SEQUEL TO THE EPOCH OF HIPPARCHUS. 173 



cyclical theory, was, for several reasons, an important step in astron- 

 omy ; some of these reasons may be stated. 



1. It obviously suggested, or confirmed, the suspicion that the mo- 

 tions of the heavenly bodies might be subject to many inequalities : 

 that when one set of anomalies had been discovered and reduced to 

 rule, another set might come into view ; that the discovery of a rule 

 was a step to the discovery of deviations from the rule, which would 

 require to be expressed in other rules ; that in the application oi 

 theory to observation, we find, not only the stated phenomena, for 

 which the theory does account, but also residual phenomena, which 

 i emain unaccounted for, and stand out beyond the calculation; that 

 thus nature is not simple and regular, by conforming to the simplicity 

 and regularity of our hypotheses, but leads us forwards to apparent 

 complexity, and to an accumulation of rules and relations. A fact 

 like the Evection, explained by an Hypothesis like Ptolemy's, tended 

 altogether to discourage any disposition to guess at the laws of nature 

 from mere ideal views, or from a few phenomena. 



2. The discovery of Evection had an importance which did not 

 come into view till long afterwards, in being the first of a numerous 



O ' O 



series of inequalities of the moon, which results from the Disturbing 

 Force of the sun. These inequalities were successfully discovered ; 

 and led finally to the establishment of the law of universal gravita- 

 tion. The moon's first inequality arises from a different cause ; from 

 the same cause as the inequality of the sun's motion ; from the mo- 

 tion in an ellipse, so far as the central attraction is undisturbed by any 

 other. This first inequality is called the Elliptic Inequality, or, move 

 usually, the Equation of the Centre?'* All the planets have such in- 

 equalities, but the Evection is peculiar to the moon. The discovery 

 of other inequalities of the moon's motion, the Variation and Annual 

 Equation, made an immediate sequel in the order of the subject to 



S5 The Equation of the Centre is the difference between the place of the Planet in 

 its elliptical orbit, and that place which a Planet would have, which revolved uni- 

 formly round the Sun as a centre in a circular orbit in the same time. An imagi- 

 nary Planet moving in the manner last described, is called the mean Planet, while 

 the actual Planet which moves in the ellipse is called the true Planet. The Longi- 

 tude of the mean Planet at a given time is easily found, because its motion is uni- 

 form. By adding to it the Equation of the Centre, we find the Longitude of the 

 true Planet, and thus, its place in its orbit. Littrow's Note. 



I may add that the word Equation, used in such cases, denotes in general a quan- 

 tity which must be added to or subtracted from a mean quantity, to make it 

 to the true quantity; or rather, a quantity which must be added to or subtracts; 1 

 from a variably increasing quantity to make it increase equally. 



