From Ancient Roman Omens, New Data on Solar Activity A modern analysis of historical texts sheds new light on the seasons of the sun |
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| Enlarge ImageThis image of the Sun in extreme ultraviolet light, as viewed by the Solar and Heliospheric Observatory, shows bright active regions and a prominence—signs that the Sun may be ready to blast aurora-causing energy and particles at the Earth. Some researchers believe that solar activity may have an impact on global climate. (Courtesy of the European Space Agency and NASA) |
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| Enlarge ImageAuroras, or northern lights, were viewed by the Romans and other ancient peoples as omens. Records of great auroras from 2000 years ago have become a modern data set for space scientists and climate researchers. (Courtesy of Jan Curtis) |
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| Enlarge ImageA plot of the number of sunspots (yellow curve) and of geomagnetic storms – which provoke auroras (red curve) – reveals how the cycle of auroral activity lags behind the solar cycle, but keeps the same 11-year pace. (Courtesy of Joe Allen/SCOSTEP) |
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The ancient Romans looked to the heavens for signs of what
might happen on Earth. According to historical documents, an aurora in the
shape of horses and soldiers was said to appear in the sky in 44 BC,
foretelling the fall of Julius Caesar. Another celestial event 114 years later
presaged the sack of Jerusalem by Titus Vespasian.
Because auroras, comets, and meteors were omens, the Romans
and other ancient peoples observed the sky fastidiously, recording heavenly
observations alongside detailed accounts of earthly events and military
triumphs. Twenty centuries later, those historical records have become
scientific data for researchers such as WHOI Senior Scientist Andy Solow.
Auroras, or “northern lights,” are caused by eruptions and
flares from the sun that stir up Earth’s magnetic field and atmosphere. These
heavenly fireworks coincide with the appearance of sunspots on the face of our
star. Three centuries of modern observations have shown that these spots and
storms occur in a rhythmic pattern, with the number of sunspots waxing and
waning in an 11-year cycle. But with only 300 years of observations, scientists
can’t say whether this cycle is recent or abiding in the life of a
5-billion-year-old sun.
In the late 1970s, astronomer Richard Stothers of NASA’s
Goddard Institute for Space Studies analyzed classical writings from 467 BC to
333 AD, particularly Livy’s history of Rome. He found dates and descriptions of
celestial events that sound much like “great auroras”—northern lights that
extend to middle latitudes (including Rome) during intense periods of solar
activity. Stothers uncovered an 11-year cycle of solar activity at work in
ancient times.
Some years later, Solow read Stothers’ paper and felt
compelled to do another statistical analysis. He filed the problem away for
another day, hoping to come back to it. In late 2003, he found the paper in his
office, rekindling the urge.
“To me, data are data,” said Solow, who is trained in
mathematics and statistics and leads the WHOI Marine Policy Center. “Research
is driven by curiosity, and interesting statistical problems come up in all
kinds of fields.”
Solow applied some new statistical methods to the dates
Stothers gathered from ancient Rome. In a paper published in Earth and
Planetary Science Letters in April
2005, Solow confirmed the 11-year solar activity cycle seen by Stothers.
But he went further, finding that the cycle of solar activity and resulting
auroras was bimodal—that is, the ancient cycle had two peaks. Solow’s finding
jibes with modern observations that auroras occur in an 11-year cycle, but the
peaks of activity tend to lag behind the peaks of the sunspot cycle (see third
image at right).
For space physicists and earth scientists,
establishing the stability of the 11-year pattern is important because solar
variability has been linked to climate changes in the past. By finding both the
solar and auroral cycles, Solow has provided twice as much confidence that the
ancient solar cycle mirrored the modern one.
— Mike Carlowicz
Posted: June 24, 2005 [top] |
