Origins
The English adjective 'eccentric' began its career not as a description of quirky human behavior but as a precise term in mathematical astronomy. Its journey from the geometry of planetary orbits to the psychology of unconventional personalities is a striking example of how scientific vocabulary can be metaphorically appropriated for human description.
The word enters English in the 1550s from Medieval Latin 'eccentricus,' borrowed from Greek 'ekkentros' (ἔκκεντρος), meaning 'out of the center' or 'not having the same center.' The Greek compound joins 'ek-' (out of, away from) with 'kentron' (center). Greek 'kentron' originally meant 'a sharp point' — specifically the pointed leg of a compass, the fixed point around which a circle is drawn. From this concrete meaning, it generalized to 'center' in the geometric and then the ordinary sense.
In ancient and medieval astronomy, 'eccentric' described a geometric model used to explain the apparently irregular motions of the planets. In the Ptolemaic system, each planet was imagined to move on a circular path (an epicycle) whose center was itself moving along a larger circle (the deferent). When the center of this larger circle did not coincide with the Earth, the orbit was called 'eccentric' — off-center. This model, refined by Ptolemy in the second century CE, remained the dominant astronomical framework for nearly 1,400 years.
Figurative Development
The application of 'eccentric' to people began in the 1620s, and the metaphor is brilliantly apt. A conventional person's behavior revolves around the social center — the norms, expectations, and habits of their community. An eccentric person's behavior revolves around a different center, one offset from the social norm. They are not random or chaotic (that would be a different astronomical metaphor); they have a center, a logic, a consistency — but it is not the common center. Their orbit is regular but displaced.
This metaphor explains why 'eccentric' carries a more affectionate tone than 'weird,' 'strange,' or 'abnormal.' Eccentricity implies a kind of systematic deviation — the eccentric person has principles and patterns, just unusual ones. English culture, particularly the British variety, has long celebrated eccentricity as evidence of individualism and independence of mind. The eccentric aristocrat, the eccentric professor, the eccentric inventor — these are stock characters in British literature and cultural mythology, treated with fond tolerance rather than disapproval.
The mathematical meaning of 'eccentricity' — the degree to which an ellipse deviates from a perfect circle — survives in modern astronomy and engineering. An orbit with eccentricity 0 is a perfect circle; eccentricity approaching 1 indicates an increasingly elongated ellipse; eccentricity of exactly 1 is a parabola, and greater than 1 is a hyperbola. Comets typically have high orbital eccentricity, which is why their appearances are irregular and their paths dramatically different from the nearly circular orbits of planets.
Greek Origins
The word family surrounding 'kentron/center' is large and transparent. 'Concentric' (sharing the same center), 'epicenter' (the point on the surface above the center of an earthquake), 'centrifugal' (fleeing the center), 'centripetal' (seeking the center), 'concentrate' (to bring to a center) — all preserve the Greek root in various configurations. 'Eccentric' is the outlier of the family, the word that defines itself by its distance from the center that all the others reference.