專四晨讀美文：Kepler's World

Kepler's World
Much has been made of the 400th anniversary this year
of Galileo pointing a telescope at the moon
and jotting down what he saw.
But 2009 is also the 400th anniversary
of the publication by Johannes Kepler,
a German mathematician and astronomer,
of "Astronomia Nova".
This was a book that contained an account of his discovery
of how the planets move around the sun,
correcting Copernicus's own more famous
but incorrectly formulated description of the solar system.
And it established the laws for planetary motion
on which Isaac Newton based his work.
Four centuries ago the received wisdom was that of Aristotle,
who asserted that the Earth was the centre of the universe,
and that it was encircled by the spheres of the moon,
the sun, the planets and the stars beyond them.
Copernicus had noticed inconsistencies in this theory
and had placed the sun at the centre,
with the Earth and the other planets travelling around the sun.
Some six decades later
when Kepler tackled the motion of Mars,
he proposed a number of geometric models,
checking his results against the position of the planet
as recorded by his boss.
Kepler repeatedly found that his model
failed to predict the correct position of the planet.
He altered it and, in so doing,
created first egg-shaped "orbits" and,
finally, an ellipse with the sun placed at one focus.
Kepler went on to show that an elliptical orbit
is sufficient to explain the movement of the other planets
and to devise the laws of planetary motion that Newton built on.
A.E.L. Davis this week told astronomers and historians
that it was the rotation of the sun that provided Kepler
with what he thought was one of the causes
of the planetary motion that his laws described,
although his reasoning
would today be considered entirely wrong.
In 1609 astronomy and astrology
were seen as intimately related;
mathematics and natural philosophy,
meanwhile, were quite separate areas of endeavor;
however, Kepler sought physical mechanisms
to explain his mathematical result.
He wanted to know how it could be that
the planets orbited the sun.
Once he learned that the sun rotated,
he comforted himself with the thought
that the sun's rays must somehow sweep the planets around it
while some magnetism accounted for the exact elliptical path.
As today's astronomers struggle to determine
whether they can learn from the past,
Kepler's tale provides a salutary reminder
that only some explanations stand the test of time.