The King of Siam predicted an eclipse more accurately than France
In short: In 1868, a total solar eclipse revealed a new element to science. But the most precise calculations for its arrival didn't come from the established European academies—they came from the court of King Mongkut of Siam, who outpaced French astronomers by two seconds.
The solar eclipse of August 18, 1868, was a landmark event for both diplomacy and spectroscopy. Known as "The King of Siam's eclipse," it occurred at the Moon's ascending node with a magnitude of 1.0756. While several international expeditions were dispatched to observe the phenomenon—including German teams led by Gustav Spörer and Friedrich Tietjen—the most notable scientific breakthrough was the discovery of helium.
During the totality, French astronomer Pierre Janssen observed the Sun's chromosphere from Guntur in British India. Using a spectroscope, he identified a bright yellow spectral line at 587.49 nm. While previous assumptions attributed this to sodium, Janssen realized it was a distinct element. This discovery was independently confirmed by British astronomer Norman Lockyer in England. On 26 October 1868, the French Academy of Sciences received their results.
The event also served as a triumph for Siamese science. King Mongkut (Rama IV) had predicted the eclipse's time, place, and type two years in advance. His calculations were so precise that contemporary French astronomers acknowledged they were superior to their own by approximately two seconds. The King used the event to host foreign dignitaries and scientists, signaling Siam's sophisticated engagement with modern science before his death just a month and a half later.
The eclipse was part of Saros series 133, which has been repeating every 18 years, 11 days since 1219. It is also linked to complex celestial cycles, including the Metonic series (repeating every 19 years) and the tritos cycle (repeating every 11 years minus one month). These overlapping cycles ensure that such rare alignments provide unique windows into the chemical composition of our solar system.
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- Summary by Intellect · source: Solar eclipse of August 18, 1868
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