Astronomers have watched sunspots come and go on the sun’s surface for at least 400 years. But to learn about the history of the sun’s activity before the time of telescopes, they have to turn to historical references to phenomena linked to solar activity, like the northern lights.

Now, a team of scientists have discovered what may be the oldest written records of aurorae to date. These three Assyrian and Babylonian cuneiform tablets from about 680 to 650 B.C. seem to refer to aurorae lighting up the skies. Chemical analysis of tree rings also show that there was likely a spike in solar activity around that time. This extends the known timespan of aurora records to about 2,700 years, as the team reported earlier this month in The Astrophysical Journal Letters.

“Historical records of aurorae improve our understanding of the history of solar activity,” study author Hisashi Hayakawa said via email. He's an astrophysicist at Osaka University in Japan and a visiting researcher at the Rutherford Appleton Laboratory in the U.K.

Digging for Deeper Sun Records

Before the new findings, the oldest known record of an aurora was a Babylonian tablet from 567 B.C. referring to a red glow flaring in the sky. Babylonian and Assyrian astrologers from this era commonly recorded celestial events to interpret them as omens. So, Hayakawa and his team decided to search tablets from the seventh century B.C. to find any earlier reports.