[tt] NS: Histories: Eclipses solved in ancient Greece

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Histories: Eclipses solved in ancient Greece
http://www.newscientist.com/article.ns?id=mg19826602.100&print=true

11 June 2008

Once phenomena that inspired fear and foreboding, lunar and solar
eclipses can now be predicted down to the second, forecast centuries
into the future, and "hindcast" centuries into the past. The person
who started us down the path from superstition to understanding has
been called the "Einstein of the 5th century BC", and was known to
his contemporaries as "The Mind". He went on trial for his impious
notions, was banished from his adopted home, but nevertheless
influenced generations of later scholars. He was Anaxagoras, a
native of Ionia in what is now Turkey, and the first great
philosopher to live in Athens. Now this little-known scholar is
being seen by some as the earliest known practitioner of the
scientific method.

NICIAS was in a quandary. It was the summer of 413 BC, the 19th year
of the Peloponnesian war, and the Athenian army he commanded was
many stadia from home, camped in a marsh near the Sicilian city of
Syracuse. His campaign of shock and awe - forced on him by
demagogues in the Athenian government - had turned into a protracted
siege. And now the siege was failing as reinforcements from all over
the Hellenic world arrived to help the beleaguered city. With hope
of victory rapidly vanishing, Nicias cut his losses and decided to
sail for home.

On the night of 27 August, the historian Thucydides wrote, "All was
at last ready, and they were on the point of sailing away when an
eclipse of the moon, which was then at the full, took place...
Nicias, who was somewhat over-addicted to divination and practices
of this kind, refused even to take the question of departure into
consideration, until they had waited the thrice nine days prescribed
by the soothsayers."

The delay proved disastrous. Syracuse, under the command of the
Spartan general Gylippus, seized the opportunity to block the
harbour entrance, trapping the Athenian fleet. Nicias was forced to
surrender. According to Thucydides, the defeat was "the most
calamitous" in Greek history.

Had Nicias and his fellow citizens cast their minds a little wider,
it needn't have turned out that way. The Athenians had been given a
perfectly good explanation of eclipses 65 years earlier. Anaxagoras
of Clazomenae - who lived in Athens - had tried to persuade them
that an eclipse was a purely physical phenomenon and had nothing to
do with the gods.

No document survives to tell us how Anaxagoras arrived at his
revolutionary insight. But philosopher Daniel Graham of Brigham
Young University in Utah argues that Anaxagoras witnessed a solar
eclipse that was visible from Athens in 478 BC and put this together
with a then-novel theory that the moon was opaque to conclude that
the moon could cast a shadow on Earth.

People had seen and recorded eclipses for centuries before
Anaxagoras, of course. What made his observations different, Graham
says, was that he actively collected data to test his notions about
what caused eclipses, adopting what today we would call the
scientific method. Clues lie scattered in Anaxagoras's own writing
and in computer simulations of the eclipse by Graham's colleague,
physicist Eric Hintz.

In 478 BC, Anaxagoras was 22 years old, and recently arrived in
Athens. He had clearly read the work of Parmenides, a Greek
philosopher who flourished in Italy around 490 BC. Parmenides wrote
that the moon is illuminated by "borrowed light": in other words,
the light of the sun. It follows that if the borrowed light goes out
- if, for instance, Earth blocks the sunlight - the moon will be
extinguished. This would account for lunar eclipses. Parmenides's
idea also implied that the moon was opaque, because we do not see
the sun shining through it at new moon. It took only a small leap of
imagination to realise that the opaque moon could also cause the
sun's light to fail - and so create a solar eclipse.

There is no evidence that Parmenides made these deductions, but
apparently Anaxagoras did. Although his writings are now lost, they
were available to scholars in the Hellenic world for several
centuries. Most agree that Anaxagoras explained the cause of
eclipses. But none says how.

Before 478 BC, the idea of a solid, opaque moon casting a shadow on
Earth could only be considered a hypothesis. The essence of the
modern scientific method is to test a hypothesis against
observations. And to a degree unique among Greek philosophers, that
was also Anaxagoras's method. The few quotes of his that have been
preserved attest to this. "Phenomena are a view of that which is
unseen," he said, in his cryptic fashion. In other words, to
understand things you can't see, observe carefully the things you
can. Forget gods and superstitions.

