AYUTTHAYA HISTORICAL RESEARCH



THE FIRST SCIENTIFIC OBSERVATION OF A LUNAR ECLIPSE IN SIAM





“It can be said that the modern study of astronomy happened for the first time here in Thailand at this yen pavilion of Lopburi.” (1)


Some scholars, as well as Thailand’s Fine Arts Department, put the first observation of a lunar eclipse in Siam in the morning of 11 December 1685 CE in Lopburi by Father Jean de Fontenay (1643–1710 CE) and his fellow Jesuit mathematicians and astronomers. The latter were sent by the French King Louis XIV on a scientific expedition to the Indies and China and had joined the first French Embassy to Siam led by Chevalier Alexandre de Chaumont. In fact, the first lunar observation was made in Ayutthaya three years earlier, in 1682 CE, by Father Antoine Thomas.


Antoine Thomas was born on 25 January 1644 CE in Namur, Spanish Netherlands (present-day Belgium). He joined the Society of Jesus (a Jesuit Order) on 8 September 1660 CE in Namur.


Between 1662 and 1671 CE, he studied in Namur, Huy, Douai, and Lille. Thomas studied theology in Douai from 1671 to 1675 CE. Thereafter, Thomas professed philosophy at the College de Marchiennes in Douai, but was especially interested in mathematics. Thomas was ordained as a priest by 1678 CE. Between March 1678 and January 1680 CE, he studied mathematics in Coimbra, Portugal. He published a short account of his observation of a lunar eclipse in Coimbra on 29 October 1678 CE in the Journal des Sçavans. [1]





(King Narai and French Jesuits observing a lunar eclipse at Lopburi, 1685 CE. Engraving by Cornelis Martinus Vermeulen in Guy Tachard, "Voyage de Siam des Pères Jésuites envoyez par le Roy aux Indes et à la Chine". Paris, 1686 CE)



Thomas arrived at Ayutthaya on 1 September 1681 CE via the Portuguese enclave of Goa (where on 2 May of that year he determined the latitude of Goa) to continue from here his travel to China, as requested by the Flemish Jesuit missionary Ferdinand Verbiest (1623-1688 CE), whom he was later to succeed as chief mathematician and astronomer of the Chinese empire.

Waiting for a ship to Macao, Thomas stayed in the Portuguese settlement, as he was familiar with the Portuguese. In the Portuguese settlement, he resided at the House of the Society of Jesus, the residence of the Portuguese Jesuits in Ayutthaya (São Paulo community). During his temporary stay, Thomas conducted solar observations to determine the latitude of the City of Ayutthaya on 14 October 1681 CE. Thomas did not use a telescope (2) in his possession, but a “simple pendulum" (an iron wire with a lead ball - not a pendulum clock) and a wooden quadrant that, while “precisely divided”, could be easily read to the minute and was mounted with pinnules rather than telescope sights. Instead of “shooting” the sun's meridian altitude with his quadrant, Thomas deployed the time-honoured practice of constructing gnomons in situ, piercing a wall or plate mounted to allow sunlight to pass through a small aperture to find the sun's Zenith distance by trigonometry. [Ref 2]

Hereby citing Thomas: “I used, to take the meridian height of the sun, a gnomon of about forty Roman feet: I did it, advancing a pierced wooden board on the top of the wall of our Chapel and placing on this board an iron plate parallel to the plane of the horizon, pierced in the middle by a little round hole, through which passed the ray of the sun, which would fall on another board which had been placed at the foot of the wall parallel to the plane of the horizon by means of a channel full of water so that the meridian line drawn on this board made a right angle with a wire which fell perpendicularly in the middle of the small hole through where passed the ray which formed the image of the sun on this board.” (3) [Ref 3]





(Detail from Ref 3)



He determined the latitude of his location on his first observations at 14° 17’ 45” N latitude. This is the first known scientific astronomical observation made from Siam.


Thomas conducted further observations on 30 December 1681 CE in better weather conditions and found that his position was 14° 18′ 27″ N, a difference of 42’ with his first observations. He concluded from these observations that his position was 14° 18’ 20” N (which is plotted at present, south of Wat Kai Tia, about 2.5 Km south of the presumed location of the São Paulo Church).





(Detail from Ref 3)



Thomas followed “the old astronomers” in assuming refraction to be negligible for the sun if observed at altitudes higher than 45°. (4) Using his editorial privileges, the Jesuit priest Thomas Gouye (1650-1725 CE) adapted Thomas' values for the latitude of Ayutthaya, using tables of refraction drawn up by Philippe de la Hire (1640-1718 CE) and Giovanni Domenico Cassini (1625-1712 CE). He recalculated 14° 18′ 5″ N and 14° 19′ 7″ N for 14 October and 30 December 1681 CE, respectively, and set the average to 14° 18′ 36″.


