Sundial, calendar and Khmer temples

     The article below was published in June 2007 in the Muang Boran Journal: English text,
which also has a Thai translation:
สุริยัน จันทรา และราหูที่ปราสาทพนมรุ้ง: ข้อสังเกตทางโบราณดาราศาสตร์


The Sun, the Moon, and Rahu at Prasat Phanom Rung

Solar-lunar events at Prasat Phanom Rung in Spring 2007 AD (2550 BE)

Notes on archaeo-astronomy

Test, photos, and illustrations Asger Mollerup

Photo above: The setting sun seen through all 15 gates of Prasat Phanom Rung, 7 March 2000.

Archaeo-astronomy. Aligned sunrises/sunsets. Naksatra. Total lunar eclipse. Sidereal lunar month. Synodic lunar month. Draconic lunar month. Lunar periods.

     Four annual solar events can be observed at Prasat Phanom Rung: Two in the spring and two in the autumn. The two events in spring consist of a sunrise and a sunset visible through all 15 doorways of the ancient Hindu sanctuary.
     The solar events are actually solar-lunar events: The period between the two events in spring (and autumn as well) is approximately 28 days, which is close to a sidereal lunar month (the period it takes for the moon to return to the same point on the celestial sphere).
     The orientation of Prasat Phanom Rung also depicts another kind of lunar period: A draconic lunar month: Knowledge of draconic months is essential for predicting solar and lunar eclipses. Around the sun rise in September 2006 there was a lunar eclipse (partial). And around the coming sunset in March 2007 there will be a second lunar eclipse, this time total. In the autumn 2007 a second total lunar eclipse will occur.
     The 15 doorways of the ancient Shiva temple can be used as an astronomical device called a dioptra. We do not know whether the contemporary Khmer astronomers/astrologers actually did so, but a trained astronomer living at the sanctuary would certainly have noted the correlation between the orientation of the doorways of the temple and the described aspects of astronomy.

     Prasat Phanom Rung is publicised as an ancient Khmer temple that allows the rays of the sun to penetrate the 15 doorways of the sanctuary illuminating the linga in the central sanctuary.
     Initial focus was only on the sunrise in April. The author's observations of the sunset in March 2000 and calculations of two similar events in the autumn were published in the Bangkok Post in March 2000, and in Silapa Watthanatham in 2003. Since 2004 Prasat Phanom Rung has been promoted for its four annual solar events.
     Unnoticed by the public is that the solar events also have lunar aspects and accordingly should be termed solar-lunar events: When the sun sets straight through the doorways of Prasat Phanom Rung, wait until it is dark and note where the moon is located on the celestial sphere (near what star or in what zodiacal sign). Four weeks later the sun will rise and be visible straight through the doorways of the temple and in the hours before sunrise the moon will be located at the same spot again.
     A third aspect is eclipses of the sun and the moon. In this case the intervals between the events ca be 14 days, 28 days, 6 months, 19 years etc. The coming total lunar eclipses in 2007 will illustrate the eclipse aspect: The night before the sun rose visible through the doorways of Prasat Phanom Rung in September 2006 a partial lunar eclipse was observed. Six lunar months later, at the coming sunset in March, the moon will be totally eclipsed. The lunar eclipse will happen in the hours before sunrise and surely be announced in radio and TV.
  Above: Eastern facade of Prasat Phanom Rung  

Prasat Phanom Rung
     Prasat Phanom Rung is an ancient Khmer temple dedicated the Hindu god Shiva and is located on the top of an extinct volcano in Buriram Province, NE-Thailand. The location above the surrounding plains gives a unique view of the surrounding plains and of the celestial sphere as well.

