The aurora borealis of 1737 unveiled

It's the oldest known reproduction of the aurora borealis: this 18th-century black-and-white engraving has just been restored to its original colors thanks to a team from LIRMM, CNRS, the Bibliothèque Universitaire des Sciences and theIUT de Béziers.
"There were columns, jets of light and several sprays 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. Their drawing, the first of its kind, came down to us in the form of a black-and-white engraving in Jean-Jacques Dortous de Mairan's Traité physique et historique de l'aurore boréale (1754). Gone are the shimmering colors of the spectacular phenomenon...

Graphic puzzle

But when Elizabeth Denton looked at this work from the University Science Library's old collection, she made a discovery. The engraver used a graphic code to note the colors: a system of hatchings and dots, used since the Middle Ages in heraldry," describes the head of the BU's heritage department. And where there's a code, there's a computer..."
To restore the colors of the aurora borealis of 1737, all that remained was to entrust the engraving to the computer. " Not so simple," explains William Puech, head of the ICAR project at the Laboratoire d'Informatique, de Robotique et de Microélectronique de Montpellier (LIRMM). "In addition to textures, the engraver had to render shades of grey. The gradations had to be rendered at the same time as the colors. A real graphic enigma to solve!

Long-explained phenomena

The riddle was solved with the help of Gérard Subsol from CNRS, as well as two computer science Master's students and a trainee from the IUT, Antoine Noto. " By passing the image through another color space - YCrCb, which uses the luminance signal (black and white), plus two chrominance informations (blue and red) - and by superimposing several image treatments, we were able to render both the color informations and the intensity differences". The result is the reappearance of an aurora borealis observed almost 300 years ago... A method that could be systematized for other ancient engravings of the same type.
The adventure also fascinates the historian. " In writing his book, an impressive scholarly work compiling 1,441 observations since the 6th century, de Mairan was attempting to shed scientific light on apparitions as impressive as they were unexplained," says Elizabeth Denton. A way of reassuring the public about these phenomena, which were on the increase in the 18th century... and whose explanation would not be confirmed until 2008, thanks to NASA's THEMIS mission.

The aurora borealis, original - Credit: B. Py, B.I.U. Montpellier
The aurora borealis after computer processing - Credit: W. Puech, LIRMM