Kepler's world

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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.

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.

Aug 13th 2009 | RIO DE JANEIRO
From The Economist print edition

http://www.astronomy2009.com.br