18 August 1868

French astronomer Pierre Janssen discovers helium.

Pierre Janssen, a French astronomer, is known for his significant contribution to the field of astronomy and for his role in the discovery of helium. Born on February 22, 1824, in Paris, Janssen’s work was instrumental in advancing our understanding of the Sun and its composition.

In the mid-19th century, Janssen became interested in solar observations and spectroscopy, the study of the interaction between light and matter. In 1868, he traveled to India to observe a total solar eclipse. During this eclipse, he used a spectroscope to analyze the Sun’s spectrum, which is the pattern of colors produced when sunlight is split into its component wavelengths. This technique allowed him to identify the presence of an unknown yellow line in the Sun’s spectrum.

Janssen initially believed that this yellow line was indicative of a new element present in the Sun. He named this new element “helium,” after the Greek word “helios,” which means “Sun.” However, Janssen did not have the means to isolate helium on Earth, and it wasn’t until later that helium was actually discovered as a rare gas on our planet.

Coincidentally, around the same time, the English astronomer Norman Lockyer also observed this yellow line during a solar eclipse and independently identified it as a new element. The discovery of helium is often attributed jointly to Janssen and Lockyer.

It wasn’t until the late 19th and early 20th centuries that scientists were able to isolate and study helium on Earth. Helium was found to be a colorless, odorless, and inert gas, making it valuable for various applications, such as in airships and as a cooling agent in scientific research.

Pierre Janssen continued to make important contributions to astronomy throughout his career. He founded the Meudon Observatory in France and conducted extensive research on solar prominences, the corona, and other solar phenomena. He also played a role in popularizing scientific knowledge and raising public interest in astronomy.

Janssen passed away on December 23, 1907, leaving behind a legacy of groundbreaking research in solar spectroscopy and the discovery of helium’s presence in the Sun. His work laid the foundation for our understanding of the composition and behavior of stars and greatly influenced the field of astrophysics.