Exploring the First Scientific Observations of Lunar Eclipses Made in Siam

Cassini's 1679 Map of the Moon and French Jesuit Observations of the Lunar Eclipse of 11 December 1685

“…Their voyage to China was delayed in Siam, and fortuitously there was a lunar eclipse during their sojourn. Their observations of this event were facilitated by Siam's King Narai (1633 -1688), who had a progressive foreign policy and a keen personal interest in astronomy (for details see Orchiston et al, 2016). Before they left France, the six Jesuit astronomers (Landry-Delon, 2011) … were admitted to the Académie Royale des Sciences, and supplied with astronomical instruments on the understanding that these would be used-among other things-to determine the latitude and longitude of different geographical features and population centres.…”

“…For the French astronomers, successful observations of the 11 December 1685 eclipse would reveal the longitude of Lop Buri. It also would allow them to compare and contrast the position of Lop Buri with respect to nearby Ayutthaya, which another Jesuit astronomer, Father Antoine Thomas, had successfully pinpointed in 1681 and 1682 (see Orchiston et al, 2016;2018;2019b).…”

“…In Figure 5 we reproduce a drawing that was published by Tachard in 1686 and supposedly shows the Jesuit astronomers and King Narai observing the eclipse, in the presence of prostrated Siamese court astrologers. However, we have shown elsewhere (Orchiston et al, 2016: Figure 5: A drawing showing King Narai and the Jesuit astronomers observing the 11 December 1685 total lunar eclipse from the King's country retreat which was on an island in the water reservoir that was located to the northeast of his palace in Lop Buri (after Tachard, 1686). 36-37) that this incorporates substantial artistic licence and cannot be accepted … as a realistic ‗photographic' depiction of the eclipse observations, and this tallies with Michel Jacq-Hergoualc'h's evaluation.…”

“…When we first wrote about these eclipse observations (Orchiston et al, 2016) we could not locate any copies of the aforementioned cadastral map showing the boundaries of the water reservoir and the precise location of King Narai's country retreat, but a copy has now come to hand, and was used to create the map in Figure 9.…”

“…In a series of research papers we have shown how eclipses and Royal patronage played a key role in the development of astronomy in presentday Thailand (Orchiston et al, 2018;Soonthornthum, 2017). Lunar and solar eclipses largely were responsible for the birth of Western astronomy in Siam during the eighteenth century (Gislén et al, 2019;Orchiston et al, 2016;2019a;2019b;cf. Bhumadhon, 2000), and an association with total solar eclipses and Thai Royalty continued during the nineteenth century (Orchiston and Orchiston, 2018;cf.…”