This Month in Astronomical History: August 2024

Lauren Kokinakis
University of Texas at Austin
(Astronomy Undergraduate)
 

Each month as part of this series from the AAS Historical Astronomy Division (HAD), an important discovery or memorable event in the history of astronomy will be highlighted. This month, Lauren Kokinakis writes about the total solar eclipse of 1868. Interested in writing a short (500-word) column? Instructions along with previous history columns are available on the HAD web page.

James Tennant and "The King of Siam’s Eclipse"

The 1868 solar eclipse, known as “The King of Siam’s Eclipse,” exemplified the significant impact of solar eclipses on scientific discovery and international collaboration. King Mongkut, whose regnal name was Rama IV of Siam, had an avid interest in astronomy. He correctly calculated when the eclipse would occur two years before the event, more accurately than the best European astronomers at the time (Figure 1).¹ This prediction, computed without the highly accurate European technology, was so impressive that the eclipse would later be named after him.

Solar eclipse viewings have long been an essential part of astronomical history, both scientifically and socially. They have led to numerous discoveries concerning the Sun’s structure, composition, and activity. The viewings also brought scientific communities together, as scientists and other dignitaries traveled thousands of miles to view, record, and report on the events. One good example was the total solar eclipse in southeastern and eastern Asia on 18 August 1868.

Happening right at the dawn of solar astronomy and astrophysics in the 1860s, the total solar eclipse of 1868 is also remarkable for Pierre Janssen and Norman Lockyer’s discovery of helium using spectrohelioscopes.^2,3 While that discovery proved to be a paradigmatic revelation, the eclipse also drew expeditions from scientific societies and academic institutions across Europe to venture into India and Thailand along the path of totality. This was part of a broader 19th-century trend of solar eclipse expeditions driven by burgeoning discoveries and advancements in astrophotography, telescope technology, and solar astrophysics. Eclipses, such as the one in 1868, provided insights into the functioning of the prominences, photosphere, and corona in the Sun's atmosphere, as well as into planetary physics and celestial mechanics. The planning involved calculating the day the eclipse would occur, where totality would occur, and how long it would last. These trips also required coordination between the visiting country and the hosting country.

The first modern scientific expeditions were launched in the late 17th century. Fueled by the increased interest in science generally and astronomy in particular, expeditions became frequent in the Victorian Era. Charles Darwin’s service as the naturalist aboard the Beagle exemplified the rise of the United Kingdom and that astrophysical research hallmarked that ascendency. At the time, anyone remotely interested in solar physics or astrophysics attended at least one solar eclipse viewing in their careers.² This trend can be seen with the numerous expeditions England sent for the 1868 eclipse, with the most prominent team being led by James Francis Tennant.

James Francis Tennant was a geological and astronomical research scientist, a lieutenant general in the Royal Army, and a government official.⁴ He had various connections to the Indian government because he was also an Indian government official, a former engineering officer, and, for a short stint, the director of the Madras Observatory.⁴ His connections to both the Indian and British governments allowed him to persuade the Royal Astronomical Society to grant him funds, a team to observe the 1868 eclipse, and a place to stay in India.

With permission from the Indian government, Tennant and other officials and astronomers traveled to Guntur to stay and view the eclipse. The team brought a variety of instruments, including a Troughton and Simms spectroscope and a 4.6-inch aperture f/13 refractor for spectroscopy; a 7.5-inch aperture f/16 refractor for polarization viewings; and a 9-inch clear aperture f/8 Newtonian reflector for photography (Figure 2).⁶ On the 17th of August, Tennant used the spectrographic setup to view the Sun’s corona, in which he saw a faint continuous spectrum with colors ranging from red to blue, with a mysterious yellow line no one of the team could identify.⁵ On the day of the eclipse, the polarizer and 7.5-inch telescope were used to view prominences at totality, a structure that would be named the “Great Horn” by the group (Figure 3).⁶ Photography was also used to view the Sun’s corona, with six photographs taken in the six minutes of totality. The photos helped reveal that the Great Horn's prominence was over ten times the diameter of the Earth alongside other significant prominences.⁵

Many teams who used spectroscopes to view the eclipse noticed a mysterious yellow line in the sun’s spectrum.⁵ Tennant himself actually saw the emission line associated with helium but could not identify it. Other teams at the time guessed that it was likely sodium, but Pierre Janssen and Norman Lockyer later disproved this.

After the eclipse, Tennant continued to focus on astronomy and expeditions in India, observing the 1871 solar eclipse and the transit of Venus in 1874.

Fig. 1: King Mongkut and other eclipse viewers in 1868.

Fig. 2: Tennant’s 9-inch Newtonian astrograph.

Fig. 3: The Great Horn prominence and others.

Notes & References

1. Bunplod, Worachate. (2004). 200 years of His Majesty King Mongkut Father of Thai Science. The Thai Astronomical Society. https://thaiastro.nectec.or.th/library/kingmongkut_bicentennial/kingmon…
2. Pang, A. S.-K. (1993). “The Social Event of the Season: Solar Eclipse Expeditions and Victorian Culture,” Isis, 84(2), 252–277.
3. Agrawal, Alan. (2022). “The Discovery of Helium,” This Month in Astronomical History, October 2022. https://aas.org/posts/news/2022/10/month-astronomical-history-october-2…
4. Hollis, Henry P. (1916). “Obituary Notices: Fellows:- Tennant, James Francis,” Monthly Notices of the Royal Astronomical Society, Volume 76, Issue 4, Pages 272–276.
5. Orchiston, Wayne & Lee, Eun-Hee & Ahn, Young-Sook. (2017). “British Observations of the 18 August 1868 Total Solar Eclipse from Guntoor, India.” doi:10.1007/978-3-319-62082-4_25.