100 PHILOSOPHICAL TRANSACTIONS. [aNNO 1666» 



ebbing and flowing : had I not been already too tedious, I should now proceed 

 to give a further reason, why I do introduce this consideration of the common 

 centre of gravity in reference to astronomical accounts. For indeed that 

 which may possibly seem at first to be an objection against it, is with me one 

 reason for it. 



: It may be thought, perhaps, that if the earth should thus describe an epicycle 

 about the common centre of gravity, it would, by this its change of place, dis- 

 turb the celestial motions, and make the apparent places of the planets, espe- 

 cially some of them, different from what they would otherwise be. For though 

 so small a removal of the earth as the epicycle would cause, especially if its se- 

 midiameter should not be above li of the earth's semidiameter, would scarce 

 be sensible, if at all, to the remoter planets, yet as to the nearer it might. 



Now though what Galilaeo answers to a like objection in his hypothesis, that 

 it is possible there may be some small difference which astronomers have not 

 yet been so accurate as to observe, might here perhaps serve the turn ; yet my 

 answer is much otherwise, to wit, that such difference hath been observed, 

 and hath very much puzzled astronomers to give an account of. About which 

 you will find Mr. Horrocks, in some of his letters, whereof I did formerly, 

 upon the command of the Royal Society, make an extract, was very much per- 

 plexed ; and was fain, for want of other relief, to have recourse to somewhat 

 like Kepler's amicable fibres, which did, according to the several positions of 

 the moon, accelerate or retard the moon's motion ; which amicable fibres he 

 had no affection to at all, as there appears, if he could any other ways give 

 account of those little inequalities; and would much rather, I doubt not, have 

 embraced this notion of the common centre of gravity, to solve the phenome- 

 non, had it come to his mind, or been suggested to him. And you find that 

 other astronomers have been seen to bring in, some upon one supposition some 

 upon another, some kind of menstrual equation, to solve the inequalities of the 

 moon's motion, according to her sy nodical revolution, or different aspects, of 

 new moon, full moon, &c. beside what concerns her own periodical motion. 



For which this consideration of the common centre of gravity of the earth 

 and moon, is so proper a remedy, especially if it shall be found precisely to 

 answer those phaenomena, which I have not examined, but am very apt to be- 

 lieve, that it is so far from being with me an objection against it, that it is one 

 of the reasons which make me inclinable to introduce it. 



I must, before I leave this, add one consideration more ; that if we shall 

 upon these considerations think it reasonable, thus to consider the common 

 centre of gravity of the earth and moon ; it may as well be thought reasonable,, 

 that the like consideration should be had of Jupiter and his four satellites, which. 



