The great astronomer Johannes Kepler (—) was no exception. His book Harmonice mundi, published in , is considered the last serious attempt to. His search for order in the universe led Johann Kepler () to the five with the publication of Harmonices mundi [Harmony of the Worlds] and noted in. In Johannes Kepler published Harmonices Mundi (The Harmony of the World). The book contains his definitive theory of the cosmos.

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The Greek words for “harmony” harmonia and “number” arithmos both derive from the Indo-European root a ri, recognizable in such English words as rite and rhythm. In Vedic India, rta meant unity or cosmic order. The text relates his findings about the concept of congruence with respect to diverse categories of the physical domain: Here we seek to highlight issues that provide a flavor of his thought.

Plato is credited with discovering that only five three-dimensional, convex solids can be formed using regular convex polygons the so-called Platonic Solids. When a sphere is circumscribed around each shape touching all its corners, the vertices mark off spherical polygons that define the only possible equal divisions of the sphere’s surface area. Earlier, in the Mysterium Cosmographicum Kepler proposed that a nested arrangement of the Platonic Solids determines the spacing between the planetary orbits.

He expanded his investigation of three dimensional geometric shapes, notably of the semi-regular Archimedean Solidsin The Harmony of the World. By the 6th century BCE, the Kep,er discovered that the length of the diagonal of a square can not be expressed by any multiple of its side.

No whole number ratio can express precisely the magnitude of the diagonal with respect to the side. The diagonal’s length can only be approximated by an infinitely repeating fraction. The discovery of these irrational numbers marks a major departure in the history of human thought. Although finite in extension, these numbers are inexpressible or incommensurable meaning they share no common measure with the whole or natural numbers.

The irrationals represent a completely different species of magnitude. The Greeks were careful to use entirely different words to denote mjndi number in Greek, arithmos and a magnitude megethos.

The diagonal of the unit square above produces a square of doubled area. This is true of all diagonals of squares. These diagonals, which are incommensurable magnitudesrepresent kep,er generating powers for squares of doubled area. Kepler disagreed and argued that the magnitudes are essential to understanding the cosmos, and knowable by their relations.

Kepler devised a system harmonkces classifying both commensurable and incommensurable magnitudes into a keplet of “degrees of knowability,” using the circle as a relative unit of measure.

The Mindi philosopher and mathematician Pythagoras of Samos c. Dividing the vibrating string of a musical instrument in the ratio 1: A “fourth,” the difference between do and fa, represents a 3: Today, we recognize that these musical intervals are produced by, in the case of an octave, doubling the rate of vibration harmonicrs a string from vibrations per second to A “fifth,” the hxrmonices between do and solwould be produced by two strings vibrating in the ratio of to Kepler rigorously investigated auditory space through experimentation.

Mundj followers of Pythagoras limited their musical system to the three intervals mentioned above. Kepler sought to determine all of the possible harmonic ratios for sound, and to inquire as to their causes in the domain of geometry and mathematics. Refuting the musical theories of the ancient Greeks, Kepler questioned if there truly were a “unit” or the “one” common to all the harmonic divisions of the string? Did there exist a smallest interval or “common factor” from which each other harmony could be constructed?

Without some system of “tempering” to adjusting for these discrepancies, the “commas” would accumulate on an instrument based on equal steps, such as the piano, throwing off harmonies as one continued up the range.

Vincenzo GalileiGalileo Galilei’s father and a musical theorist, championed a system of equal tuning that “split the difference” the “commas” represented in order to facilitate composition and performance on the piano. Kepler regarded this idea as simplistic and mistaken.

A more elegant system for tuning, he thought, needed to be found.

### The Harmony of the World – Johannes Kepler – Google Books

Just as planetary orbits were not based on perfect circles, the principles underlying harmony could not be reduced to musical “atoms” without sacrificing true consonance. Continuing the search for organizing principles first taken up in the Mysterium Cosmographicumhe asked if the greatest and least distances between a harmonicex and the Sun might approximate any of the harmonic ratios, but found they did not.

He looked at the speed of the planets at the points where they move fastest and slowest, noting that movement represents a better analogue to harmonic vibration than distance. In fact, he found that planets did seem to approximate harmonies with respect to their own orbits. The orbits of Mars, the Earth, and Venus approximated the following harmonies: Walker writes, “Kepler’s celestial harmonies are unique in several respects.

## Kepler, “Harmonices mundi” (Harmonies of the World)

First, they are real but soundless,” whereas the Greek and medieval music of the spheres is “metaphorical. Seeking still more precise harmonies, he examined the ratios between the fastest or slowest speed of a planet and slowest or fastest speed of its neighbors which he calls “converging” and “diverging” motions.

Kepler discovered, he believed, that harmonic relationships structure the characteristics of the planetary orbits individually, and their relationship to one another. Kepler, writing indid not know of the existence of the asteroid belt, in the region of interplanetary space between the orbits of Mars and Jupiter. The asteroid belt would not discovered until The gap in Kepler’s theory of the harmonic theory of the planetary orbits represented by the asteroid belt seems to indicate that he had discovered something consequential about the ordering of our Solar System.

After Kepler sent The Harmony of the World to the printer inhe attained the end of a separate but related, and longstanding, quest: To discover some definitive relationship between the periods of the planets, or the time it takes planets to complete a revolution around the Sun, and their distance from the Sun. From his earliest investigations Kepler knew that the speed of the planets decreases with increasing distance from the Sun, and he sought some rule or principle that connected speed and distance.

Believing as he did that the same immaterial species of universal gravitation produced all planetary motionhe was convinced that a regular relationship must exist. Thus, in the course of his concluding investigations into the harmonic structure of the cosmos, Kepler discovered that the ratio of the square of a planet’s period to the cube of its semi-major axis see below is constant for all orbits.

Today, we know this relationship as Kepler’s Third Law. Many consider it to be one of the most elegant results in all of astronomy.