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.

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