south and the horizontal dial illustrates the relationship between the two. Stepping around the object, one can see that the pole-style always covers the two corresponding hour lines simultaneously. In doing so, one may also discover why a vertical dial can show only 12 hours at most, but a horizontal dial can read the time at all daylight hours.
One can also learn how to distinguish a horizontal from a vertical dial when finding one in a garden center or antique shop: the hour numbers run clockwise or counterclockwise (trips to the Southern Hemisphere excluded).
Using a sheet of paper, one can also understand how the hour lines come out on any plane, flat or curved.

A combination of a vertical south and a horizontal dial with a common pole-style is called a diptych dial. These were once popular as folding travelling dial, with a built-in compass. The unfolding of an enlarged version formed the opening ceremony of the Sundial Park on 19 March 2000.
Mayor Jef Gabriels of the City of Genk (at the right) and TV-weatherman Frank Deboosere, godfather of the project, open the diptych. Johan Gijsenbergs (to the left of Deboosere) lends a helping hand.

Official opening of the Sundial Park

Polar dial and vertical east and west dials

These three dials are closely related. Even their hour line patterns are identical. Only the hour numbers are shifted. If they all would read the same hour type, the shift would be 6 hours from east to polar and from polar to west dial. In this case, the polar dial reads EET* and the vertical dials CET*, so that the shift between east and polar dial is 7 hours and between polar and west dial 5 hours.

The east dial has hour lines from 4 hr onwards and the west dial has lines up to 22 hr. That is correct, as the earliest sunrise is at about 4:20 hr and the last sunset at about 21:00 hr. In practice, however, the sun will not reach this polyhedron in the early and late hours. The park is located somewhat in a valley, as is evident from the small stream that runs close by. Moreover, the park rightly is so named, as it has beautiful high-rising trees.

Vertical north dial

The vertical north dial is closely related to the vertical south dial. If their dial faces - now one meter apart - would be joined, they would have several hour lines in common, and other lines would line up. There would be one pole-style penetrating the dial face.

Which hours to include on a north-facing dial can be read from the following graph. It depicts the sun's path across the sky throughout the year, for the latitude of Genk. Only the morning hours are depicted, for reasons of graphical resolution. At the summer equinox the sun is north of east from dawn (just before 4:00 hr local time) to about 7:20 hr, or from 5:20 to almost 9:00 hr EET*. Hence, hour lines from 5 to 9 hr would be appropriate. The 5 hr line is probably omitted because of the trees around.

Solar trajectories for 51° N

The same graph answers a sundialing riddle: On which date sees a south-facing vertical dial the longest daylight? Perhaps counter-intuitively, the answer is: at the equinoxes. In the summer half-year the sun shines longer, but it spends less time south of the east-west plane: hardly more than 9 hours at the summer solstice at this latitude.

The Equation of Time

EoT graph The Equation of Time graph on the south face shows the difference between local time and mean time in the course of the year. Two definitions of the EoT exist, the one the inverse of the other. Fortunately, the definition used here is specified on the various dial faces: "-E = CET" or "-E = EET". More about the EoT at this page.

As the information panel explains, E, read from the graph, is the correction to convert local time into civil time. What a shame!!
It would have made sense, though, to combine the longitude correction and the EoT into the graph, by adding 38 min to the vertical scale. That would also have eased the sign problem.

Anja Roemer is the name... Poor Anja

On the information panel as well as in the brochure, the name of the sculptor is given as Anja Römer. In the new folder this has been 'corrected' into Anja Romers. Her own spelling is clearly readable from the base plate...

Other polyhedron dials

Cubical sundial A common type of polyhedron dial is a cube with 5 dials. A nice example is found in the Sundial Village of Rupelmonde (Belgium). It is located in Mercator Square, in front of the statue of this great geographer and cartographer. In fact the cube has beveled corners, which actually makes it a 14-sided polyhedron. Eight more dials could have been installed!

Ten-sided polyhedron in Celle Other polyhedrons may have even more faces. Subtracting the bottom face for obvious reasons, and sometimes the top face for some form of decoration, this leaves lots of faces for sundials. A craftman's challenge! These are examples of a 10-sided polyhedron with 8 dials in Celle (Germany), in which not all gnomons are present anymore; an 18-sided, recently restored specimen in Bad Münstereifel (Germany) with 17 dials, and (my favorite!) a 26-sided stone in Horn (Netherlands).

18-sided polyhedron in Bad Münstereifel 26-sided polyhedron in Horn

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