One of the oldest cold cases in the world — the mystery of a plume of light that burst over the skies of Japan — has finally been solved.

You'll be forgiven if you don't remember the strange phenomenon. It took place in the year 620, long before celestial phenomena could be photographed and shared on social media.

(It's also the reason why the image you see in this post is an approximation of what it looked like.)

Still, long after it painted the sky an eerie red, the "red sign" — as historical records described it — remained the subject of heated scientific inquiry. What exactly was that lingering burst of spectacular light? And why was it shaped, as records suggest, like a pheasant's tail, complete with dazzling feathers stretching across the sky?

"It is the oldest Japanese astronomical record of a 'red sign,'" Ryuho Kataoka, a researcher at Japan's National Institute of Polar Research notes in a statement. "It could be a red aurora produced during magnetic storms. However, convincing reasons have not been provided, although the description has been very famous among Japanese people for a long time."

Back in the day, according to records, the only thing stargazers could agree upon was that this couldn't be good. No deity would ever paint the sky blood red as a positive sign.

As time went on, the discussion became somewhat more scientific. Was it an aurora? A comet?

Recently, however, Kataoka, along with colleagues at the National Institute of Polar Research conducted a rigorous analysis of the pheasant's tail to determine once and for all whether it was a comet, an aurora or sky-scrawl from an angry god.

Their work, published this month in the Sokendai Review of Cultural and Social Studies, indicates Japan experienced a rare kind of aurora on Dec. 30, 620 — the kind that really did look like the blazing backside of a pheasant.

For their study in scarlet, researchers combed through historical accounts of the red sign, comparing its features to those of auroras. For one thing, red isn't a typical hue for auroras. These electrically charged particles entering the Earth's atmosphere usually manifest in green and yellow. But they're also known to appear pink, blue, and, yes, even red.

Researchers also noted other, more recent auroras that somewhat resembled a pheasant's tail. And finally, they developed a historical magnetic field — a key factor in determining where auroras are seen.

Japan, in the early seventh century, would have been around 33 degrees of magnetic latitude, which is the angular distance between a region and the magnetic equator. That's a substantial drift from its current perch at 25 degrees. All signs pointed to an interesting aurora.

"Recent findings have shown that auroras can be 'pheasant tail' shaped specifically during great magnetic storms," Kataoka explains. "This means that the 620 A.D. phenomenon was likely an aurora."

One of Japan's most spectacular ancient sky mysteries has been solved
Researchers pored over historical accounts to crack the case of a 1,400-year-old light show.