Hostname: page-component-77c89778f8-sh8wx Total loading time: 0 Render date: 2024-07-22T13:02:09.489Z Has data issue: false hasContentIssue false

“The Whole Moon was Eaten”: Southeast Asian Eclipse Calculation

Published online by Cambridge University Press:  24 August 2009

J.C. Eade
Australian National University
Lars Gislén
Lund University


One might classify earthquakes and volcanic eruptions as “natural disasters”, whereas comets and eclipses might be described merely as “natural curiosities”. In earlier times, however, all were equally frightening, signs that the gods were angry.

A form of protection against eclipses was available, however, in the sense that prediction provided a means of countering the event. Consequently one main function of the astronomer/astrologer (henceforth “hora”) in his society was to warn his patron that the sky would darken and the birds stop singing at such and such a time on such and such a day.

Copyright © The National University of Singapore 1998

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)


1 In popular thinking, an eclipse was a time when the monster Rahu was expected to try and devour either the sun or the moon. This disaster had to be avoided, so at the appropriate time (which it was the duty of the hora to predict in advance) as much noise as possible had to be generated, in order to scare Rahu away.

2 We make a distinction here between Thai Chulasakarat and Burmese Thekkarit. Although the year-values are numerically the same for the two groups, their systems of intercalation are so different that it is misleading to lump the two systems together. For instance the year CS 803 begins on Caitra 7 waxing, but BE 803 begins on Kason 8 waxing (one month and a day, not just one day, later).

3 The standard procedure in finding the position of a planet is first to calculate where it would be if it appeared to circle the Earth always at an even speed (its “Mean” position). Correction formulae are then applied to this position to discover where it actually will be when viewed from the Earth (its “True” position).

4 This date falls within the reign of the Khmer ruler, Suryavarman II, and at a period when the Khmer empire had secure bases in Siam.

5 There is a lesson here for those who suppose that the application of Southeast Asian astronomy had anything to do with real-time observation or with the possibility of making it. The sun's mean motion of slightly less than one degree per day is here multiplied by ten million! In the 86400 seconds of time that make up 24 hours, at this rate the sun would pass through about 120 of these millions of units in one second of time. The hora's power lies in his ability mechanically to calculate numbers on paper.

6 We know of no method of assessing the transmission lines from expert to hora historically. But whatever they were, they were certainly efficient. Our data comes mainly from a Thai expert in the mid-twentieth century and from a French civil servant taught in Cambodia in the early twentieth century. By some accident they both use the same eclipse as a worked example and they both come to essentially the same results. Our third source, a calculation of the Mongkut eclipse (albeit of uncertain date and provenance) follows exactly the same procedures with exactly the same values as one would predict of it. See further below.

7 For those who wish to pursue how the reckoning is made here: the values from Aries (Mesa) to Virgo (Kanya) are given respectively as 120, 96, 72, 120. 144, 168 = 720 minutes = 12 hours. The values in the other half of the circle, from Libra (Tula) to Pisces (Mina) are 168, 144, 120, 72, 96, 120 = 720 minutes = 12 hours.

8 The low geographical latitudes of the region, where the extremes between the shortest day and the longest day experienced, for example, in Europe, do not apply — this proximity to the Equator makes the volvelle values workable. They give tolerable results in Bangkok, Rangoon, or Phnom Penh: they would generate rubbish in London, Paris, or New York.

9 Wisandarunkorn's values for day-length are operative at 15° 45′N; Faraut's two sets of values are operative at 16° 45′N and at 9° 30′N. They are, in short, markedly different from those that should have been adopted for Bangkok (13° 45′N) or for Phnom Penh (11° 30′N).

See Wisandarunkorn, Luang, Kamphi Horasasat Thai (Bangkok, 1965), p. 180Google Scholar and Faraut, F.G., Astronomie Cambodgienne. (Saigon, 1910), pp. 166 and 174Google Scholar.

10 See, e.g. the standard work, McFarland, G.B., Thai-English Dictionary (Stanford University Press, 1944), p. 588, col 2, ad initGoogle Scholar.

11 Monier-Williams, Monier, A Sanskrit-English Dictionary, col. 172c: “the apex of the orbit of a planet”Google Scholar.

12 The programme is Gislén's “Planets” (available at Sum-Aim Archive either for Macintosh with floating point unit or for Mac Power PC).

13 The manuscript is held, we understand, by the Library of Chiang Mai University.

14 This (Macintosh) computer programme, written by Eade and Gislén, is available, free of charge, from

15 I have reproduced the data, together with verifications where possible, in Appendix VII of The Calendrical Systems of Mainland Southeast Asia (E.J. Brill, Leiden, 1995)Google Scholar. The sources drawn on are three different examples of the genre known as “chotmaihet hon”.

16 This time is replicated exactly by the commercial programme “Starry Night” (Sienna Software).

17 A conspicuous case is where, in the 1920s, the Superintendent of the Archaeological Survey in Burma had to rely in his Reports upon the findings of Pillai, a noted (and justly respectable) expert on Indian chronology, but who had no inkling that Burmese chronology might run on a different basis from the Indian one.

18 The nadi and vinadi are the equivalent of hours and minutes, where there are 60 nadi instead of 24 hours to a day, and 60 vinadi to a nadi.