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.[1]
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 eclipse was predicted more precisely by King Mongkut of Thailand than it was by French astronomers. The eclipse allowed for the discovery of helium by both Pierre Janssen and Norman Lockyer, who observed Solar prominences with spectroscopes.
Several expeditions were sent to observe the eclipse.
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.[6] [7]
King Mongkut, also known as Rama IV of Siam, 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, then developed chills and fever. He died on October 1, 1868.[8] According to the Thai Astronomical Society and NASA, this eclipse is known as "The King of Siam's eclipse".[9] [10]
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.[11]
| 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 |
| 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 |
See also: Eclipse cycle. 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.
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, 1866 Partial | 1.4241 | 113 | ||||
| 118 | March 6, 1867 Annular | 0.7716 | 123 | August 29, 1867 Total | −0.7940 | ||
| 128 | February 23, 1868 Annular | 0.0706 | 133 | August 18, 1868 Total | −0.0443 | ||
| 138 | February 11, 1869 Annular | −0.6251 | 143 | August 7, 1869 Total | 0.6960 | ||
| 148 | January 31, 1870 Partial | −1.2829 | 153 | July 28, 1870 Partial | 1.5044 | ||
All eclipses in this table occur at the Moon's ascending node.
