The words "secular" and "siecle" [as in "fin de siècle"] have the same Latin origin.
The OED documents the first recorded usage of the word "secular" in the sense of "belonging to an age or long period" in the late sixteenth century (the first recorded usage of the same word in the sense of "belonging to this world" is traced back to the late thirteenth century).
At the beginning, though, "secular" meant "occurring or celebrated once in an age, century, or very long period," whereas from the seventeenth century, you begin to see the recorded usage of the same word in the opposite sense of "living or lasting for an age or ages."
<blockquote>II. Of or belonging to an age or long period.
5. Occurring or celebrated once in an age, century, or very long period. secular games, plays, shows [L. ludi sæculares]: in ancient Rome, games continuing three days and three nights celebrated once in an 'age' or period of 120 years. secular poem [L. carmen sæculare], a hymn composed to be sung at the secular games.
1599 PONT Right Reckoning of Years 34 Supposing that they celebrate their secular solemnities at the precise end and periode of every hundreth yeare. 1601 HOLLAND Pliny VIII. xlii. I. 221 The secular solemnities, exhibited by Claudius Cæsar, in the Circensian games. 1606 {emem} Sueton. 52 He restored againe..the Sæcular playes. 1696 B. KENNETT Antiq. Rome II. V. vii. 292 The famous Secular Poem of Horace was compos'd for this last Day, in the Secular Games held by Augustus. 1697 EVELYN Numism. iii. 62 To..divert the People..during the Secular Shews. 1706 HEARNE Collect. 3 Apr. (O.H.S.) I. 215 A letter sent to our University from the University of Francfort..inviting them to celebrate the secular day of the Foundation of their University, wch will happen in this month, it being now just two Hundred years since that University was Founded. 1716 ADDISON Free-holder No. 46 {page}1 When Augustus celebrated the secular year, which was kept but once in a century. 1790 GIBBON Misc. Wks. (1814) III. 418 Had a fortnight more been given to the philosopher, he might have celebrated his secular festival [sc. his hundredth birthday]. 1862 MERIVALE Rom. Emp. lxviii. (1865) VIII. 332 One man asserted that the secular fire would descend at the moment when..he should be seen transformed into a stork. 1869 RAWLINSON Anc. Hist. 509 M. Julius Philippus..celebrated the secular games in commemoration of the thousandth year from the founding of the city. 1884 Q. Rev. July 1 Changes in..the City..have been going on at a rate..unknown to any former generation, except those distant generations which have witnessed the rare and secular phenomena of siege, fire, and plague.
6. Living or lasting for an age or ages. Now chiefly with reminiscence of the scientific sense 7. Also (of trees, etc., after F. séculaire), centuries old.
1629 DONNE Serm. cxxxi. Wks. 1839 V. 435 If I had a secular glass, a glass that would run an age.., it would not be enough to tell the godly man what his treasure is. 1671 MILTON Samson 1707 And though her body die, her fame survives, A secular bird ages of lives. 1847 EMERSON Poems, Monadnoc 311 Slowsure Britain's secular might. 1850 TENNYSON In Mem. xli, I shall be thy mate no more, Tho' following with an upward mind The wonders that have come to thee, Thro' all the secular to-be. 1868-9 TYNDALL Fragm. Sci. v. (1871) 103 The improvement of man is secular{em}not the work of an hour or of a day. 1870 LOWELL Among my Bks. Ser. I. (1873) 253 We envy the secular leisures of Methusaleh. 1876 R. F. BURTON Gorilla L. I. 36 A fern field surrounded by a forest of secular trees. 1879 STEVENSON Trav. with Donkey 186 Mankind outlives saecular animosities, as a single man awakens from the passions of a day. 1888 BRYCE Amer. Commw. III. VI. cxv. 653 The centripetal forces are permanent and secular forces, working from age to age.
7. In scientific use, of processes of change: Having a period of enormous length; continuing through long ages. a. Astr. Chiefly of changes in the orbits or the periods of revolution of the planets, as in secular acceleration, equation, inequality, variation. The terms secular acceleration, secular variation were formerly also used (with reference to the sense 'century' of L. sæculum) for the amount of change per 100 years; similarly {dag}secular precession (see quot. 1812). secular equation is also used more widely to designate any equation of the form |aij-bij{lambda}| = 0 (i,j = 1,2, . . ., n), in which the left-hand side is a determinant and which arises in quantum mechanics.
1801 Monthly Rev. XXXV. 537 M. De La Place..found the secular equation of the moon to be due to the action of the sun on the moon. 1812 WOODHOUSE Astron. ix. 63 The secular precession, that is, the accumulated precessions of 100 years. 1812-16 PLAYFAIR Nat. Phil. (1819) II. 275 In the orbit of Mars, the eccentricity is diminishing. The secular variation of the greatest equation of the centre is{em}37´´. 1834 M. SOMERVILLE Connex. Phys. Sci. iii. (1849) 16 Secular inequalities. 1862 CAYLEY Math. Papers (1890) III. 522 On the Secular Acceleration of the Moon's Mean Motion. 1937 E. C. KEMBLE Fund. Princ. Quantum Mech. x. 361 Its components must yield a nontrivial (i.e., nonvanishing) solution of the set of g equations {Sigma}n(Amn - a{delta}mn)xn = 0... Such a solution exists only if the determinant of the coefficients vanishes, i.e., if a is a root of the so-called 'secular' equation det (A - aI) = ..0. 1974 GILL & WILLIS Pericyclic Reactions i. 21 To obtain the wave functions corresponding to these energies it is necessary to solve the secular equations using the appropriate values of E.
b. Geol., Physical Geogr., Meteorol., etc.
1833 LYELL Princ. Geol. Gloss., Secular Refrigeration, the periodical cooling and consolidation of the globe, from a supposed original state of fluidity from heat. 1856 KANE Arct. Expl. I. xxiii. 308 A secular elevation of the coastline. 1861 TYNDALL Fragm. Sci. xiii. (1871) 399 The earth's magnetic constituents are gradually changing their distribution. This change is very slow; it is technically called the secular change. 1867 H. MACMILLAN Bible Teach. xvi. (1870) 320 Those grand secular tides which have punctually recurred every ten thousand years. 1872 {emem} True Vine v. 176 The earth has its secular seasons as well as its annual. 1880 HAUGHTON Phys. Geog. ii. 53 The contraction of the globe due to secular cooling. 1887 ABERCROMBY Weather 312 Annual and Secular Variations.
8. Econ. and Statistics. Of a fluctuation or trend: occurring or persisting over an unlimited period; not periodic or short-term.
1895 A. MARSHALL Princ. Econ. (ed. 3) I. V. v. 470 There are secular movements of normal price, caused by the gradual growth of knowledge, of population and of capital, and the changing conditions of demand and supply from one generation to another. 1926 L. D. EDIE Econ. II. iv. 49 Economic fluctuations fall into four major types: seasonal, secular, cyclical, and residual. 1971 H. S. SHRYOCK et al. Methods & Materials Demography II. xiii. 377/2 If the observations are made at different times of the year, seasonal movements may also be apparent. When we are trying to describe the growth of a population over a relatively longer period of time (for example, India from 1872 to 1961) we are generally interested in the secular trend only. 1973 Daily Tel. 15 Jan. 17/6 This is the first time the Government has had to pay so much for money but the secular trend of interest rates will stop rising only if the rate of inflation is brought down. 1976 Sci. Amer. Sept. 107/1 The secular trend of workers migrating out of agricultural jobs as a result of technological change in agriculture has recently slackened.</blockquote> -- Yoshie
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