ON ASTRONOMY. 



113 



periodic inequalities which stretches 

 over an immense cycle of ages. The 

 nature of this change will be readily- 

 seen by a reference to fig. 9. A B, 

 the major axis, remains always the 

 Mime, while C D, the minor axis, 

 5 increases in length, and the sun 

 comes nearer to the centre. If we 

 suppose the full line, A C B D, to 

 represent the orbit as it was some 

 ages back, thebroken line, AG BF, 

 will represent it now. The earth 

 and moon, by being carried into 

 a wider curve, are less affected by 

 the disturbing force of the sun. The earth's power upon the moon 

 becomes relatively increased, and hence, as before stated, the gradual 

 acceleration. 



The eccentricity of the earth's orbit is now about 0.0168 ; that is, 

 the distance from the centre to one of the foci is yj o o o P^'''^ o^ ^^^^ mean 

 radius. Leverrier, in a remarkable memoir upon the secular varia- 

 tions of the orbits of the seven principal planets, has shown that the 

 eccentricity of the earth's orbit will continue to diminish for about 

 24,000 years, when it will be reduced to 0.0033, or about -! of its 

 present value. It will then begin to increase, and will go on in- 

 creasing for many thousands of years. The moon will then begin to 

 wind out instead of winding in, and will enter upon her appointed but 

 immense cycle of secular retardation. The anticipated catastrophy of 

 a collision between the earth and moon will be happily prevented. 



3. I select as a third illustration the discovery of the planet Nep- 

 tune. This will ever be regarded as one of the grandest achievements 

 of science. 



The planet Uranus was discovered by the elder Hersch 1 in 1781. 

 On searching the records of previous observers it was found that the 

 planet had been observed and its place recorded nineteen times before 

 it was suspected to be a planet. By this means it was possible to 

 determine the orbit and construct tables much sooner than could other- 

 wise have been done. The French Academy made its orbit the subject 

 of a prize in 1790. Delambre was the successful competitor. He 

 subsequently computed tables of this planet. It was pretty soon 

 found that Delambre's tables did not well represent the observed 

 places of the planet. These inequalities were so great that new tables 

 were computed by Bourvard in 1821. But these, again, in a few 

 years, were found to be nearly as defective as Delambre's, and required 

 correcting. From 1795 to 1822 the observed longitude was in advance 

 of the computed. The planet then fell back till 1830 and 1831, when 

 the observed and computed longitudes agreed. The observed place 

 then rapidly fell behind the computed place. The error of the tables 

 in giving the geocentric place, in 1835, was 30"; in 1838, 50"; in 

 1841, 70". The attempt further to correct the elements as given by 

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