Solar eclipse of August 18, 1868


A total solar eclipse occurred at the Moon's ascending node of orbit on Tuesday, August 18, 1868, 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, 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 Siam 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.

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 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, the 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.
EventTime
First Penumbral External Contact1868 August 18 at 02:34:50.2 UTC
First Umbral External Contact1868 August 18 at 03:27:59.0 UTC
First Central Line1868 August 18 at 03:29:29.6 UTC
First Umbral Internal Contact1868 August 18 at 03:31:00.1 UTC
First Penumbral Internal Contact1868 August 18 at 04:24:13.7 UTC
Greatest Duration1868 August 18 at 05:08:35.7 UTC
Ecliptic Conjunction1868 August 18 at 05:11:42.5 UTC
Greatest Eclipse1868 August 18 at 05:12:09.6 UTC
Equatorial Conjunction1868 August 18 at 05:13:17.9 UTC
Last Penumbral Internal Contact1868 August 18 at 06:00:03.7 UTC
Last Umbral Internal Contact1868 August 18 at 06:53:18.8 UTC
Last Central Line1868 August 18 at 06:54:49.0 UTC
Last Umbral External Contact1868 August 18 at 06:56:19.2 UTC
Last Penumbral External Contact1868 August 18 at 07:49:29.4 UTC

ParameterValue
Eclipse Magnitude1.07561
Eclipse Obscuration1.15693
Gamma−0.04434
Sun Right Ascension09h51m00.1s
Sun Declination+13°02'06.8"
Sun Semi-Diameter15'48.4"
Sun Equatorial Horizontal Parallax08.7"
Moon Right Ascension09h50m57.4s
Moon Declination+12°59'28.9"
Moon Semi-Diameter16'42.8"
Moon Equatorial Horizontal Parallax1°01'20.3"
ΔT2.2 s

Eclipse season

This eclipse is part of an eclipse season, a period, roughly every six months, when eclipses occur. Only two eclipse seasons occur each year, and each season lasts about 35 days and repeats just short of six months 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.
August 3
Descending node
August 18
Ascending node
September 2
Descending node
Penumbral lunar eclipse
Lunar Saros 107		Total solar eclipse
Solar Saros 133		Penumbral lunar eclipse
Lunar Saros 145

Related eclipses

Eclipses in 1868

Metonic

Tzolkinex

Half-Saros

Tritos

Solar Saros 133

Inex

Triad

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 and December 22, 1870 occur in the next lunar year eclipse set.

Metonic series

All eclipses in this table occur at the Moon's ascending node.