Solar eclipse: A strange yellow line appeared during the 1868 phenomenon in India, sparking a mystery that took nearly 27 years to solve and eventually led scientists to helium

In August 1868, astronomers gathered in India to observe a rare total solar eclipse. What began as an effort to study the Sun's atmosphere would lead to one of the most intriguing discoveries in astronomy: evidence of an element that had not yet been found on Earth.

Among the scientists was French astronomer Pierre Jules Janssen. During the eclipse, he directed a spectroscope at the bright solar prominences visible around the Moon's darkened disc. The instrument revealed a strange yellow spectral line that appeared close to the familiar lines produced by sodium, reported TOI.

The line did not exactly match sodium, however, leaving astronomers with a mystery that would take decades to solve.

What astronomers saw during the 1868 eclipse

The total solar eclipse on August 18, 1868, provided astronomers with a rare chance to examine the Sun's prominences using spectroscopy.

According to the study Observations of the total solar eclipse of 18 August 1868 carried out by Jules Janssen at Guntoor, India, spectroscopy allowed scientists to analyse the light from these glowing structures by separating it into individual wavelengths.

Janssen was particularly interested in finding a way to observe solar prominences without having to wait for a total eclipse. His work during the expedition helped establish what later became known as the prominence method.

During the eclipse, observers detected a yellow feature in the spectrum. Its precise position was not established immediately, but it appeared close to the two well-known yellow sodium lines, known as D1 and D2, reported TOI.

The English astronomer Norman Lockyer later studied the observation and determined that the unidentified line did not correspond to sodium. He labelled the feature D3 and noted that it had a slightly shorter wavelength.

Janssen subsequently confirmed that the line was extremely close to the sodium lines but was slightly more refrangible.

Why Janssen was not the discoverer of helium

The 1868 eclipse is often described as the discovery of helium, but that description oversimplifies what actually happened.

Janssen observed the mysterious spectral line, but he did not identify it as helium. At that stage, scientists did not even know whether the line belonged to an element that existed on Earth.

Janssen's major contribution was his work on observing solar prominences. He developed a technique that allowed astronomers to isolate a particular wavelength and create a monochromatic image of a prominence, reported TOI.

The method meant scientists could study these structures without having to wait for another total eclipse. It later influenced the development of instruments such as the spectrohelioscope and spectroheliograph, with astronomers including George Ellery Hale and Henri Deslandres refining the techniques.

Norman Lockyer names the unknown element

The mysterious D3 line continued to puzzle scientists after the eclipse.

Lockyer investigated its origin and worked with chemist Edward Frankland to determine whether it could be associated with hydrogen. Their laboratory experiments failed to reproduce the spectral line under the conditions they tested.

That failure raised a striking possibility: the Sun might contain an element that had never been identified on Earth.

Lockyer eventually named the hypothetical element helium, derived from Helios, the Greek name associated with the Sun.

But helium remained known only through its spectral signature. No one had isolated the element in a laboratory, and there was no terrestrial sample to study, reported TOI.

The episode demonstrated the power of spectroscopy, allowing scientists to identify the fingerprints of substances in the Sun long before they could isolate those substances on Earth.

William Ramsay finally finds helium on Earth

The mystery was not solved until 1895, when Scottish chemist William Ramsay extracted helium from the mineral cleveite.

The discovery confirmed that the element first identified through its spectral signature in the Sun also existed on Earth.

Nearly 27 years had passed since the 1868 eclipse, reported TOI.

The development changed the way Janssen's contribution is understood. He did not claim to have discovered helium, and his most lasting contribution from the 1868 expedition was his work on the observation of solar prominences.

The mysterious D3 line was nevertheless a crucial clue. What began as an unexplained yellow line during an eclipse in India eventually led scientists to recognise helium as a new element — first in the light of the Sun and, decades later, in material found on Earth.