The above article about archaeoastronomy can be downloaded from the archives of the Bangkok Post

or read here in its original and nearly unchanged form - with better graphics:

Archaeoastronomical touring, 2007

The sun, the moon and Rahu (the Dragon's head)
 

Above: Prasat Phanom Rung, 2000 March 3.

Above: Prasat Phanom Rung, 2004 March 6.

Above: 2000 March 7.		Above: 2003 March 7.		Above: 2004 October 5.

    The aim of this article is to inform the reader about when to be where in order to observe sunrises and sunsets aligned with the structure of some ancient Khmer temples – with emphasis on Prasat Phanom Rung, Buriram Province.
     A special astronomical feature in 2007 is the coming total full-moon, 4 March, which will set aligned with the gates of some other Khmer temples and their associated barays, water reservoirs.
     In an article in Horizon, 23 March 2000, titled New Light on an Ancient Site, the author published his observations of the sunset in the gates of Prasat Phanom Rung and calculations on all four annual solar events at the ancient Khmer Shivaite temple.
This article also discloses that the solar events actually are solar-lunar events and furthermore that the orientation of Prasat Phanom Rung might be related to lunar and solar eclipses.

     The most well-known ancient structures with a likely archaeoastronomical relevance are Stonehenge in England, the pyramids in Egypt and Mesoamerica, and Angkor Wat and Banteay Srey in Cambodia. These structures are oriented straight east-west and the solar events are related to the equinoxes and in some cases also the solstices.
     Prasat Phanom Rung in Buriram province, Thailand, is being promoted for its solar events when the sun rises or sets visible through all 15 gates of the sanctuary.
     The orientation of Prasat Phanom Rung is not straight east-west, so the solar events will not happen at the equinoxes. The orientation is 84.5 degrees – differing 5.5 degrees from straight east, resulting in that the solar events will occur 14 days before and after equinox. In 2007 the sun will set aligned with the doorways of Prasat Phanom Rung the 7th of March and rise straight the 4th of April. Around autumnal equinox there will be another two solar events.
     The orientation of 84.5 degrees is not unique for Prasat Phanom Rung; some 20 % of all Khmer temples share this orientation.
     The day the sun rises minus 5.5 degrees from straight east (at 84.5 degrees east) it will set close to plus 5.5 from straight west. This means that all structures aligned plus/minus 5.5 degrees from straight east-west share one feature: 14 days and 14 days after equinox the sun will either rise or set aligned with the centre-line of the temple. One example is Prasat Phanom Rung (84.5 degrees) and the nearby Prasat Kuti Rishi Ban Khok Muang (app. 95.5 degrees): On the 7th of March the sun will set visible through the gates of Phanom Rung and on the same day the sun will rise from the barai visible through the gates of Prasat Kuti Rishi Ban Khok Muang, giving the photographer two opportunities for a unique photo.
     Vat Phou in southern Laos has an orientation close to Prasat Kuti Rishi Ban Khok Muang so another tour-option is to watch the sun rise from the barai east of Vat Phou and the same day see the sun set through the gates of Phanom Rung – 320 km away as the crows flow...

     The basic data for archaeoastronomical calculations are orientation, location and age of a site. The author has the last four years been conducting a GPS-based field research on Khmer temples. The results from 270 sites indicate that the group of Khmer temples oriented plus/minus 5.5 degrees from straight east amounts to app. 30 %. The temples in this group all have solar events plus minus 14 days from equinox.
     The time span between one solar event to the next is thereby 28 days, which is close to a lunar month. Not the lunar month counted from full moon to full moon (synodic lunar month on 29.5 days) but the period it takes the moon to revolve one time on the celestial sphere (sidereal lunar month on 27.3 days). The sidereal lunar month can be observed the following way: After observing the sunset straight through the doorways of Prasat Phanom Rung 7 March, 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 (see figure A and B).

