Jules Janssen was a 19th-century umbraphile: He traveled the world to witness solar eclipses. During one such event in 1868 in India, he saw something that would help to shape the understanding of both the Sun’s inner workings and the periodic table. When he sent the highly attenuated sunlight through his spectrometer, he found a yellow line that no one had ever seen before. It turned out to be an atomic emission from an as-yet undiscovered element: helium.
The discrete colors of light absorbed and emitted by atoms have long been integral to our understanding of the physical world. A few decades after Janssen’s discovery, the pioneers of quantum mechanics looked to atomic energy levels, deduced from their spectra, to work out the laws that govern the dynamics of electrons in atoms. And modern atomic clocks, the best of which waver by less than a second over the age of...
