$$284286] Solar Parallax : Variation of Latitude 367 



aberration, unite in giving values not differing from 8"'8o 

 by more than two or three hundredths of a second. The 

 results of the last transits of Venus, the publication and 

 discussion of which have been spread over a good many 

 years, point to a somewhat larger value of the parallax. 

 Most astronomers appear to agree that a parallax of 8"'8, 

 corresponding to a distance of rather less than 93,000,000 

 miles, represents fairly the available data. 



285. The minute accuracy of modern observations is 

 well illustrated by the recent discovery of a variation in 

 the latitude of several observatories. Observations taken at 

 Berlin in 1884-85 indicated a minute variation in the latitude; 

 special series of observations to verify this were set on 

 foot in several European observatories, and subsequently at 

 Honolulu and at Cordoba. A periodic alteration in latitude 

 amounting to about |" emerged as the result. Latitude 

 being defined (chapter x., 221) as the angle which the 

 vertical at any place makes with the equator, which is 

 the same as the elevation of the pole above the horizon, 

 is consequently altered by any change in the equator, and 

 therefore by an alteration in the position of the earth's poles 

 or the ends of the axis about which it rotates. 



Dr. S. C. Chandler succeeded (1891 and subsequently) 

 in shewing that the observations in question could be in 

 great part explained by supposing the earth's axis to undergo 

 a minute change of position in such a way that either pole 

 of the earth describes a circuit round its mean position in 

 about 427 days, never deviating more than some 30 feet 

 from it. It is well known from dynamical theory that a 

 rotating body such as the earth can be displaced in this 

 manner, but that if the earth were perfectly rigid the period 

 should be 306 days instead of 427. The discrepancy 

 between the two numbers has been ingeniously used as a 

 test of the extent to which the earth is capable of yielding 

 like an elastic solid to the various forces which tend to 

 strain it. 



286. All the great problems of gravitational astronomy 

 have been rediscussed since Laplace's time, and further 

 steps taken towards their solution. 



Laplace's treatment of the lunar theory was first developed 

 by Marie Charles Theodore Damoiseau (1768-1846), whose 



