Sundial - Isan

Equinox path - at 16*40'25 North, 104*15'25 East

A path oriented straight east-west towards equinox sunrise and sunset

Experimental archaeology on how to construct a straight east-west oriented path
demonstrating equinoctial point without using compass - only using the sun.

 

     The sun rises nearly straight 90 degrees east on equinox mornings and sets the same days nearly straight west. The yearly deviation is small, but must be taken into account when remembered that the angle of the sun is 0,52 degree and that the width of the eastern doorway of e.g. That Phanom Rung is 0,87 deg. The sun does not rise straight up; it will move 0,4 deg. towards south in those few minutes it is visible in the 2.20 meter high doorframe.

     Next morning the sun will rise on a new position 0.38 deg. more northerly (at spring equinox).

 

Method

1) First determine equinox day on the sundial. At equinox the shadow of the sun follows a straight line east-west line.

Left:
     In February the shadow of the sun will follow a curved line throughout the day.
     The daily line will become less and less curved until equinox-day.
The daily path of the shadow is most curved at solstice and goes in a straight east-west line at equinox.

Right:
     23rd March 2000. Two days after passing the equinox-line. The path of the shadow is slightly curving. The straight line of coins in the middle is laid on equinox day. The the black line marks the path two weeks before/after equinox day - when the rays of the sun penetrates That Phanom Rung.

 
 

2) On equinox day: Mark a line oriented towards the rising sun.
    The process has to be repeated the following equinoxes in order to get an average.		   3) Raise interim gates: Make a levelled horizon.

     The horizontal baluster is levelled with water so the baluster in the other end has the same level
 

History

 


The equinox path from west

     The first attempt to make an alignment was made at vernal equinox 2000. The rising sun was not visible at the horizon at neither sunrise nor sunset, so only approximate orientation possible. The sun was first visible 3-4 degrees above horizon.

     The next attempts was made at autumnal equinox 2000 and at spring equinox 2001, but again the sun was obscured by clouds. At autumnal equinox 2001 the author was back in Denmark. Vernal equinox 2002 brought better luck and autumnal equinox again Denmark.
     With new gates vernal equinox 2003 (2546) will be exciting.
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Vernal equinox 2002 - March 21
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The 20th : After 2 years of waiting for the sun, it was finally observed at the mountain horizon.

     The blue line indicates ecliptic horizon ('ocean level', altitude = 0 deg.) and is leveled with the western gate.
     The red line indicates the path of the rising sun = latitude of location. Distance between gates = 80 meters.
     The white lines mark the contour of future gates: 140 cm = 1 deg.

     Azimuth at sunrise the 20th was 90*11'32'' (0*11'32'' after 90*00'00''), and azimuth at sunrise the 21st was 89*46'47'' (0*13'13'' before 90*00'00''), so a fictive 90*00'00'' degrees sunrise would be close to mid between the 2 sunrises.
     0*11’32’’ is approximately 1/6 of a degree, which seems to look like the distance from the left white pole to the red line. 

     Conclusion: The pole is oriented straight.
     Action: Construction of a new interim western gate in May 2002.

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 Left: Sunrise
Right:       New eastern gate
               - with local astronomical alignments:

a) Two vertical poles 1,40 m. between (1 degree).
b) Two horizontal sticks (not shown).
c) One equinox pole (16,35 degrees = local latitude)
d) One northern pointer (towards the polar star).

     The blue line determines the height of the two vertical poles. With the shown height the shadow of the right pole will pass the foot of the left pole at noon - at equinox. Calculate trigonometrically.

     
Sunset

The sun disappeared in clouds before reaching the tree-horizon (green stippled line). The black arrow is the projection of the setting sun continuing down with an angle of 17 degrees. It meets the (water-) horizon (on the western gate) around 10 cm from the bamboo stick marking test equinoctial point. The sun set at azimuth 279*00'59'', marked by the stippled orange line, which is the new centre-line and a new test equinoctial point. The western gate was torn down and re-build using aimingpoint-bamboostick as alignment for east-west (see left photo). The blue line is perpendicular to the black, rising 17 degrees from the horizon pointing exactly at the Northern Star. For construction is used 16,65 degrees = the angle of noon shadow = angle of setting and rising sun = local latitude on equinox days.

 

 

New western gate
raised before the rainy season, so the construction can set.
   
Analysing the photos with a professional drawing programme the results are more precise.

With differences between computed time and photo is so small as 0*03'28'' and 0*00'54'' the method seems reliable.

 

 

Vernal equinox 2002:
 2002/03/20 at 19:16 (U.T.), 
local time:  21 March at 02:16
Sunrise, March 20
Azimuth at horizon: 
Computed: 90*11'32''
On photo:  90*15'00''
Difference = 0*03'28''
6
6		 Sunset, March 20
Azimuth at horizon: 
Computed: 270*00'54''
On photo:  270*00'00''
Difference = 0*00'54''

 

Remarks and conclusions - October 2002:

     It is possible by simple means to construct long corridors with a straight east-west orientation. And it's simplicity indicates that man must have been able to do so from very early times.

     Taking into consideration that the Khmers in the Angkor period by Indian influence were capable of predicting equinox days, eclipses etc., constructing temples with a straight eastern orientation would have caused no difficulty. The orientation of Vedic alters is essential in Hindu religious practise, and Indian astronomical works as Surya Siddhanta (6.th century A.D.) give detailed instructions in determining the equinoctial point. So when That Phanom Rung (9-12th century A.D.) was orientated at 84,5 degrees instead of 90 degrees, then it must have been based on another concept that orienting the corridors after the rising sun on equinox morning.

     The angle of the noon-shadow (at equinox) = the latitude of the location!
     This angle (and it's perpendicular) could be interesting features in modern building construction: A roof could point at the Northern Star, another roof could be constructed, so that it would be in shadow from vernal equinox and six months on to autumnal equinox and tell equinox days too. A construction could be orientated, so that it acted as a calendar for those, who knows the shadows. See construction examples at Astronomical Alignments in Constructional Designs

...

 

Computations from 2000

Obstacles encountering the rising sun at the horizon let to computer animated photos with lines indicating sunrise/sunset. This method has also shown valuable when analysing photos made by a third person.

 

     The photo to the left was taken at the centreline and at horizon-level when the sun first became visible at equinox the 20th of March 2000 at 06:24:00 A.M.
     Using ASTRONOM (see link page) and putting in time and position of the site, one gets informed that the azimuth of the sun was 91*05'57'' degrees (right).
     Drawing a line from the position of the sun to the horizon using the angle of 16,6736111 degrees, which is the latitude of the location, one gets the point, where the sun would have been seen if the site was located by the ocean.
     The distance between the vertical lines is approximately 1 degree.

 

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Last updated: 24 November 2003