A total solar eclipse occurred at the Moon's ascending node of orbit on Tuesday, August 18, 1868 (also known as "The King of Siam's eclipse"), with a magnitude of 1.0756. A solar eclipse occurs when the Moon passes between Earth and the Sun, 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 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. Occurring about 6.5 days after perigee (on August 17, 1868, at 22:35 UTC), the Moon's apparent diameter was larger.
The path of totality was visible from parts of modern-day Ethiopia, Eritrea, Djibouti, Yemen, India, the Andaman and Nicobar Islands, Myanmar, Thailand, Cambodia, Vietnam, Malaysia, Brunei, Indonesia, and Papua New Guinea. A partial solar eclipse was also visible for parts of East Africa, the Middle East, Central Asia, South Asia, Southeast Asia, Australia, and western Oceania.
The total solar eclipse of 18 August 1868 was accurately predicted by King Mongkut of Siam, whose calculations were acknowledged as more precise than those of contemporary French astronomers. During observations of the eclipse, Pierre Janssen in India and, independently, Norman Lockyer in England detected a new yellow spectral line in the Sun's chromosphere using spectroscopes, leading to the discovery of the chemical element helium.
Observations
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. 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, sketching the appearance of the corona, while Gustav Fritsch accompanied an expedition to Aden.Standard Encyclopaedia of Southern Africa
Discovery of helium French astronomer Pierre Janssen observed the eclipse from Guntur in Madras State, British India. It was the first total eclipse since Gustav Kirchhoff's 1859 theory that the Fraunhofer lines 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. He noticed a bright yellow line (λ = 587.49 nm) in the spectra of the solar prominences 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, and both Janssen's and Lockyer's communications were presented to the French Academy of Sciences on October 26, 1868...
King Mongkut's calculation
King Mongkut, Rama IV of Siam, predicted and calculated the solar eclipse two years prior. The calculations were correct as to the place, time, and type of the solar eclipse that would occur. His calculations were better — by about two seconds — than those of the French astronomers, who acknowledged his accuracy. Mongkut invited foreign dignitaries and scientists, including Sir Harry Ord, Governor of the Straits Settlements, and the French expedition led by Pierre Janssen that led to the discovery of helium. The event demonstrated Siam's engagement with modern science and diplomacy.
Mongkut was exposed to malaria around this time, developing chills and fever. He died a month and a half after the eclipse, on 1 October 1868. According to the Thai Astronomical Society and NASA, this eclipse is known as "The King of Siam's eclipse".๒๐๐ ปี พระบาทสมเด็จพระจอมเกล้าเจ้าอยู่หัว พระบิดาแห่งวิทยาศาสตร์ไทย.
Eclipse details Shown below are two tables displaying details about this particular solar eclipse. The first table outlines times at which the Moon's penumbra or umbra attains the specific parameter, and the second table describes various other parameters pertaining to this eclipse.
+August 18, 1868 Solar Eclipse Times Event Time (UTC) First Penumbral External Contact 1868 August 18 at 02:34:50.2 UTC First Umbral External Contact 1868 August 18 at 03:27:59.0 UTC First Central Line 1868 August 18 at 03:29:29.6 UTC First Umbral Internal Contact 1868 August 18 at 03:31:00.1 UTC First Penumbral Internal Contact 1868 August 18 at 04:24:13.7 UTC Greatest Duration 1868 August 18 at 05:08:35.7 UTC Ecliptic Conjunction 1868 August 18 at 05:11:42.5 UTC Greatest Eclipse 1868 August 18 at 05:12:09.6 UTC Equatorial Conjunction 1868 August 18 at 05:13:17.9 UTC Last Penumbral Internal Contact 1868 August 18 at 06:00:03.7 UTC Last Umbral Internal Contact 1868 August 18 at 06:53:18.8 UTC Last Central Line 1868 August 18 at 06:54:49.0 UTC Last Umbral External Contact 1868 August 18 at 06:56:19.2 UTC Last Penumbral External Contact 1868 August 18 at 07:49:29.4 UTC
+August 18, 1868 Solar Eclipse Parameters Parameter Value Eclipse Magnitude 1.07561 Eclipse Obscuration 1.15693 Gamma −0.04434 Sun Right Ascension 09h51m00.1s Sun Declination +13°02'06.8" Sun Semi-Diameter 15'48.4" Sun Equatorial Horizontal Parallax 08.7" Moon Right Ascension 09h50m57.4s Moon Declination +12°59'28.9" Moon Semi-Diameter 16'42.8" Moon Equatorial Horizontal Parallax 1°01'20.3" ΔT 2.2 s
Eclipse season
This eclipse is part of an eclipse season, a period, roughly every six months, when eclipses occur. Only two (or occasionally three) eclipse seasons occur each year, and each season lasts about 35 days and repeats just short of six months (173 days) later; thus two full eclipse seasons always occur each year. Either two or three eclipses happen each eclipse season. In the sequence below, each eclipse is separated by a fortnight.
