"harmony, knowledge of the eternal perfection of the world, and unity"
A heliocentric universe showing the planets' correct distances and the zodiacal signs
Public DomainA heliocentric universe showing the planets' correct distances and the zodiacal signs - Credit: Ebenezer Sibly's Astrology", in A New and Complete Illustration of the Occult Sciences by Ebenezer Sibly [1751-1800

The Greek word harmonia derives from the Indo-European root ri, and means “congruent,” or “fitting together.” In Sanskrit, the root, rta means unity or cosmic order.

As a term harmony, derived from harmonia, is often used in the field of music and the arts in general. In Ancient Greece, the term was used to describe the combination of contrasted elements of higher and lower notes.  In the Middle Ages, harmonia as a term came to be used for two pitches or tones, sounding in combination, and in the Renaissance the concept was expanded to denote three pitches sounding together.

In the year 1619, German scientist and mathematician, Johannes Kepler, wrote “The Harmony of the World (Harmonices Mundi)”, in which he discussed harmony and congruence in geometrical forms and physical phenomena.