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The history of science
Kepler's world
Aug 13th 2009 | RIO DE JANEIRO
From The Economist print edition
Celebrating the work of a neglected astronomer
MUCH has been made of the 400th anniversary this year of Galileo pointing
a telescope at the moon and jotting down what he saw (even though this
had previously been accomplished by an Englishman, Thomas Harriot,
using a Dutch telescope). But 2009 is also the 400th anniversary of the
publication by Johannes Kepler, a German mathematician and astronomer,
of “Astronomia Nova”. This was a treatise 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 establishing 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.
Science Photo Library
Kepler moved it elliptically
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, Tycho Brahe, a
Danish astronomer. 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” (he coined the term) 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, of Imperial College, London, this week told astronomers and historians at the International
Astronomical Union meeting in Rio de Janeiro that it was the rotation of the sun, as seen by Galileo and
Harriot as they watched sunspots moving across its surface, 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 endeavour. Kepler, however, 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 a quasi magnetism accounted for the exact elliptical path. (Newton did
not propose his theory of gravity for almost another 80 years.) 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.
Copyright © 2009 The Economist Newspaper and The Economist Group. All rights reserved.
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