A TRAWL through two ancient texts written almost a thousand years ago has
unearthed a link between sunspots and an aurora sighted at roughly the same time
on opposite sides of the Earth. The link shows that solar activity hit a peak in
1128, and lends credence to the observations of medieval astronomers.
Richard Stephenson, a physicist from the University of Durham, noticed a
picture of sunspots in The Chronicle of John of Worcester, a diary kept
by an English archivist in the Middle Ages. The drawing shows the Sun with two
black dots encircled with brown and red rings. The entry is dated 8 December
1128, making it the oldest known drawing of sunspots.
Hearing about Stephenson’s discovery, a solar physicist from the University
of Warwick, David Willis, then went in search of any reports of the Northern
Lights from that time. Such displays of the aurora borealis often follow periods
of intense solar activity, which are marked by sunspots. In a Korean chronicle
called Koryo-sa he found written accounts of a red aurora on 13
December. The lag of five days is typical, he says. It takes about that time for
charged solar particles to reach Earth.
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The link helps confirm the accuracy of both the records, says Willis, so
other bits of astronomical information buried in their pages can also be
trusted. This means astronomers can pin down events to specific days. It’s
possible to track solar activity by looking at tree rings, but this can only
average the Sun’s behaviour over six months.
However, the two researchers aren’t confident that they will glean much
information from the picture itself. “We don’t know how much artistic licence
was involved,” says Willis. Apparently, the spots have “teeth” around them,
which don’t show up with modern telescopes.