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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.







... a Moondial.

Here in the tropics, especially as the September 22 equinox nears, we have about twelve hours of daylight and twelve hours of darkness. Monday, September 8, 2014 was a rare night in that the full moon:


  • Rose exactly as the sun set (this is to be expected as the moon must be 180 degrees away, or on the opposite side of the earth, from the sun when full), 

  • Peaked shortly afterward (between 9 and 10pm it was at its fullest, which doesn't always happen at night),

  • Occurred within the period of the supermoon (when the moon is in the perigee of its elliptical orbit it is closer to the earth and therefore bigger and brighter),

  • Was the last one before the equinox, making it this year's harvest moon, when there is spectacularly no period of darkness between sunset and moon rise (ideal for harvesting),

  • Was extremely visible over the Pondorosi on a rare cloudless night,

  • Cast a shadow on the sundial just like the sun does, and

  • Remarkably, the time shown on the dial was correct!


Remarkable to me, because I had never been able to closely observe the dial's time during a full moon before, but really not remarkable at all when you figure that the moon is following roughly the same path over the earth as the sun, but exactly 12 hours behind (or ahead) when full. I checked it on Tuesday night and it was about a half hour slower than Monday as the moon was continuing its monthly cycle around the earth. On average, the moon shadow would be 48 minutes slower every night, but because it is at lunar perigee the moon revolves more slowly in this segment of its orbit due to the earth's gravitational pull being strongest when the moon is closest.

Here is a doctored and enhanced image of what it would look like Monday night at the peak of the full super harvest moon.


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