Sundial - Isan

The construction

 

The row in the middle marks the position of the shadow of the sun at 12 o'clock from winter solstice to May: The beginning of an analem.

In school I was thought that the sun will be in straight south at 12-o'clock noon. A truth with slight modifications. I had expected the shadow of the sun to follow a straight north-south line at noon, but as shown on the photo to the right, the marks followed an S-shape. The coins mark the position of the shadow of the sun at full hours. The curve in the middle marks the position of the sun at 12-o'clock noon on  various days from winter-solstice to mid May.

Continuing until November next year the s-shape turned into an figure eight-shape.

November the 9th. at 12:07
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At equinox the shadow of the sun follows a straight east-west line (the equinox-path). Next was to draw a north-southern line 90 degrees to the equinox-path.
On the north-south line I constructed a 'compass-rose', in which I put my compass (see photos above). The compass deviated 1-2 degree from the 'compass-rose', which seemed like a satisfying error-margin. Two years later I found out, that the deviation not was an error, but the difference between geographic and magnetic north.

In November I marked the time-lines for a week and left the site puzzled about how the shape would turn out. Around winter solstice I was back again (the full-drawn line) for some weeks. The shape now became evident. Before leaving in February it seemed like the half-hour lines overlapped one another one to two minutes. On the Net I later found out the form is called an analemma. 

February 2nd at 10:30 The gnomon February 2nd at 12:07
     
Continued analemma marking

The analemma marking continued in vacant periods in 1999, 2000 and 2001 until a double lining of the analemmas  made it clear that the construction of the terrace, where the sundial is drawn on, was unstable. In 2001 a termite attack on the base of the gnomon made the this unstable too. All marking were then suspended.
In March 2002 the construction was re-enforced and the gnomon 'put back on time'. Whether this has been successfully done time will soon tell.
The gnomon might not be as vertical as before. Zenith-day in May will tell. But it makes no difference whether vertical or not, because new simple tools to determine zenith-day are under construction.
Drawing of day-lines as well as analemma lines were started up again primo April 2002 and will hopefully be finished for final painting around autumnal equinox 2003.

The analemma figures are of no significance for the studies of cardinal orientation of Khmer structures. For cardinal orientation I only need a few days before and after equinox to draw day-lines of the shadow of the sun. I only need a straight E-W and a N-S alignment.

 

Sunset at autumn equinox. 
Right under the sun there is a small spear marking spring equinox.

 

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Last updated: 24 November 2003