Although the Galileian school still included scholars of the first rank like Evangelista Torricelli  (1608–1647) and Vincenzo Viviani  (1622–1703), the barycentre of European scientific evolution had now shifted to the North. The most important characters for our history had already appeared in the correspondence of Huygens . Through the one with whom our Dutchman had the closest relationship, Leibniz  (1646–1716), we can take up the thread of music which we prefer to follow here. Actually for his model of the solar system, the renowned German philosopher and diplomat had used a term which too few modern astronomers would expect: "circulation harmonique" ["harmonic circulation"].