Musica universalis (literally universal music), also called Music of the spheres or Harmony of the Spheres, is an ancient philosophical concept that regards proportions in the movements of celestial bodies--the Sun, Moon, and planets--as a form of musica (the Medieval Latin term for music). This "music" is not usually thought to be literally audible, but a harmonic, mathematical or religious concept. The idea continued to appeal to thinkers about music until the end of the Renaissance, influencing scholars of many kinds, including humanists. Further scientific exploration has determined specific proportions in some orbital motion, described as orbital resonance.
The discovery of the precise relation between the pitch of the musical note and the length of the string that produces it is attributed to Pythagoras. The Music of the Spheres incorporates the metaphysical principle that mathematical relationships express qualities or "tones" of energy which manifest in numbers, visual angles, shapes and sounds - all connected within a pattern of proportion. Pythagoras first identified that the pitch of a musical note is in inverse proportion to the length of the string that produces it, and that intervals between harmonious sound frequencies form simple numerical ratios. In a theory known as the Harmony of the Spheres, Pythagoras proposed that the Sun, Moon and planets all emit their own unique hum based on their orbital revolution, and that the quality of life on Earth reflects the tenor of celestial sounds which are physically imperceptible to the human ear. Subsequently, Plato described astronomy and music as "twinned" studies of sensual recognition: astronomy for the eyes, music for the ears, and both requiring knowledge of numerical proportions.
From all this it is clear that the theory that the movement of the stars produces a harmony, i.e. that the sounds they make are concordant, in spite of the grace and originality with which it has been stated, is nevertheless untrue. Some thinkers suppose that the motion of bodies of that size must produce a noise, since on our earth the motion of bodies far inferior in size and in speed of movement has that effect. Also, when the sun and the moon, they say, and all the stars, so great in number and in size, are moving with so rapid a motion, how should they not produce a sound immensely great? Starting from this argument and from the observation that their speeds, as measured by their distances, are in the same ratios as musical concordances, they assert that the sound given forth by the circular movement of the stars is a harmony. Since, however, it appears unaccountable that we should not hear this music, they explain this by saying that the sound is in our ears from the very moment of birth and is thus indistinguishable from its contrary silence, since sound and silence are discriminated by mutual contrast. What happens to men, then, is just what happens to coppersmiths, who are so accustomed to the noise of the smithy that it makes no difference to them. But, as we said before, melodious and poetical as the theory is, it cannot be a true account of the facts. There is not only the absurdity of our hearing nothing, the ground of which they try to remove, but also the fact that no effect other than sensitive is produced upon us. Excessive noises, we know, shatter the solid bodies even of inanimate things: the noise of thunder, for instance, splits rocks and the strongest of bodies. But if the moving bodies are so great, and the sound which penetrates to us is proportionate to their size, that sound must needs reach us in an intensity many times that of thunder, and the force of its action must be immense.
Musica universalis, which had existed since the Greeks, as a metaphysical concept was often taught in quadrivium, and this connection between music and astronomy intrigued Johannes Kepler, and he devoted much of his time after publishing the Mysterium Cosmographicum (Mystery of the Cosmos) looking over tables and trying to fit the data to what he believed to be the true nature of the cosmos. In 1619 Kepler published Harmonices Mundi (literally Harmony of the Worlds), expanding on the concepts he introduced in Mysterium and positing that musical intervals and harmonies describe the motions of the six known planets of the time. He believed that this harmony, while inaudible, could be heard by the soul, and that it gave a "very agreeable feeling of bliss, afforded him by this music in the imitation of God." In Harmonices, Kepler laid out an argument for a creator who had made an explicit connection between geometry, astronomy, and music, and that the planets were arranged intelligently.
Harmonices is split into five books, or chapters. The first and second books give a brief discussion on regular polyhedron and their congruences, reiterating the idea he introduced in Mysterium that the five regular solids known about since antiquity define the orbits of the planets and their distances from the sun. Book three focuses on defining musical harmonies, including consonance and dissonance, intervals (including the problems of just tuning), their relations to string length, and what makes music pleasurable to listen to. In the fourth book Kepler presents a metaphysical basis for this system, along with arguments for why the harmony of the worlds appeals to the intellectual soul in the same manner as the harmony of music appeals to the human soul. Here he also uses the naturalness of this harmony as an argument for heliocentrism. In book five, Kepler describes in detail the orbital motion of the planets and how this motion nearly perfectly matches musical harmonies. Finally, after a discussion on astrology in book five, Kepler ends Harmonices by describing his third law, which states that for any planet the cube of the semi-major axis of its elliptical orbit is proportional to the square of its orbital period.
