Archaeoastronomy in the Khmer Heartland

So far, so good for the Angkor Wat orientation. It is now obvious, however, that if a similar phenomenon must be observed in a temple whose main access is to the east, observation will occur at sunset, and the azimuth of the temple must be slightly misaligned to the north of east. In fact, in this way the direction of the observer looking along the temple axis will point slightly to the south of west, where the equinoctial sun will be seen to disappear just above the temple.

This is the likely explanation for the temples’ orientation slightly to the north of east, and so in particular for the state temples Bakong, Phnom Bakheng and Bayon, whose azimuth (“exactly” like that of Angkor Wat, but in the opposite direction) is 89.5°. The phenomenon of the sun disappearing vertically behind the temple at the equinoxes in this latter case can be verified using Google Earth 3D visualization. The dimensions of the sun in Google Earth simulations are relatively big, but despite this the effect is unmistakable, as shown in Fig.3.

What is the meaning of such spectacular hierophanies? For Angkor Wat, Stencel et al. 1976 proposed a rather complex calendrical function, trying to frame the phenomenon into a series of supposed astronomical functions of the monument which, when viewed from different, suitably chosen points of the esplanade supposedly furnish, for example, the extrema of the motion of the moon at the horizon. This idea, however, is clearly biased by a strong selection effect of the observation points, and in any event there is no evidence that the Khmer monuments were used for making precise astronomical observations. The key is instead symbolic: the beautiful hierophany of the sun suspended just above the mountain temples at the equinoxes was very probably intended as a materialization of the connection of the temple itself with the heavens, since it brings about a match between the zenith in the sky and the cardinal directions on the ground.

The recent discovery that the zenith passages were probably also made visible inside the temples [Barnhart and Powell 2013] supports this interpretation, as well as the well-known general connection of the temple mountain architecture with the axis orthogonal to the earth's surface—the zenith-nadir axis to which, respectively, the temple and its image reflected in the waters in front of it explicitly allude. In fact, although the simplest way to observe the zenith passages is to look for the days in which the shadow of a post vanishes at noon, another effective way is to use a straight vertical tube leading from the open sky into a dark chamber. If the tube is sufficiently long and narrow, the identification of the zenith passage will be accurate and, most important of all, the effect inside the room will be spectacular. This method, which was developed for example in pre-Columbian Mesoamerica, was very likely in use also in the Angkor temples. Today, their roofs are open, but the capstones are missing. Many such stones, however, are present in the rubble near the temples, and all have a hollow tube running down their axes. The holes allowed rain to hit the holy stone lingas located at the center of many chambers, and also allowed the sun passing overhead to light the same stones in spectacular hierophanies occurring twice a year (zenith passages at Angkor occur on April 26 and August 17).

The temples of the Angkor heartland were thus anchored in the cycle of the sun in two ways: the orientation, related to the equinoxes, and the vertical openings of their chambers, related to zenith passage. Yet another way to connect a temple with the zenith passages is, of course, by orienting the building to the sun rising or setting on these days, as occurs, for instance, for the world famous post-classic Maya pyramid of Chichen Itza, Yucatan. The azimuth of the rising sun on the zenith passages at Angkor is 76°, so apparently no Angkor temple was oriented in this way. Interestingly enough, however, the cardinal orientation appears to be a pattern, a rule only for the sacred space of Angkor, since —as we shall see in section 4— a state temple oriented to the sun rising on the days of the zenith passages actually exists outside the heartland.

Archaeoastronomy in the Khmer Heartland     1:7

Finally, a few temples remain for which the misalignment is too elevated to be attributed to considerations of visual effects. In particular, the temple exhibiting the worse misalignment with respect to due east, Banteay Kdei, was probably built long after the construction of the baray which lies in front of it and has the same alignment. Thus, the original rough orientation of the baray probably influenced that of the later temple for aesthetic reasons.

Moreover, he added an island temple (the East Mebon) inside the already existing baray. The position chosen for the East Mebon is on the “same” meridian as Pre Rup. The reason for the quotation marks is that the azimuth of this alignment is slightly off-center (178°), but this is coherent with the orientation of Pre Rup, which indeed is 88°. In other words, it is clear that East Mebon was intentionally placed along the direction orthogonal to the north side of Pre Rup. Interestingly, a very long line (about 7 kilometers) almost lying on the parallel (azimuth 269°) connects East Mebon with Phimeneakeas, whose original dating is not known, but which functioned as the state temple of the successor, Suryavarman I. The Victory Gate of Angkor Thom lies along the very same direction and its placement was almost certainly chosen for the very same reason (Fig. 5).

