What does ellipticity mean?
Ellipticity is a noun describing the degree to which something deviates from a perfect circle or sphere — in other words, how elliptical or oval it is. It appears most often in technical contexts: astronomers speak of the ellipticity of planetary and cometary orbits, geophysicists of Earth's slight flattening, and engineers of ellipticity in cross-sections of pipes, beams, and optical fibres. In mathematics it can be quantified precisely through eccentricity, a number between zero and one where zero corresponds to a circle. Beyond geometry, the term surfaces in optics to describe elliptically polarized light. The word derives from Greek roots meaning 'falling short', since an ellipse was conceived as a circle that falls short of being round. Though formal in register, ellipticity is a useful term whenever precision about shape matters more than casual description, and it lends scientific writing a concise way to capture deviation from roundness without lengthy explanation.
nounThe quality or degree of being elliptical; in mathematics, a measure of how much an ellipse or ellipsoid deviates from a perfect circle or sphere.
- A measure of how much an ellipse, orbit, or ellipsoid deviates from circularity; eccentricity.
- The general state or quality of being elliptical or oval in shape.
"Satellite orbits are chosen to minimize ellipticity, keeping the craft at a nearly constant altitude."
"The comet's high ellipticity carries it from the inner solar system to the distant reaches of the Oort cloud."
"Geologists measured the ellipticity of the sedimentary basins to reconstruct ancient tectonic forces."
Rarely pluralized; usually treated as an uncountable abstract noun, though the plural occurs when comparing measures across different objects.
"The survey compared the ellipticities of thousands of galaxy images."
Earth itself is slightly pear-and-pumpkin shaped: its ellipticity — the flattening caused by its spin — means the poles sit about 21 kilometres closer to the centre than the equator does.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of ellipticity
Ellipticity comes from 'elliptic', which traces back through Latin 'ellipticus' to Greek 'elleiptikos', from 'elleipein' ('to fall short, leave out'), formed from 'en-' ('in') and 'leipein' ('to leave'). Greek geometers called the ellipse a 'falling-short' section because its conic-section construction falls short of closing into a circle. The noun suffix '-ity' turns the adjective into an abstract noun denoting a property, giving English a term attested by the early 19th century. Cognates include 'ellipse', 'ellipsis', and 'elliptical', all from the same Greek root.
How ellipticity is actually used
A technical term used chiefly in mathematics, physics, astronomy, and engineering; rare in everyday conversation. In orbital mechanics it is closely related to (and sometimes used interchangeably with) eccentricity, though eccentricity is the more precise dimensionless quantity. Also used in polarized-light optics for the ellipticity of polarization states.
Easily confused with ellipticity
Ellipticalness is a rare, non-technical word for the general quality of being oval-shaped, whereas ellipticity is the standard term, especially as a precise measure in mathematics and physics.
An ellipsis is the omission of words from a sentence (marked by three dots), while ellipticity refers to the geometric property of being ellipse-shaped.