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.
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".
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.
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.
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
This eclipse is a member of a semester series. An eclipse in a semester series of solar eclipses repeats approximately every 177 days and 4 hours (a semester) at alternating nodes of the Moon's orbit.
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.
Saros 133
This eclipse is a part of Saros series 133, repeating every 18 years, 11 days, and containing 72 events. The series started with a partial solar eclipse on July 13, 1219. It contains annular eclipses from November 20, 1435 through January 13, 1526; a hybrid eclipse on January 24, 1544; and total eclipses from February 3, 1562 through June 21, 2373. The series ends at member 72 as a partial eclipse on September 5, 2499. Its eclipses are tabulated in three columns; every third eclipse in the same column is one exeligmos apart, so they all cast shadows over approximately the same parts of the Earth.
The longest duration of annularity was produced by member 25 at 1 minutes, 14 seconds on November 30, 1453, and the longest duration of totality was produced by member 61 at 6 minutes, 50 seconds on August 7, 1850. All eclipses in this series occur at the Moon’s ascending node of orbit.
Metonic series
The metonic series repeats eclipses every 19 years (6939.69 days), lasting about 5 cycles. Eclipses occur in nearly the same calendar date. In addition, the octon subseries repeats 1/5 of that or every 3.8 years (1387.94 days). All eclipses in this table occur at the Moon's ascending node.
Tritos series
This eclipse is a part of a tritos cycle, repeating at alternating nodes every 135 synodic months (≈ 3986.63 days, or 11 years minus 1 month). Their appearance and longitude are irregular due to a lack of synchronization with the anomalistic month (period of perigee), but groupings of 3 tritos cycles (≈ 33 years minus 3 months) come close (≈ 434.044 anomalistic months), so eclipses are similar in these groupings.
Inex series
This eclipse is a part of the long period inex cycle, repeating at alternating nodes, every 358 synodic months (≈ 10,571.95 days, or 29 years minus 20 days). Their appearance and longitude are irregular due to a lack of synchronization with the anomalistic month (period of perigee). However, groupings of 3 inex cycles (≈ 87 years minus 2 months) comes close (≈ 1,151.02 anomalistic months), so eclipses are similar in these groupings.