Considering the obliquity of the ecliptic, he made another value adjustment, deriving 14° 19′ 20″ N as the position of Thomas' observation location, which was the São Paulo Church or House in the Portuguese settlement, adding 1’ to the calculation of Thomas. This location is (with present means) situated about 590 m south of the São Paulo Church at the mouth of Khlong Takhian. Gouye concluded that the latitude of the City of Ayutthaya was 14° 20′ 40″, being half a French mile further north than the São Paulo Community. (5) Father Thomas conducted further stellar observations from Ayutthaya between December 1681 and February 1682 CE.


On 22 February 1682 CE, Thomas observed a total lunar eclipse and determined the longitude of the city of Ayutthaya to be 120° 40' 30" of El Hierro or Ferro, using his simple pendulum again. El Hierro, the westernmost of the Canary Islands, was called the 'Meridian' Island. The Ferro meridian was used as the prime meridian (or zero meridian) and as a key reference on old maps, except for Anglo and North American maps. El Hierro was at that time considered exactly 20° west of the Paris meridian (the actual island of El Hierro itself is in fact 20° 23' 9" west of Paris).


According to the European longitude adjustment of Carl Theodor Albrecht (ca. 1890 CE), the Ferro meridian is 17° 39' 46.02" west of the Greenwich meridian. If we deduct the Ferro-Greenwich difference from the longitude Thomas determined for his location, we get 103° 0' 44''. The location of Thomas' lunar eclipse observations (São Paulo) is at present 100° 34' 21". The longitude calculation of Thomas had an error of 2° 26’ 23". The lunar eclipse in Ayutthaya on 22 February 1682 CE began at 03.53.49 Hr and reached complete immersion at 04.52.29 Hr. The details of Thomas’ observations can be read in Ref 3, pages 183-194.





(Detail from Ref 3)



Father Thomas made these very acceptable latitude and longitude calculations for his position south of Ayutthaya without using precision instruments. Knowledge of both latitude and longitude was required for both cartography and navigation, and in particular to provide safe sea navigation.


Antoine Thomas likely had an amicable relationship with Constantine Phaulkon, and the latter may have been present at the Portuguese Jesuit Mission during the lunar eclipse. He likely acquired a sympathy for astronomy here. Thomas led the ceremony of Phaulkon abjuring from Anglicism to Roman Catholicism on 2 May 1682 CE at the San Paulo Church (a change of faith at the request of his future father-in-law) and being his converter, he was also probably present at the marriage ceremony of Phaulkon with Maria Guyomar de Pinha, a Catholic of mixed Japanese-Portuguese-Bengali extraction a few days later at the same church. Shortly after, Thomas left Ayutthaya for China, arriving in the Portuguese settlement of Macau on 4 July 1682 CE. He remained in China, considered the leading mathematician and astronomical expert, until his death.





(Jean Dominique Cassini‘s map of the Moon was engraved by Jean Patigny and was first printed in 1679 CE)



Footnotes:

(1) Literal text quote of the Fine Arts Department information board at the Kraison-Siharat Hall in Lopburi.
(2) Thomas was in the possession of a telescope made by the renowned lens maker Giuseppe Campani (1635-1715). [Ref 1]
(3) Original French text of Thomas: “Je me suis servi pour prendre la hauteur meridienne du Soleil, d'un gnomon d'environ quarante pieds Romains: je l'ay fait, en avançant sur le haut de la muraille de notre Chapelle un ais percé & mettant sur cét ais une plaque de fer parallelle au plan de l'horizon, percée au milieu d'un petit trou rond, par ou passoit le rayon du Soleil, qui alloit tomber sur un autre ais qu'on avoit mis au pied de la muraille parallele au plan de l'horizon par le moyen d'un canal plein d'eau de sorte que la ligne meridienne tracée sur cét ais faisoit un angle droit avec un fil qui tomboit a plomb du centre du petit trou par ou passoit le rayon qui formoit l'image du Soleil sur cet ais.” [Ref 2]
(4) Thomas followed here the school of André Tacquet (1612 – 1660 CE), a Jesuit priest and brilliant mathematician of international fame from Antwerp. [Ref 1] (5) Thomas was cited in de La Loubère’s work (1692) A New Historical Relation of the Kingdom of Siam: “Its [City of Ayutthaya] Latitude, according to Father Thomas the Jesuit, is 14d. 20 m. 40 S. and its Longitude 120d. 30 m.”

References:

[1] Thomas, A. (1679). Observations de l’éclipse de lune du 29 Octobre 1678 à Coimbre. Journal des Sçavans.
[2] Florence C. Hsia (2009). Sojourners in a Strange Land: Jesuits and Their Scientific Missions in Late Imperial China. University of Chicago Press.
[3] Thomas Gouye (1688). Observations physiques et mathématiques pour servir à l'histoire naturelle et à la perfection de l'astronomie et de la géographie envoyées des Indes et de la Chine à l'Académie royale des sciences à Paris par les Pères Jésuites avec les réflexions de Mrs de l'Académie et les notes du P. Goüye de la Cie de Jésus. La veuve d'Edme Martin, Jean Boudot et Estienne Martin.



When the past no longer illuminates the future, the spirit walks in darkness.
(Alexis de Tocqueville)





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