136 PROGRESS IN ASTRONOMY. 



The measurements of arcs of parallel have l)een developed by the 

 rapid extension of telegraphic communications which now permit the 

 longitude of the terminal stations to be determined with the greatest 

 accuracy. 



Thanks to this work we now have the size of our planet to a few 

 miles. The polar diameter is 41,709,790 feet; but the equator is not a 

 circle, the ecpiatorial diameter from longitude 8'^ 15' west to longitude 

 188° 15' west is 41,853,258 feet; that at right angles to it is 41,850,210 

 feet; that is, some thousand 3'ards shorter. The earth, then, is shaped 

 like an orange slightly squeezed. 



Knowing the earth's diameter, we can obtain the sun's distance by 

 several methods — the old one l)y observing transits of Venus, one of 

 which Cooke went out to observe in 1769, and two of which recurred 

 in 1874 and 1882; new ones b}- observations of Mars or one of the 

 minor planets at a favorable opposition, and b}' determining the velocity 

 of light. 



The recent discover}^ of a minor planet. "'Eros,"^ which in one part 

 of its orbit is nearer the earth than Mars, has recently revived 

 interest in this method and a combined attack is in contemplation. 



It has been long known that light has a finite velocity; but we had 

 to wait till the sixties before Fizeau and Foucault showed us how to 

 determine its exact value. The methods introduced ])y them have 

 been recently applied l»y Cornu, Newcoml), and Michelson, and the 

 resulting value is slightly less than 3()0,000 meters per second. Com- 

 bining this with the constant of aberration, the disttmce of the sun can 

 be determined. 



It is w^onderful how these vastly dili'erent methods agree in the 

 resulting mean distance. At the beginning of the century it stood 

 roughly at 95,000,000 miles; this has l)een reduced to 93,965,000 miles. 

 The extreme difference IjetAveen the old and new values of the solar 

 parallax, two-fifths of a second of arc, is represented hy the apparent 

 breadth of a human hair viewa'd at a distance of about 125 feet. 



Knowing the distance of the sun, the way is open to us to determine, 

 by a method suggested by Galileo, the distances of those stars which 

 occupy a different position among their fellows, as seen from opposite 

 points in the earth's or])it round the sun, points 186,000,000 miles 

 apart. We now know the distances of many such stars, Bessel having 

 determined the first in 1838. The nearest star to us, so far as we know, 

 is Centauri, the light of which takes four and a half years to reach 

 us. Not many years ago Pritchard applied photography to this branch 

 of inquiry. We may therefore expect a still more rapid progress in 

 the future. 



^[Dr. E. von Oppolzer gives the rc'marka])le announcement of a variation of the 

 brightness of Eros of about one magnitude, the change taking place in a few hours. — 

 Nature LXIII, 1901, page 383.] 



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