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Sundial, calendar and Khmer temples

Determination of Vernal Equinox 2008
by use of gnomon and solar gates

เขาพระวิหาร

     This page describes in text and pictures the easiness of determining equinox day by observation of a gnomon shadow (sundial) and how to construct solar gates orientated true east-west.
     The Sundial and Solar-Gate Project is located in the Phu Phan Mountains in NE-Thailand and aims at by experimental astro-archaeology to find a method which ancient temple builders could have used for constructing temples and related structures straight true east-west.
     The area where the project is being implemented was once a part of ancient Kambuja and a majority of the Khmer temples were orientated straight east towards the rising sun at equinox. How were structures constructed towards the cardinal directions before the compass had become popular?

     In SE-Asia spring equinox is in the middle of the dry season. The rainy season ends in October and the months in the cold season from November to January are the best for naked eye astronomical observations. The months from February to April are in the warm season and normally with clear sky. In the heat of May the first clouds announce the coming rainy season.
     In 2008 the weather pattern was unusual in several respects; one was that the observations of sunrises and sunsets around equinox for the first time in the 10 years of sundial projects history were obscured by clouds and rain.
     If the methods were relying on observations of the sun at the horizon - like for example through temple corridors of Prasat Phanom Rung - the outcome would have been zero. But the use of 'equinox-pointers' and especially the sundial clearly evidence the day of equinox.

     On equinox day the shadow of the sun follows a nearly straight line (here called 'equinox-line'). On all other days it follows a curve. In the 6 months from autumnal equinox to vernal equinox the curve turns towards north; the other 6 towards south.
     In the days and weeks before equinox the afternoon shadow is approaching the equinox line, which on the sundial floor is marked by change of colour from yellow to white.
     14 days before equinox the shadow followed the black curved line (left photo). On the 18th the late afternoon shadow the indicated that there were only 2 days to equinox. The sundial floor was therefore repainted to 'clean the table' in preparation for equinox marking.    
Above: Afternoon shadow March 18   Above: Afternoon shadow March 19
Above: Sunrise 19 March in the eastern gate as seen from the western pointer. Above: Sunrise 19 March in the eastern gate as seen from the western pointer. Above: Sunrise 19 March
as seen from the sundial
During the night between the 18th and 19th it rained and clouds and morning fog obscured the rising sun until it became visible for a few minutes as shown on the pictures above.
     

The drawing (left) is a calculation of the sunrise the 19th of March 2008. The red line rises from the equinoctial point (true east) and has an angle from vertical identical to the latitude of the location = 90 - 16.676 = 73.324 degrees. The rising sun follows a path parallel to the red line. At equinox the sun rises behind the line!
     The 'pointers' in the gates on the pictures above and below have an identical angle, but are constructed by using the sun: At solar noon on a previous equinox day the pointer is fixed pointing directly at the sun (so that the pointer has no shadow in that moment). These pointers are here called equinox pointers, because an observer will by using 2 pointers see the sun rise or set at equinox behind the pointer.
     On the pictures above the sun rises right next to and south of the pointer, indicating that sun the next day will rise behind the pointer and that the next day is equinox day.
Accuracy in determination of true east: The diameter of the sun is app. 0.5 degree. The calculation shows that the distance from the line to the sun is app. 1/6 of the diameter or less than 0.1 degree.
0.1 deg. is an acceptable error-margin. Parts of the structures of Angkor Wat and Angkor Thom are more than 0.5 degrees of from true east, which is less acceptable. The ancient city, Shestrapura, at Wat Phu is apparently straight east, which is admirable.
     
Above: The 'equinox pointer' east of the gnomon. Under construction... Above: Another observer secretly observing the observer... Above: The equinox pointers of the eastern and western gates are some 70 m apart.

Above: On the 20th it was raining all morning. Only e few moments around solar noon the sun left the clouds to let a weak shadow announce that it was equinox.
     Solar noon or 'high noon' is when the shadow of the sun passes the line pointing true north from the gnomon. This line is also called the meridian and is on the photo above marked by a string. The meridian can be constructed by using the ancient method called the Indian Circle or the Vedic Circle. On the sundial here it is done by using Pythagoras on the equinox line.

Above: Sunset the 20th of March

Above: Calculated position of the pointer

 Above: The pointer has been extended.

Some 20 minutes before sunset the sun was visible for a few minutes. The picture above was taken 17 minutes before sunset and right before the sun disappeared. The sun had not reached the pointer, but it is evident that the sun is on the northern side of the pointer - and that we have passed equinox.
 

Above: On the 21st of March no sunrise or sunset were observed, but the shadow on the sundial confirms that equinox is passed. The 2 red marks were made on the 19th and the 21st. The shadow is now moving on the southern part of the sundial floor, the white part; the sun is revolting on the northern part of the hemisphere.

Important preparations for observations are:
 1. Check that the vertical and horizontal parts of the gates still are exact. For vertical lines a plump line is used. For horizontal water.
2. Check that the 'equinox pointers' still point at the sun at solar noon on equinox day having no shadow.
 

'Equinox pointer' (2007).		 End note: The first intuitive experiences of 'primitive man' on astronomy.

1. Determination of equinox day is very easy and the day becomes evident to the untrained observer within the first year of gnomon shadow reading: The path of the shadow is curved throughout the year - except at equinox, when it is (nearly straight).

2. Marking the path of the shadow on equinox day with a rope we have an E-W line. With plumb-lines we can extend the W-W line.

3. Having an E-W line; the N-S line is evident.

4. If the sundial floor has an eastern wall orientated N-S, then the equinox shadow continues on the wall - still straight - with an angle from horizontal resembling the path of the rising sun. This angle also resembles the angle of the sun at solar noon: When installing a rod pointing at the 'high noon' sun, so the rod has no shadow', then the equinox pointer is born.

NB: By 'primitive man' the author refers to himself: The first years of experiments and experiences were done without any previous literature studies. And the time before literature is pre-history.
 
   

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Last up-dated: 18  September 2008

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