The Planets by Dava Sobel Penguin, HK$101 About 2,600 years ago, Pythagoras deduced that 'the cosmic order obeyed the same mathematical rules and proportions as the tones on a musical scale', says Dava Sobel in her delightful book of essays The Planets. Plato celebrated the 'music of the spheres' in The Republic and perceived 'celestial harmony'. But it was English composer Gustav Holst who created between 1914 and 1916 'the only known example of a symphonic tribute to the solar system'. Haydn and Mozart (whose music is on the Voyager, which left our solar system a while back) played with planets, but Holst's Opus32, The Planets and its seven movements captured them all on Earth. It's an excellent accompaniment to Sobel's fascinating and agreeable musings that ultimately remind how small, and fragile, Earth is in the cosmic scheme of things, how important the planets are to culture and history, and how far science has come. Jupiter, once deified, turns out to be a mini-system all its own, managing a ballet of 60-plus planet-like moons. Holst loved Saturn, and devoted 9:21 to this 'vision of fulfilment'. Sobel has a thing for them all.