     Archaeo-astronomy is a relatively new scientific discipline. The term archaeo-astronomy is a combination of archaeology and astronomy, and is an interdisciplinary research being taught at university level only at the University of Leicester, England.
     The author of this paper is autodidact. He has conducted GPS-based (1) field research on ancient Khmer temples since 1996, having hitherto visited some 240 temples. His research also includes experimental astronomy. On his test-site in the Phu Phan Mountains in Mukdahan Province in NE-Thailand he has constructed a solar calendar and as a second step constructed cardinal orientations only using the sun.

Location, orientation, and construction date are the essential data for archaeoastronomical calculations at a given site.
     Location: The coordinates of the eastern gate of Prasat Phanom Rung is 14.53198 degrees North and 102.94086 East (2).
     The orientation of the 15 doorways of Prasat Phanom Rung is 84.5 degrees from celestial north (3), as determined by observation of the sunrise visible through the temple on 7 March 2000.
     The exact date of construction is not known. The central tower, the galleries and the 15 doorways date to the 12th century. The oldest construction at Phanom Rung is the ruined brick towers inside the surrounding galleries, which have the same orientation as the later additions. The dating of the brick towers is based on the type of door columns and assumed to date to the beginning of the 10th century. Presumed previous constructions made of perishable materials such as wood have not been found. The unique mountaintop location suggests that the site was likely used for religious ceremonies before the brick towers were built. The author has therefore conducted calculations on astronomically events such as eclipses from the 7th century on.

Right: Columns and door-frames of the 10th century brick towers


The solar events

The solar events at Prasat Phanom Rung as a rule of thumb occur 14 days before and 14 days after the equinoxes (4). At each solar event the sun will be visible through all 15 doorways over 3 succeeding days of which the day in the middle is most straight.

Sunsets in March 2007 will occur 6th, 7th and 8th.

Sunrises in April 2007 will occur the 3rd, 4th and 5th.

The moments when the sun will be aligned with the structure of the temple is illustrated on fig. 1 and 2.



Left: Fig. 1. Sunset 7 March

Right: Fig. 2. Sunrise 4 April

The lunar events

     Astronomically there are five different types of lunar months of which three are of interest in relation to the solar-lunar events at Prasat Phanom Rung:
     1 Synodic month of 29.5 days (5) is easily observable and known to everyone as the period from one full moon to the next full moon.
     2 Sidereal month of 27.3 days (6) is the period it takes for the moon to return to the same spot on the celestial sphere.
     The moon changes position every night as compared with the fixed stars and after one sidereal month returns to the same position in one of the 12 zodiacal signs (figs. 3 and 4 below). The zodiacal signs are related to the apparent annual movement of the sun round the Earth. The ancient Khmer sages would most likely have paid attention to the 27 lunar houses or asterisms, the naksatras (7). The Indian concept of naksatras was used by the ancient Khmers for calendric as well as astrological purposes. The ecliptic was divided into 27 equal parts of 13 degrees and 20 minutes of arc.
     The Moon and almost all the planets (excepting Pluto) keep within a belt eight degrees wide on either side of the ecliptic. This belt is known as the zodiac, or rasicakra. To indicate the day-to-day position of the Moon in relation to the stars, this belt was divided into 27 equal naksatra divisions from a fixed initial point in the ecliptic. Each naksatra division (8) is named after an identifying star known as yogataras. The yogataras of the naksatra named Citra is Spica-ά Virginis. (Sen, p. 274).
     The Vedic legend (9), Satapatha Brahmana, states that the 27 naksatras are the 27 wives of Chandra, the Moon, and that he spends one night with each naksatra throughout the month. After sunset 7 March Chandra will be hosted by Citra and 27.2 days later, 4 April, Chandra will have returned to Citra. (See figs 3 and 4).
Fig. 3: 7 March 2007 (late evening)   Fig. 4: 4 April 2007 (before dusk)
Above: The sidereal month around spring equinox 2007 is illustrated by the moon in the naksatra named Citra.