The eclipse of 478 BC gave Anaxagoras the chance to put his theory
to the test. If the moon casts a shadow on Earth, then that shadow
should have a finite size. The idea that some people would see the
eclipse and some wouldn't was completely novel. "All the earlier
Greek theories would have predicted that everyone would see the
eclipse," Graham says.

The clue that Anaxagoras deliberately set out to find the size of
the moon's shadow lies in another fragment of his teaching, which on
the face of it has nothing to do with eclipses. Four ancient writers
mention that Anaxagoras compared the moon and Sun to the
Peloponnesian peninsula. The moon was "as large as the Peloponnese",
Anaxagoras said, and the sun was "greater than the Peloponnese". But
why the Peloponnese?

The comparison makes perfect sense when you plot the track of the
annular eclipse of 478 BC, as Hintz and Graham did (APEIRON, vol 40,
p 319). The antumbra, the darkest part of the moon's shadow during
the eclipse, passed exactly over the Peloponnese. People in this
region would have seen an entire black disc in front of the sun,
blocking out 95 per cent of the sun's light - an unforgettable
sight. People outside the antumbra would have seen only a dimming of
the sun. "All Anaxagoras had to do was go down to the port of
Piraeus and ask some sailors what they saw last week or last month,"
says Graham. Because the moon's shadow roughly covered the
Peloponnese, it makes sense that Anaxagoras thought the moon was the
same size as the peninsula.

In order to ask those questions, though, Anaxagoras first had to
have the right hypothesis. "Only someone who already had the idea of
a limited area [of visibility] would go out and ask where the
eclipse was visible," says Graham. "To me, that's real research."

The idea that Anaxagoras witnessed an actual eclipse has been
proposed before. In 1973, David Sider of New York University argued
that Anaxagoras must have seen the total eclipse of 463 BC. But that
eclipse passed over north-west Greece, so Anaxagoras would have had
no reason to compare its shadow with the Peloponnese. Sider is now
convinced that Graham is right. The most important implication, he
says, is not the date but the fact that "Anaxagoras treated the
whole phenomenon the way a scientist would".

Nevertheless, the date adds some detail to our picture of
Anaxagoras. It makes him a 22-year-old wunderkind when he proposed
his theory, not a well-established 37-year-old sage. "He was kind of
the Einstein of the 5th century BC," says Graham. Anaxagoras later
became the personal teacher of Pericles, the leader who took Athens
to the height of its power.

Unfortunately, Anaxagoras's free-thinking ideas eventually led to
his downfall. At an unknown date, he was brought to trial on a
charge of impiety, for teaching that the moon and sun were physical
objects and not gods. According to one account, Anaxagoras was fined
5 talents and exiled from Athens. Another said he had already left
Athens and was sentenced to death in absentia, for sympathy with
Athens's enemy, the Persians. Two say that Pericles rescued him,
bringing him to court so "tottering and emaciated with disease" that
he was freed out of pity. Whatever the circumstances of his
departure from Athens, Anaxagoras's name became synonymous in Athens
with scandalous and forbidden thoughts. In his Ionian homeland,
however, he was a national hero.

We know now that many of Anaxagoras's ideas were wrong: the moon is
bigger than the Peloponnese, for instance. Nevertheless, his theory
of eclipses has stood the test of time as a model of scientific
reasoning. If only the Athenians had listened, Nicias and his army
might have made it home.

Weblinks

Apeiron journal
http://www.utexas.edu/cola/depts/philosophy/apeiron/
Nasa's eclipse page
http://eclipse.gsfc.nasa.gov/eclipse.html
The 2008 solar eclipse
http://eclipse.gsfc.nasa.gov/SEmono/TSE2008/TSE2008.html
Watch an annular eclipse
http://www.nmm.ac.uk/rog/2008/02/eclipses_in_february_2008.html