THE FIRST 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
in Lopburi by Father Jean de Fontenay (1643-1710) 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 of Chevalier Alexandre de Chaumont to Siam.
In fact, the first lunar observation was made in Ayutthaya three years earlier in 1682 by Father Antoine Thomas.

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

Between 1662 and 1671 he studied in Namur, Huy, Douai, and Lille. Thomas studied theology in Douai from 1671-1675. 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. Between March 1678 and January 1680, he studied mathematics in Coimbra, Portugal and published a short account of his observation of a
lunar eclipse in Coimbra on 29 October 1678 in the
Journal des Sçavans. [1]

Thomas arrived at Ayutthaya on 1 September 1681 via the Portuguese enclave of Goa (where on 2 May of that year he determined the latitude of
Goa) with the aim to continue from here his travel to China, as requested by the Flemish Jesuit missionary Ferdinand Verbiest (1623-1688), 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 (
San Paulo community). During his temporary stay,
Thomas carried out solar observations in order to determine the latitude of the City of Ayutthaya on 14 October 1681. 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 so
as 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]
He determined the latitude of his location on his first observations at 14° 17' 45" N latitude. This is the first scientific astronomical observation that is
known to have been made from Siam.

Thomas conducted further observations on 30 December 1681 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 San Paulo Church).
(Detail from Ref 3)
(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), adapted Thomas' values for the latitude of Ayutthaya, using tables of refraction
drawn up by Philippe de la Hire (1640-1718) and Giovanni Domenico Cassini (1625-1712). He recalculated 14° 18′ 5″ N and 14° 19′ 7″ N for 14
October and 30 December 1681 respectively and set as average 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 San Paulo Church in the Portuguese settlement,
adding 1' to the calculation of Thomas. This location is (with present means) situated about 590 m south of the San Paulo Church at the mouth of
Khlong Takhian. Gouye concluded that the latitude of the City of Ayutthaya was at 14° 20′ 40″ being half a French mile further north than the San
Paulo Community. (5)

On 22 February 1682, Thomas observed a total lunar eclipse and derived the longitude of the city of Ayutthaya, being 120° 40' 30" of El Hierro or
Ferro by using again his "simple pendulum". 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 a key reference for old maps, with an exception for the 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) 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 (San Paulo) is at present 100° 34' 21". The longitude calculation of Thomas had as thus an error of 2° 26' 23".
The lunar eclipse in Ayutthaya on 22 February 1682 started at 03.53.49 Hr and full immersion occurred at 04.52.29 Hr. The details of Thomas'
observations can be read in Ref 3 page 183-194.

Father Thomas made thus very acceptable latitude and longitude calculations for his position south of Ayutthaya, without working with 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 had likely an amicable relationship with Constantine Phaulkon and the latter could have been present at the Portuguese Jesuit
Mission at the time of the lunar eclipse. He likely acquired here sympathy for astronomy. Thomas led the ceremony of Phaulkon abjuring from
Anglicism to Roman Catholicism on 2 May 1682 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 as he arrived on 4 July 1682 at the
Portuguese settlement of Macau. He remained in China considered as the main mathematician and astronomical expert of China until his death.

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), 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, & Estienne Martin.