     3 Draconic month of 27.2 days (10) is a third feature related to the orientation of Prasat Phanom Rung. The draconic month is vital in calculations of eclipses. It is defined as the period after which the moon returns to the same node of its orbit. The nodes are the invisible points on the celestial sphere where the moon passes the ecliptic. An eclipse can only occur when the moon is close to one of its nodes. If the moon comes from below the ecliptic, the node is the ascending node, and if the moon comes from above the ecliptic, the node is the descending node (11).
     The draconic month is 'embedded' in the orientation of Prasat Phanom Rung and is illustrated by the partial full moon in September 2006 and the coming full moon in March 2007.
     App. four hours before sunrise 8 September 2006 the partial lunar eclipse was at its maximum. At sunrise the sun rose visible through all 15 doorways of the sanctuary. Ten minutes later the moon set and would have been visible through all 15 door-ways if not obscured by clouds.
     The 9th of September the sun rose aligned with the structure of the sanctuary.

The partial lunar eclipse 8 September 2006 at its maximum and the beginning of the total lunar eclipse 4 March 2007 - both observed from Phanom Rung.

     Six full moons after the partial lunar eclipse there will be a total lunar eclipse in the hours before sunrise, 4 March 2007. At 05:05 (1 hour before sunrise) the moon will be nearly eclipsed. The eclipse is total at 06:21 and the moon will set some five minutes later at sunrise. The sun and the moon will be observable directly opposite one another.
     At sunset the same day the rays of the setting sun will illuminate the linga in the central sanctuary and 20 minutes after sunset the linga will be illuminated by the rising (nearly) full moon. Neither the sun nor the moon will be observable through all 15 door-ways. The day when the sun sets aligned with all 15 doorways is the 3 days later.

     This '6 months full-moon eclipse interval' was well known to ancient astronomers: ''The insight that the Babylonians had at an early date is that observed lunar eclipses are separated either by 6 synodic months, or by one less than multiples of 6 synodic months (reflecting an occasional 5-month interval)'' (Goldstein, p. 2).
     Based on recorded observations 2500 years ago the Babylonians also discovered that an eclipse will be repeated after 18 years and 10.33 days. They called it Saros, meaning 'repetition'.
     Eclipses close to the annual solar events at Prasat Phanom Rung come in 'bunches' with a frequency of 8-10 years: The 2006/07 'bunch' consist of the two above described lunar eclipses
(12). The next 'bunch' will consist of a total lunar eclipse on 8 October 2014 and six full moons later total lunar eclipse, 4 April 2015. Next again are the total lunar eclipses 8 September 2025 and 3 March 2026, also separated by 6 full moons.

Was Prasat Phanom Rung consecrated coinciding with a solar eclipse?
     The author has often been asked if Prasat Phanom Rung could have been consecrated coinciding with a solar eclipse on a day when the sun rose aligned with the structure of the temple. If the question implicates a total solar eclipse, the answer is: No! Total solar eclipses are much rarer than total lunar eclipses. From 700 AD to 931 AD
(13) only one total eclipse was visible from Prasat Phanom Rung: 23 March 768, and did not coincide with the day when the sun rose illuminating the deity inside the central brick tower.
     18 full moons months later, 5 September 769, the sun rose partially eclipsed (6%) and nearly aligned with the structure.
In the 8th century AD we only have two candidates
(14): Both partial solar eclipses and both one day after the sun rose aligned with the structure.
     None of the above described solar eclipses seem to be significant enough to connect the consecration of Prasat Phanom Rung to a past solar eclipse.
     Extending the above question to include lunar eclipses, the conclusion is negative as well.
     Even lunar eclipses are more frequently observed, none of the eclipses in the 8th and 9th seem significant enough to connect the consecration of Prasat Phanom Rung to one of them.
     The orientation might rather be connected to all those, who appear in a '6 months full-moon eclipse interval' as described above.

