Pondorosi

 

I planned to construct a giant horizontal sundial in front of our house overlooking the Orosi Valley in Costa Rica, as a tribute to the ancient Meso-Americans' religious fascination with the sun.  And what better time to do it than 2012?

 

But it turns out that the garden sundial design so familiar in the States will not work in the tropics for the whole year.  At less than ten degrees of latitude north of the equator, the sun is to the south of Orosi most of the year, casting the gnomon's shadow to the north across a range of hours laid out on the dial. But from April 14 through August 27, the sun moves to the north of us and warms the United States and the northern hemisphere.
 
I searched for sample images of horizontal sundials in the tropics, reasoning that this problem had been solved centuries ago.  And indeed it was, but there were no pictures to illustrate.  Sundials are apparently very scarce in the tropics, which made me all the more interested. With the help of some French software and the generous advice of America’s foremost sundial designer, I came up with a design which has been laid out in red and gray brick in a bed of crushed stone.
 
The pattern consists of a curved line (hyperbola) tracing the path of the shadow of the gnomon’s tip at the winter solstice, a second curved line for the summer solstice, and a straight east-west line in between marking the two equinoxes. These three lines are crossed by nine straight hour lines identifying 8am through 4pm that would all converge at a point over the hill to the south if extended far enough. To simplify construction, the hyperbolas representing each solstice were not laid out in brick but can be visualized by connecting the tips of the hour lines. The lines of the dial connect carefully measured points determined by the software along an x-y axis aligned to true north.  Gnomon placement was calculated to account for the latitude of the site and its east-west location within the time zone, so it approximates Central Standard clock time (we have no daylight savings time here).
 
The Greeks of 400 BCE called this pattern a pelekinon (derived from the Greek word for double-edged axe).  I have referred to it as a misshapen bow tie. The sundial is considered to be the first scientific instrument. It makes it possible to visualize our four-dimensional world, the three physical dimensions plus time.
 
What was I to use as a gnomon, the post in the ground casting the shadow?  Egyptians commonly used obelisks, and of course there is no limit to the designs which can adequately cast a shadow.  But since this was to be a tribute to the Meso-Americans, I chose an eight-inch replica of the mysterious Costa Rican stone spheres, somehow created 2000 years ago by an unknown indigenous people, mounted on top of a vertical steel pipe.  The spheres, now known as Las Bolas, are thought by some to have been placed to represent heavenly bodies such as the sun and the planets.  Unfortunately, in 1930 they were discovered, dynamited and moved by workers from the Chiquita Banana Co. to make room for a new plantation south of here.  
 
I was quite fortunate to find Ricardo, a local artisan who meticulously carved bolas by hand out of gabbro, a rock formed from molten magma.  I was also fortunate to find Ali, a maestro albañil (master bricklayer) who executed my reloj de sol (sun clock) design impressively, once he suspended disbelief.
 
I have linked a slideshow of the photographs I took at each stage of the construction process, the design, and the dimensions in Excel. The files should be downloaded for better resolution. Free Shadows software was utilized.

 

​

Addendum: In Aug 2012, The sundial was  briefed at the North American Sundial Society annual meeting as one of the interesting new dials registered in 2012. Briefing linked below.

 

 

CLICK OROSI SUNDIAL PROJECT FOR SLIDESHOW: