The Northern Lights of 1737 Revealed
This is the oldest known depiction of the aurora borealis: this 18th-century black-and-white engraving has just been restored to its original colors thanks to a team comprising the LIRMM, the CNRS, the University Science Library, andthe Béziers University Institute of Technology (IUT).
“There were columns, jets of light, and several bursts of rays…”. On December 16, 1737, astronomers in Montpellier described a fantastic curtain of colors. An exceptional phenomenon observed in Paris and as far away as Italy. While their drawing—the first of its kind—has survived to this day, it exists only as a black-and-white engraving included in Jean-Jacques Dortous de Mairan’s *Traité physique et historique de l’aurore boréale* (1754). Gone are the shimmering colors of this spectacular phenomenon…
Graphic Puzzle
But while poring over this volume from the old collection of the University Science Library, Elizabeth Denton made a discovery. “The engraver used a graphic code to represent colors: a system of hatching and dots, used in heraldry since the Middle Ages,” explains the head of the university library’s heritage department. “And where there’s a code, there’s computer science…”
To restore the colors of the 1737 aurora borealis, all that remained was to entrust the engraving to a computer. “Not so simple!” explains William Puech, head of the ICAR project at the Montpellier Laboratory of Computer Science, Robotics, and Microelectronics (LIRMM). “In addition to the textures, the engraver took great care to render the shades of gray. So we had to recreate the gradients along with the colors. It was a real graphic puzzle to solve!”
Phenomena That Remained Unexplained for a Long Time
The mystery was solved with the help of Gérard Subsol of the CNRS, as well as two master’s students in computer science and an intern from the IUT, Antoine Noto. “By converting the image to a different color space—YCrCb, which uses the luminance signal (black and white) plus two chrominance components (blue and red)—and by layering several image processing steps, we were able to restore both the color information and the differences in intensity.” And to see a northern lights display observed nearly 300 years ago reappear… A method that could be applied systematically to other ancient engravings of the same type.
The project is also sure to fascinate historians. “In writing his book—an impressive scholarly work compiling 1,441 observations dating back to the 6th century—de Mairan sought to shed scientific light on phenomena as striking as they were unexplained,” says Elizabeth Denton. It was a way to reassure the public about these phenomena, which were on the rise in the 18th century… and for which an explanation would not be confirmed until 2008, thanks to NASA’s THEMIS mission.
