Peering at Mars

Abstract

From his home on the island of Rhodes in the Aegean, Hipparchus, the greatest of the ancient Greek astronomers, compiled a catalogue of the positions and motions of the objects in the sky. He interpreted his observations as meaning that Earth was at the centre of everything, the planets circled around it, and the stars were fixed. Claudius Ptolemaeus, a fellow Greek living in Alexandria in Egypt a century later, observed that the planets did not precisely follow their predicted paths. However, since the circle was considered to be perfect, he proposed an ‘epicycle’ scheme in which, as each planet progressed around its orbit, it simultaneously traced a small circle around its mean position. Having studied mathematics at the University of Cracow, Nicolaus Copernicus realised in 1507 that the complexity of the epicycle scheme could be eschewed if it was assumed that the planets revolved around the Sun. Although Copernicus worked out his heliocentric theory in detail, it was not published until after his death in 1543, in the form of the book De Revolutionibus Orbium Coelestium.