In the final book of Harmonices, Kepler explains how the ratio of the maximum and minimum angular speeds of each planet (its speeds at the perihelion and aphelion) is very nearly equivalent to a consonant musical interval. Furthermore, the ratios between these extreme speeds of the planets compared against each other create even more mathematical harmonies. These speeds explain the eccentricity of the orbits of the planets in a natural way that appealed to Kepler's religious beliefs in a heavenly creator.
While Kepler did believe that the harmony of the worlds was inaudible, he related the motions of the planets to musical concepts in book four of Harmonices. He makes an analogy between comparing the extreme speeds of one planet and the extreme speeds of multiple planets with the difference between monophonic and polyphonic music. Because planets with larger eccentricities have a greater variation in speed they produce more "notes." Earth's maximum and minimum speeds, for example, are in a ratio of roughly 16 to 15, or that of a semitone, whereas Venus' orbit is nearly circular, and therefore only produces a singular note. Mercury, which has the largest eccentricity, has the largest interval, a minor tenth, or a ratio of 12 to 5. This range, as well as the relative speeds between the planets, led Kepler to conclude that the Solar System was composed of two basses (Saturn and Jupiter), a tenor (Mars), two altos (Venus and Earth), and a soprano (Mercury), which had sung in "perfect concord," at the beginning of time, and could potentially arrange themselves to do so again. He was certain of the link between musical harmonies and the harmonies of the heavens and believed that "man, the imitator of the Creator," had emulated the polyphony of the heavens so as to enjoy "the continuous duration of the time of the world in a fraction of an hour."
Kepler was so convinced in a creator that he was convinced of the existence of this harmony despite a number of inaccuracies present in Harmonices. Many of the ratios differed by an error greater than simple measurement error from the true value for the interval, and the ratio between Mars' and Jupiter's angular velocities does not create a consonant interval, though every other combination of planets does. Kepler brushed aside this problem by making the argument, with the math to support it, that because these elliptical paths had to fit into the regular solids described in Mysterium the values for both the dimensions of the solids and the angular speeds would have to differ from the ideal values to compensate. This change also had the benefit of helping Kepler retroactively explain why the regular solids encompassing each planet were slightly imperfect.Kepler was convinced "that the geometrical things have provided the Creator with the model for decorating the whole world" and wanted to further explored the aspects of the natural world specifically being involved with astronomical and astrological concepts of music. When Kepler published Harmonices Mundi, Kepler was held liable in a dispute with Robert Fludd, who also published his own harmonic theory at the time. To Kepler, the celestial physics of the spheres were seen as geometrically spatial regions that consisted of each planetary orbit rather than its physical form. 
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The connection between music, mathematics, and astronomy had a profound impact on history. It resulted in music's inclusion in the Quadrivium, the medieval curriculum that included arithmetic, geometry, music, and astronomy, and along with the Trivium (grammar, logic,and rhetoric) made up the seven liberal arts, which are still the basis for higher education today. A small number of recent compositions either make reference to or are based on the concepts of Musica Universalis or Harmony of the Spheres. Among these are Music of the Spheres by Mike Oldfield, Om by the Moody Blues, The Earth Sings Mi Fa Mi album by The Receiving End of Sirens, Music of the Spheres by Ian Brown, and Björk's single Cosmogony, included in her 2011 album Biophilia. Earlier, in the 1910s, Danish composer Rued Langgaard composed a pioneering orchestral work titled Music of the Spheres. Music of the Spheres was also the title chosen for the musical foundation of the video-game Destiny, and was composed by Martin O'Donnell, Michael Salvatori, and Paul McCartney. Paul Hindemith wrote an Opera (1957), and a Symphony using the same music, called 'Die Harmonie der Welt' based upon the life of the Astronomer Johannes Kepler (1571-1630).