Music of the Irregular Spheres

This blog post is contributed by Mark McCaughrean, senior science advisor in the Directorate of Science and Robotic Exploration at ESA. The rather tortured title of this post is intended as a humorous reference to the decidedly non-spherical shape of Comet 67P/Churyumov-Gerasimenko, the target of our Rosetta mission. It also refers to the philosophical concept of ‘musica universalis’ or ‘music of the spheres’, usually attributed to Pythagoras, Ptolemy, and other ancient philosophers and scientists. They suggested that the proportions of the distances of the celestial bodies in the Solar System thought to be orbiting the Earth could be assigned tones, much as the pitch of a note coming from a plucked string is related to the length of that string. This theory was later developed further by Johannes Kepler in his works ‘Mysterium Cosmographicum’ (1596) and ‘Harmonices Mundi’ (1619). In these, Kepler proposed that a series of nested regular polygons provided the basis for the proportions observed in the heavens in a Copernican, Sun-centred Universe, and that the geometrical properties of these polygons could be linked to the ‘music of the spheres’. Kepler’s work on Tycho Brahe’s astronomical data later led him to his famous laws of planetary motion, which describe the elliptical orbits of objects around the Sun; astronomers would later realise that they even apply to the very elliptical orbits of comets like 67P/C-G. It's interesting to note that one of Kepler’s formative astronomical experiences was seeing the Great Comet of 1577 when he was just six years old. (To read a brief history of comets, please see our earlier blog post "Chasing comets - across history"). In the theory of the ‘music of the spheres’, the sounds are not supposed to be directly audible. That provides a nice link to the now famous “Singing Comet” as presented […]



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