     Ancient Khmer astronomy/astrology and calendric system was influenced by Indian philosophy. Khmer inscriptions refer to the 27 lunar houses called the naksastras, and the Vedic legend, 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 in the hours before sunrise on the 4th of April, Chandra will have returned to Citra.
This relation between the solar events and the lunar month suggests that the event should be termed solar-lunar event. These solar-lunar events are likely to have been significant for the ancient Khmer sages for astrological purposes.

	Above: The location of the moon 2007 March 7 some 4 hours after sunset		Above: The location of the moon 2007 April 7 app. 1 hour before sunrise.

     The time-interval between the two solar-lunar events is nearly identical to a draconic lunar month on 27.2 days. The draconic month is vital in calculations of eclipses and 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. The ascending and descending nodes are in western astronomy known as the dragon's head and the dragon's tail. In Khmer concept they are called Rahu and Ketu. And the old people still tell about Rahu holding the moon and also tell how the monster is chased away by shooting at the full moon and by knocking various utensils at the columns of the house.
     In Khmer mythology the nodes of the moon were personalized as Rahu (the ascending node) and Ketu (the descending node). Rahu was an asuras: an enemy of the gods, the devas. During the Churning of the Ocean of Milk he 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 discus shaped cakra. 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: The partial lunar eclipse, 2006 September 6, as observed from Prasat Phanom Rung. Visible all over Thailand.		Above: Rahu and Ketu, Prasat Ban Ben, Ubon. Exhibited at Ubon Museum. Rahu emerges from the clouds (or whirlwind) holding the moon. Ketu is depicted on his vehicle, a simha (a mythological lion).		Above: The coming total lunar eclipse will be reddish in colour. Photo: With courtesy to Frank Espenak.

     The draconic month is also 'embedded' in the orientation of Phanom Rung, which will be beautifully demonstrated by the total lunar eclipse before sunrise 4 March, which was preceded by a partial lunar eclipse six full moons before, the 8 September 2006, just before a lunar-solar event. This leads to: If an eclipse happens around the time of a solar-lunar event at Prasat Phanom Rung, then there is a 70 % chance that another eclipse will happen around the next event. Or more popularly: If Rahu is observed in the corridors of Phanom Rung, then he will most likely be back after six full moons.

     The coming total full moon will be observable at Phanom Rung in the hours before sunrise 4 March, but will not be aligned with the doorways of the temple. The red-coloured eclipsed moon will set at 7 degrees north of straight west and therefore be nearly aligned with the group of temples oriented plus 5.5 degrees from straight east-west.

	Above: Kuti Rishi Ban Khok Muang, 2004 March 8. Sunrise.		Above: Kuti Rishi Ban Khok Muang, 2004 April 1. Sunset.

     The lunar eclipse will be visible from all over Thailand, actually most of the world. The author will go for the photo of the moon setting at the spire of the central sanctuary and mirrored in the barai of Prasat Kuti Rishi Ban Khok Muang (next to Prasat Muang Tam) – and a few minutes later to take photos of the rising sun mirrored in the barai as seen through the eastern gate of this Jayavarman VII 'hospital'.

	Above: Vat Phou (Wat Phu), Champassak, Laos, 2006 March 3. Sunrise over the eastern barai.		Above: Vat Phou (Wat Phu), Champassak, Laos, 2006 March 3. Sunrise over the processional road.

     The central barai of Vat Phou would be another beautiful option to watch the eclipsed moonset and the sunrise mirrored in the water, but the author wants to stay near Prasat Phanom Rung in order to catch the full moon in the gates of the sanctuary– and also to watch the sunset the 7th of March.

     For photo enthusiasts elsewhere the advise is to be on the eastern side of a reservoir or river an hour or two before sunrise. One beautiful location in Bangkok will be opposite the Temple of Dawn, Wat Arun.

	Above: Wat Arun (the Temple of Dawn), Bangkok, at dawn 2007 February 23.