Rahu and Ketu
     In Khmer mythology the nodes of the moon were personalized as Rahu (the ascending node) and Ketu (the descending node). Rahu was an asuras
(15) who, during the Churning of the Ocean of Milk, tried to steal some of the elixir of immortality, the amrita, which made the gods immortal. The Sun and the Moon noted the theft and alerted Vishnu, who immediately decapitated Rahu with his cakra (16). But too late; Rahu already had amrita in his mouth and his head became immortal. Rahu has since then revengefully tried to swallow the Sun and the Moon, which we see as eclipses.
Above: Rahu and Ketu, India.
11th century AD. Rahu holds the moon.
Ketu got a tail of a snake - or a dragon (17)
  Above: Rahu and Ketu, Prasat Puai Noi, Khon Kaen. 11th century AD.
Rahu ascends from a whirlwind and holds the crescent moon.
Ketu rides the simha, a mythological lion.

     Rahu and Ketu are in iconography depicted as two of the Nine Celestial Deities, the Navagraha (lit. the 'nine planets').
     The Indian navagraha friezes starts from left with the Sun and the Moon followed by the five 'old planets' (18) and in the end Rahu and Ketu. The friezes are mostly associated with Shiva temples.
     The Khmer navagraha differs from its Indian origin. The series also starts with the Sun and the Moon and ends with Rahu and Ketu, but the five graha between have been replaced with some of the eight Guardians of the Cardinal Directions, the dikpalas. Indra is the leader of the dikpalas and is often depicted on his elephant, Airavata, above eastern doors of Khmer temples, the orientation of which he guards. On Khmer depictions of the Nine Deities (19), he is always presiding in the middle.
     A rather ruined frieze has been excavated at Prasat Phanom Rung. Of the four deities only two can be identified: Rahu holding the moon and Ketu on his simha (20).

The Doors of Prasat Phanom Rung
     Nearly all Khmer Hindu sanctuaries are oriented towards east allowing the rising sun to illuminate the deity in the central tower.
     One third of the Khmer temples are oriented straight east resulting in sunrises and sunsets aligned with the structure on the equinox days. Spring equinox, visuvat, was Vedic solar New Year and dated Khmer stone inscriptions count their solar-lunar calendar from the new moon before equinox.
     Prasat Phanom Rung is orientated 5.5 degrees from straight east, an orientation it shares with some 20% of the Khmer temples (21). Unique for Prasat Phanom Rung is that is it located on a mountain top and has doorways opening towards east as well as west allowing the rising as well as the setting sun to illuminate the central deity, Shiva's phallic symbol, the linga.

The astronomical aspects are:

1. Solar: The illumination of the linga occurring close to 14 days before and after the two equinox days. Annually there are 4 solar events. At each event the sun will be visible the day before and after as well. On each day the sun will be visible for some 8 minutes.
2. Solar-lunar: The period from the solar event before equinox to the event after equinox is close to one synodic month. The moon will be located at the same location (zodiacal sign, naksatra) at both events.
3. Eclipses: When a solar or lunar eclipse occurs at a solar event it will most likely be repeated with some interval at a coming solar event. The most common interval is 6 synodic months (app. 164 days).

Right: The vehicle of Shiva, Nandin, and the illuminated linga.



Conclusive remarks:
     No past astronomical event indicates that Prasat Phanom Rung was constructed coinciding with a celestial events (e.g. eclipse).
     The 15 door-ways form a 76 m. long tunnel, which can be used for calendric observations by an observer with some knowledge in astronomy, but we do not know if it was intended by the constructor. None of the stone inscriptions found at Phanom Rung inform us about whether the ancient sages living at the temple paid any attention to the various types of astronomical aspects of the orientation of the temple.
     Other contemporary Khmer inscriptions praise various kings and their royal gurus for their exceptional knowledge in various Vedic sciences; astronomy listed as one of them.
     This, together with the abundant sculptural manifestations of the Guardian of the Cardinal Directional and the Nine Deities, indicate that the astronomical aspects would not have gone unnoticed by the contemplating sages, who daily worshipped the linga on the mountain temple.