+ Eclipse season of August–September 1868 August 3Descending node (full moon) August 18Ascending node (new moon) September 2Descending node (full moon) Penumbral lunar eclipseLunar Saros 107 Total solar eclipseSolar Saros 133 Penumbral lunar eclipseLunar Saros 145
Related eclipses Eclipses in 1868 A penumbral lunar eclipse on February 8. An annular solar eclipse on February 23. A penumbral lunar eclipse on March 8. A penumbral lunar eclipse on August 3. A total solar eclipse on August 18. A penumbral lunar eclipse on September 2.
Metonic Preceded by: Solar eclipse of October 30, 1864 Followed by: Solar eclipse of June 6, 1872
Tzolkinex Preceded by: Solar eclipse of July 8, 1861 Followed by: Solar eclipse of September 29, 1875
Half-Saros Preceded by: Lunar eclipse of August 13, 1859 Followed by: Lunar eclipse of August 23, 1877
Tritos Preceded by: Solar eclipse of September 18, 1857 Followed by: Solar eclipse of July 19, 1879
Solar Saros 133 Preceded by: Solar eclipse of August 7, 1850 Followed by: Solar eclipse of August 29, 1886
Inex Preceded by: Solar eclipse of September 7, 1839 Followed by: Solar eclipse of July 29, 1897
Triad Preceded by: Solar eclipse of October 17, 1781 Followed by: Solar eclipse of June 20, 1955
Solar eclipses of 1866–1870
The partial solar eclipses on April 15, 1866 and October 8, 1866 occur in the previous lunar year eclipse set, and the solar eclipses on June 28, 1870 (partial) and December 22, 1870 (total) occur in the next lunar year eclipse set.
Solar eclipse series sets from 1866 to 1870 Descending node Ascending node Saros Map Gamma Saros Map Gamma 108 March 16, 1866Partial 1.4241 113 118 March 6, 1867Annular 0.7716 123 August 29, 1867Total −0.7940 128 February 23, 1868Annular 0.0706 133 August 18, 1868Total −0.0443 138 February 11, 1869Annular −0.6251 143 August 7, 1869Total 0.6960 148 January 31, 1870Partial −1.2829 153 July 28, 1870Partial 1.5044
Saros 133
Metonic series All eclipses in this table occur at the Moon's ascending node. 24 eclipse events between March 25, 1819 and August 20, 1906March 25–26January 11–12October 30–31August 18–20June 6–7107109111113115March 25, 1819January 12, 1823October 31, 1826August 18, 1830June 7, 1834117119121123125March 25, 1838January 11, 1842October 30, 1845August 18, 1849June 6, 1853127129131133135March 25, 1857January 11, 1861October 30, 1864August 18, 1868June 6, 1872137139141143145March 25, 1876January 11, 1880October 30, 1883August 19, 1887June 6, 1891147149151153March 26, 1895January 11, 1899October 31, 1902August 20, 1906
Tritos series
Inex series
Notes
References NASA chart graphics Googlemap NASA Besselian elements www.starwrite.org: Solar Astronomy in 1868 Observations of the Total Solar Eclipse of August 18, 1868 by Charles G. Perrins Drawing of Corona