Definition of "obliquity"

The Planet Earth, so stedfast though she seem, / Insensibly three different Motions move? / Which else to several Sphears thou must ascribe, / Mov'd contrarie with thwart obliquities

1667, John Milton, “(please specify the book number)”, in Paradise Lost. […], London: […] [Samuel Simmons], […]; republished as Paradise Lost in Ten Books: […], London: Basil Montagu Pickering […], 1873, lines 766-769

Screwed at its axis against the side, a swinging lamp slightly oscillates in Jonah’s room; and the ship, heeling over towards the wharf with the weight of the last bales received, the lamp, flame and all, though in slight motion, still maintains a permanent obliquity with reference to the room; though, in truth, infallibly straight itself, it but made obvious the false, lying levels among which it hung.

1851 November 14, Herman Melville, chapter 9, in Moby-Dick; or, The Whale, 1st American edition, New York, N.Y.: Harper & Brothers; London: Richard Bentley

She wore glasses which, in humble reference to a divergent obliquity of vision, she called her straighteners, and a little ugly snuff-coloured dress trimmed with satin bands in the form of scallops and glazed with antiquity.

1897 October 16, Henry James, What Maisie Knew, Chicago, Ill., New York, N.Y.: Herbert S. Stone & Co.

Calculating by spherical trigonometry, and assuming the same obliquity, I obtain 3 dundas and 40 pulas for the ascensional arc, giving a difference in time of 3 pulas, or about one of our minutes; an error so small, that even were the Indian astronomer aware of its existence he would disregard it, satisfied that the practical purposes which his labours subserve, are, notwithstanding, carried out with sufficient accuracy.

1839 October, J.J. Middleton, “Description of an Astronomical Instrument presented by Raja Ram Sing, of Khota, to the Government of India”, in Journal of the Asiatic Society, volume 8, number 94, page 837

Habitually living with the elements and knowing little more of the land than as a beach, or, rather, that portion of the terraqueous globe providentially set apart for dance-houses, doxies and tapsters, in short what sailors call a "fiddlers'-green," his simple nature remained unsophisticated by those moral obliquities which are not in every case incompatible with that manufacturable thing known as respectability.

1888–1891, Herman Melville, “[Billy Budd, Foretopman.] Chapter II.”, in Billy Budd and Other Stories, London: John Lehmann, published 1951

Stray's [friends], apt to keep more to the shadows, tended to be practitioners of obliquity—as it quite often came down to, varieties of pimp.

2006, Thomas Pynchon, Against the Day, Vintage, published 2007, page 404

That spiked my gun. I could not say anything. I was entirely out of verbal obliquities; to go further would be to lie, and that I would not do; so I simply sat still and suffered,—sat mutely and resignedly there, and sizzled,—for I was being slowly fried to death in my own blushes.

1880, Mark Twain [pseudonym; Samuel Langhorne Clemens], chapter 25, in A Tramp Abroad; […], Hartford, Conn.: American Publishing Company; London: Chatto & Windus