The earliest record of an astronomical event observed in Japan has left an important mark on Japanese history and classical literature. The event dates back to December 30, 620 AD, and has been a mystery to scientists for centuries.
Partially mythical, partly factual, Nihon shoki is Japan’s oldest official history, describing the country’s origin and subsequent development until the end of the 7th century. It contains a description of a highly unusual astronomical event that scientists have been unable to explain.
The strange light appeared on December 30, 620 AD, when “red spirit” appeared in the form of a “long and long” pheasant tail. The chronicle also uses two words in the description that correspond, in other historical texts, to a type of comet and auroras.
On the one hand, comets are not known to produce spectacles of red lights in the sky. On the other hand, the northern lights (aurora borealis) do not usually resemble a pheasant’s tail.
A team of scientists from Sokendai University in Japan studied several aspects of the issue, such as astronomical, geophysical, linguistic and even ornithological features, before concluding that the event observed in Japan in the 7th century was the first northern lights ever documented. The study’s findings were published in March in the scientific journal Sokendai Review of Cultural and Social Studies.
Normally, auroras do not resemble a bird’s tail and rarely have a reddish color, something that made it difficult to identify. Ryuho Kataoka, the study’s first author, noted in a statement that “recent discoveries have shown that auroras can be shaped like a pheasant’s tail” and that they adopt it “specifically during major magnetic storms.”
However, the shape of this aurora – which today would be described as a fan – is not the most concrete detail of the chronicle. The team considered that the described “long length” is roughly equivalent to an angular distance of 10 degrees, which would be the width of the old Japanese aurora.
In addition, the researchers compiled a map of the Earth’s magnetic field in the year 620 and estimated that the observation point was then 33º latitude, instead of the current 25º. At that time, it would be easier to see the northern lights from Japan, because it was closer to the North Magnetic Pole.
The northern lights are caused by charged particles from the Sun that collide with the Earth’s atmosphere. As these particles interact with different elements, they produce different colors of light – red being a possibility. Usually, the display is visible only in the Arctic and Antarctic circles, which are located about 66.5º north and south of the Equator, although they are occasionally seen in non-polar regions.