[f. ELLIPTIC + -ITY.] Elliptic form; degree of deviation (of an orbit, etc.) from circularity, (of a spheroid) from sphericity.
1753. Phil. Trans., XLVIII. 84. For the case of beds supposed of the same ellipticity,… I have taken greater care.
1833. Sir J. Herschel, Astron., iii. 109. Its deviation from the circular form, arising from so very slight an ‘ellipticity.’
1864. Athenæum, No. 1926. 402/2. The ellipticity of Mars.
1870. Jevons, Elem. Logic, xxxiii. 291. An orbit of slight ellipticity.
b. as a measurable quantity.
The ellipticity of a spheroid (e.g., of the figure of a planet) is expressed by some mathematicians as the ratio of the difference of the axes to the major axis, and by others as the ratio of this difference to the minor axis. (With reference to orbits this mode of expressing ellipticity is not used; see ECCENTRICITY 3 b.)
1753. Phil. Trans., XLVIII. 77. The diminution of the gravity having been found greater than 1/230, the ellipticity or difference of diameters ought to be less than that fraction.
1831. Brewster, Newton (1855), I. xiii. 361. The ellipticity of the earth … has been found to be 1/299.
1867. Denison, Astron. without Math., 7. Its ellipticity … means the proportion between the difference of the two axes … of an ellipse, and the greater of them.
