The work of Nicolaus Copernicus

Abstract

It is commonly supposed that Copernicus placed the Sun at the centre of the solar system, letting the planets move around the Sun in simple circles, thereby recovering the theory of Aristarchus of Samos. This popular view is quite wrong, as it will be the purpose of this lecture to show. The six planets known to Copernicus have the eccentricity values given in the following table: planet eccentricity major-axis (in terms of Earth) Mercury 0.2056 0.387 Venus 0.0068 0.723 Earth 0.0167 1.000 Mars 0.0933 1.524 Jupiter 0.0484 5.203 Saturn 0.0558 9.539 The departures from simple circles are particularly serious for Mercury and Mars. If the eccentricity is ignored for these planets it will be found that the predictions of a simple circular model are in very serious disagreement with observation. The errors for Mars in the worst circumstances would be more than 15°, errors so gross as to have been unacceptable to astronomers even 1500 years before Copernicus. Ptolemy, working between A. D. 100 and 150, developed a geocentric theory which reduced the discrepancies by more than an order of magnitude - in the case of Mars, from more than 15° to about 1°. It was this far more sophisticated theory of Ptolemy which Copernicus had to match in his heliocentric theory.