Finally, an interesting combination of alignments also repeats for the largest of the barays, the West Baray. Here the north side is on the same parallel as the state temple of its builder, the Baphuon, which lies at a distance of 2.2 kilometers from the corner, to the east. The prolongation of the south side to the east also lies on the same parallel (azimuth 89.5°) of a temple, the pre-existing Bat Chum, at some 7.6 kilometers from the south-east corner. Finally, at the center of the West Baray, the island temple West Mebon was constructed by Udayadityavarman II (it is not completely clear if the project of the baray was already initiated by his predecessor Suryavarman I). The West Mebon is connected with Pre Rup, since a “parallel” line (azimuth 89.5°) connects the two monuments (later, Ta Prohm was also to be built on this line). This line is very long (9.6 kilometers), but Pre Rup is 12 meters high. Clearly, since the Mebon is at the center of the baray, it is the baray itself that was planned by taking into account three reference points corresponding to the parallels of the sides and the mid line (Fig. 5).

The reasons for the construction of the barays (whether they were functional, or ritualistic, or both) is a complex issue of Khmer archaeology as a whole and certainly cannot be addressed by the present author. I would only note here that the existence of the above-mentioned alignments, which are certainly intentional, reinforces

of the obliquity of the ecliptic since AD 1100 is negligible, being about 6'). Of course, this does not necessarily mean that the temple was deliberately oriented to another astronomical phenomenon, but topographical reasons are difficult to imagine, and invoking “chance” is equally unsatisfactory, especially when taking into account the strict astronomical pattern which was the rule in Angkor's heartland. Furthermore, a very clear astronomical solution does exist, involving the moon. As is well known, the plane containing the earth and the moon’s orbit is not the ecliptic, but forms with it an angle of 5° 9'. This has the consequence that the maximal and minimal declinations which the moon can attain are greater/lesser than those of the sun (which of course equal ± the obliquity of the ecliptic, 23° 30') by such an amount. This means that the moon at the horizon can attain azimuths lesser/greater than those of the sun at the solstices. Owing to a series of physical reasons, the extreme declinations (also called maximal standstills) are encountered only once every 18.6 years. Of particular interest is the full moon closest in time to the winter solstice, since it always attains a declination close to the maximal one in the year of the standstill, culminating very high in the sky and remaining in the sky almost the whole night.

Preah Khan of Kompong Svay is definitely oriented to the moon rising at the maximal northern standstill. Indeed, azimuth 61° with a flat horizon at this latitude yields a declination +28°.The true lunar declination at the site is actually slightly greater, 28° 14', because parallax must be taken into account. This value must be compared with the standstill declination of +28° 39' and again, as is the case of Koh Ker for the zenith passages, the match is impressive.

Of course, the role of the moon is quite relevant both in Hinduism—where it is identified with the God Chandra—and in Buddhism, since festivals and recurrences associated with Buddha's life are timed by the full moon. The choice of orientation to the extrema of the moon might thus have arisen from this and/or from other specific messages the builder wanted to associate with the temple. To be sure, further archaeological/historical research is needed to clarify this point. There is, however, a second issue related to astronomy which is worth discussing about Preah Khan of Kompong Svay. It is known that the temple is located on the same latitude as Angkor Wat. The accuracy of this coincidence is astonishing: the center of Angkor Wat is within 1' at the same latitude (13° 24') of the entrance to the inner enclosure of Preah Khan. The linear distance between Angkor Wat and Preah Khan of Kompong Svay is about 100 km, and therefore no visual connection is conceivable, and it is certainly possible to think that the connection occurs by chance. Indeed, to proceed along a “straight” line between non-intervisible positions is a thing which generally makes no sense at all since there are no straight lines on a curved surface (as pseudo-archaeologists in search of “leylines” usually forget). However, the special case of sites placed at the same latitude does make sense, because—although the parallel circle is not the shortest path between two points at the same latitude—the parallel in itself is, in principle, easy to determine using astronomy, and the Khmer architects certainly possessed the necessary skills. It is indeed possible to establish latitude by measuring the height of the celestial pole or the height of the sun at midday on fixed days, and this second method was clearly favored in the present case. Ancient people interested in the zenith passages of the sun appear to have also been interested in developing precise measures of latitude, as shown, for instance, by the archaeological site of Altavista in Central Mexico. However, even admitting intentionality, obtaining the accuracy exhibited by the Angkor-Preah Khan alignment must have been quite a daunting task, which clearly demands a sound historical explanation and a symbolic, rather than functional, interpretation for the site in connection with Angkor Wat.