768 GEOGRAPHICAL LATITUDE. 



work in 1800, determines the diminution of the obliquity at 0".505 per 

 annum. He adds however that one can not say that the Arabian 

 observation was exact to the minute.' Remarking that their result did 

 not agree with that of the Greeks, the Arabs thought that the " obli- 

 quity of the Ecliptic oscillates."^ 



However, it was not until toward the middle of the eighteenth century 

 that the diminution of the Ecliptic obliquity was generally admitted.^ 

 Louville was the first who attempted to measure the amount of the move- 

 ment and announced in 1716 the result, GO" a century.* The elder Cas- 

 sini believed it to be 45",^ which is practically that at present accepted.^ 

 However, opinions have varied much; Ximines in 1756 calculated it to 

 be 30".^ Hornsby (1769) believed it to be no more than 32" or 34" in 

 the century, although his observations had given ^" per annum.^ La- 

 lande gives 33".33, Delambre, 46" to 48", as the result of observations ; 

 but the latter accepts as the best those of La Caille, 44" per century.^ 



It belongs also to the childhood of the race to have noticed that some 

 stars never go below the horizon and that one seems to be stationary, 

 round which the others revolve. This star marks the prolongation of 

 the earth's axis to the north, and pre-supposes its rays to come from 

 an infinite distance and fall on the earth parallel to each other. It 

 happens then, from the nature of a sphere, that it can not be seen from 

 the equator, or further to the south ; but in i)roportion to the distance 

 of the observer north from the equator that star appears above the 

 horizon. This fact gave a second principle founded in nature on which 

 to build the science of geography. This star however is not abso- 

 lutely stationary in the sky, but itself describes daily a small circle, so 

 a simple method was invented to find the center of this circle, which 

 then should be the absolute north pole. This was to observe the alti- 

 tude of any one of the circumscribing stars when at its highest and 

 lowest points, i. e. at its culminations, and halve the result, which gives 

 a truer north point. It was the good fortune of the English astron- 

 omer-royal, Bradley (1692-1761^), to discover that even this point is not 

 absolutely fixed, but describes periodically a small ellipse. Even this 

 is not a regular curve, for the line is a wavy one. 



Still a third method of determining latitude was emjdoyed as early 

 as the time of Poseidonius (first century B.C.) which was however at 

 first so crude as to be perfectly unreliable in its results, but which in 

 the end was so far useful as to call attention to the fact that other stars 



' Delambre, Astrou. du xviil"'" siecle, pp. 13, 14. 



"-Ihid., p. 200. 



^ Ihid., vii. 



• Ihid., p. 317. 



■'Ihid., p. 262. 



6 Ibid>, p. 406. 



■I Ihid., p. 405. 



'^/ftt(?.,pp. 697,698. 



9/Wrf., p. 594, 



