A total solar eclipse occurred on August 18, 1868, also known as "The King of Siam's eclipse". A solar eclipse occurs when the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

passes between

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

and the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

, thereby totally or partly obscuring the image of the Sun for a viewer on Earth. A total solar eclipse occurs when the Moon's

apparent diameter The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it is ...

is larger than the Sun's, blocking all direct sunlight, turning day into darkness. Totality occurs in a narrow path across Earth's surface, with the partial solar eclipse visible over a surrounding region thousands of kilometres wide.

Observations

Solar eclipse 1868Aug18-Bullock.png, Bullock sketch of the eclipse, ''Total Eclipses of the Sun'', 1900. Sketches of the total solar eclipse, 1868 (19745966082).jpg, M. Stephan sketches of the eclipse, ''Archives des missions scientifiques et littéraires'', 1868. Map of the solar eclipse of August 18, 1868 (19757912091).jpg, Map of the forecast path of the total eclipse, ''Archives des missions scientifiques et littéraires'', 1868. Several expeditions were sent to observe the eclipse. * One of two expeditions from Germany was sent to Aden. The expedition was led by

Gustav Spörer Friederich Wilhelm Gustav Spörer (23 October 1822 – 7 July 1895) was a German astronomer. He is noted for his studies of sunspots and sunspot cycles. In this regard he is often mentioned together with Edward Maunder. Spörer was the first to ...

. * The second expedition was sent to the west coast of India. The expedition was led by Friedrich Tietjen. *Captain Bullock observed from the

Celebes sea The Celebes Sea, (; ms, Laut Sulawesi, id, Laut Sulawesi, fil, Dagat Selebes) or Sulawesi Sea, of the western Pacific Ocean is bordered on the north by the Sulu Archipelago and Sulu Sea and Mindanao Island of the Philippines, on the east b ...

, sketching the appearance of the corona, while

Gustav Fritsch Gustav Theodor Fritsch (5 March 1838 – 12 June 1927) was a German anatomist, anthropologist, traveller and physiologist from Cottbus. Fritsch studied natural science and medicine in Berlin, Breslau and Heidelberg. In 1874 he became an ass ...

accompanied an expedition to Aden.

Discovery of helium

French astronomer

Pierre Janssen Pierre Jules César Janssen (22 February 1824 – 23 December 1907), usually known as Jules Janssen, was a French astronomer who, along with English scientist Joseph Norman Lockyer, is credited with discovering the gaseous nature of the solar ...

observed the eclipse from Guntur in

Madras State Madras State was a state of India during the mid-20th century. At the time of its formation in 1950, it included the whole of present-day Tamil Nadu (except Kanyakumari district), Coastal Andhra, Rayalaseema, the Malabar region of North and ...

British India The provinces of India, earlier presidencies of British India and still earlier, presidency towns, were the administrative divisions of British governance on the Indian subcontinent. Collectively, they have been called British India. In one ...

. It was the first total eclipse since

Gustav Kirchhoff Gustav Robert Kirchhoff (; 12 March 1824 – 17 October 1887) was a German physicist who contributed to the fundamental understanding of electrical circuits, spectroscopy, and the emission of black-body radiation by heated objects. He ...

's 1859 theory that the

Fraunhofer line In physics and optics, the Fraunhofer lines are a set of spectral absorption lines named after the German physicist Joseph von Fraunhofer (1787–1826). The lines were originally observed as dark features ( absorption lines) in the optical spect ...

s in the solar spectrum correspond to the emission line of the different chemical elements present in the Sun. Correspondingly, Janssen observed the eclipse with the aid of a

spectroscope An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify mate ...

. He noticed a bright yellow line ( ''λ'' = 587.49 nm) in the spectra of the

solar prominence A prominence, sometimes referred to as a filament, is a large plasma and magnetic field structure extending outward from the Sun's surface, often in a loop shape. Prominences are anchored to the Sun's surface in the photosphere, and extend outw ...

s that could not be due to sodium as had previously been assumed, and was subsequently able to observe the same line even without the need for an eclipse. The same result was found independently by British astronomer

Norman Lockyer Sir Joseph Norman Lockyer (17 May 1836 – 16 August 1920) was an English scientist and astronomer. Along with the French scientist Pierre Janssen, he is credited with discovering the gas helium. Lockyer also is remembered for being the f ...

, and both Janssen's and Lockyer's communications were presented to the

French Academy of Sciences The French Academy of Sciences (French: ''Académie des sciences'') is a learned society, founded in 1666 by Louis XIV of France, Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French Scientific me ...

on October 26, 1868.

King Mongkut's calculation

King Mongkut Mongkut ( th, มงกุฏ; 18 October 18041 October 1868) was the fourth monarch of Siam (Thailand) under the House of Chakri, titled Rama IV. He ruled from 1851 to 1868. His full title in Thai was ''Phra Bat Somdet Phra Menthora Ramathibo ...

, also known as Rama IV of

Siam Thailand ( ), historically known as Siam () and officially the Kingdom of Thailand, is a country in Southeast Asia, located at the centre of the Mainland Southeast Asia, Indochinese Peninsula, spanning , with a population of almost 70 mi ...

, was able to calculate and predict the solar eclipse two years earlier. The calculations were correct as to the place, the time and the type of the solar eclipse that would happen. The eclipse took place precisely as the king had predicted, the total phase lasting six minutes and 46 seconds. In fact, his calculations were better — by about two seconds — than those of the French astronomers, who acknowledged his accuracy. Mongkut was exposed to

malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. S ...

, then developed chills and fever. He died on October 1, 1868. According to the Thai Astronomical Society and NASA, this eclipse is known as "The King of Siam's eclipse".๒๐๐ ปี พระบาทสมเด็จพระจอมเกล้าเจ้าอยู่หัว พระบิดาแห่งวิทยาศาสตร์ไทย

Related eclipses

It is a part of

solar Saros 133 Saros cycle series 133 for solar eclipses occurs at the Moon's ascending node, repeating every 18 years, 11 days, containing 72 events. All eclipses in this series occurs at the Moon's ascending node. Solar Saros 133 is one of the saros series ...

Inex series

Solar Inex series 2013 May 10 * Saros 131: Annular Solar Eclipse of 1810 Sep 28 * Saros 132: Annular Solar Eclipse of 1839 Sep 07 * Saros 133: Total Solar Eclipse of 1868 Aug 18 * Saros 134: Annular Solar Eclipse of 1897 Jul 29 * Saros 135: Annular Solar Eclipse of 1926 Jul 09 * Saros 136: Total Solar Eclipse of 1955 Jun 20 * Saros 137: Annular Solar Eclipse of 1984 May 30 * Saros 138: Annular Solar Eclipse of 2013 May 10 * Saros 139: Total Solar Eclipse of 2042 Apr 20 * Saros 140: Annular Solar Eclipse of 2071 Mar 31 * Saros 141: Annular Solar Eclipse of 2100 Mar 10 * Saros 142: Total Solar Eclipse of 2129 Feb 18 * Saros 143: Annular Solar Eclipse of 2158 Jan 30 * Saros 144: Annular Solar Eclipse of 2187 Jan 09

Notes

References

NASA chart graphics
*
Googlemap
*
NASA Besselian elements

''Observations of the Total Solar Eclipse of August 18, 1868'' by Charles G. Perrins

Drawing of Corona
* {{Commons category, Solar eclipse of 1868 August 18 1868 08 18 1868 in science 1868 08 18 Helium August 1868 events Astronomy in Thailand