     Hopefully this article will help not to let the many celestial events in 2007 go unnoticed.

  1. GPS = Global Position System
  2. Averaged GPS measurements done by the author.
  3. Celestial north or geographic north relates to the axis of the Earth and differs from magnetic north.
  4. Equinox (wan wisuwat in Thai, from visuvat in Sanskrit) is the day when the sun rises straight east and sets straight west and day and night are of equal length. Spring equinox: 20 or 21 March. Autumnal equinox: 22 or 23 September.
  5. A synodic month lasts 29.530589 days on average.
  6. The sidereal month is also called the orbital period and is on 27.32166155 days (average).
  7. Sometimes 28 naksatras.
  8. The beginning or initial point of naksatra divisions has changed from time to time. Surya-siddhanta astronomers changed the initial point for counting the naksatra divisions to a point in the ecliptic opposite the star Citra (Spica-ά Virginis). The spring equinox coincided with this initial point at AD 285 and by circa AD 400 when the Surya-siddhanta system became current, the equinoctial point was located not very far from this point.
  9. App. 3000 years ago.
10. A draconic month is also called a nodal month and lasts 27.21222082 days (average).
11. The ascending and descending nodes are also known as the dragon's head and the dragon's tail, and are important aspects of western and oriental astronomy. 15 days later (19 March, 2007) there will be a partial solar eclipse, which again will be followed by a second total lunar eclipse 6 synodic months later, at sunrise 28 August, the latter 13 days before the sunset in September.
12. App. construction year of the brick towers at Prasat Phanom Rung.
13. The 7th September 815 the sun rose illuminating the central deity and became 5% eclipsed 3 hours later.
14. The 7th September 834 the sun rose illuminating the central deity and became 45% eclipsed 2 hours later.
15. The asuras were the enemies of the gods, the devas.
16. A discus shaped weapon. Etymologically the ecliptic = zodiac = rasicakra.
17. The descending node, Ketu, is in western mythology called the Dragon's Tail and Rahu the Dragon's Head.
18. The planets visible by the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn.
19. The 'Nine Deities' might be a more proper designation, as the planetary gods have been replaced by members of the dikpalas in various combinations and chronology.
20. Simha: Mythological lion like creature. The lion is not indigenous to SE-Asia.
21. The orientations are based on the author's measurements of 240 temples, most of which dates from the 10th to the 13th century AD.

APPENDIX: Major celestial events in 2007:
Month Day Time Event Comment
March 04 06:20:56 Total lunar eclipse Starts at 04:30:00
March 07 18:13:55 Sunset in 15 doors Aligned with the structure. Moon in Citra
March 21 07:07:25 Equinox Vernal
March 19 08:24:00 Partial solar eclipse Starts at 07:50:55. Max. eclipsed surface: 7.0%
April 04 06:07:54 Sunrise in 15 doors Aligned with the structure. Moon in Citra
August 28 18:22:19 Total lunar eclipse Moon rises eclipsed at sunset
September 10 05:57:47 Sunrise in 15 doors Aligned with the structure. Moon in Magha
September 11 17:31:21 Partial solar eclipse Not visible in Thailand
September 23 16:51:13 Equinox Autumnal
October 07 17:52:29 Sunset in 15 doors Aligned with the structure. Moon in Magha

History of Astronomy in India. Sen, S.N and Shukla, K.S., Indian National Science Academy, New Delhi, 1985.
On the Babylonian Discovery of the Periods of Lunar Motion, Goldstein, University of Pittsburg.
The author's website: www.thai-isan-lao.com

- END of article-

Above: Fragment of the ruined navagraha frieze of Prasat Phanom Rung.
Ref.: PRASAT PHANOM RUNG (Thai) by Suriyawut Suksawat



10 June 2007 Asger Mollerup