Now though what Galilæo answers to a like Objection in his Hypothesis; (that its 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 puzzeled 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 perplexed; and was fain, for want of other relief, to have recourse to somewhat like Keplers 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 waies give account of those little inequalities; and would much rather (I doubt not) have embraced this Notion of the Common Center of Gravity, to salve the Phænomenon, 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 Æquation, to solve the inequalities of the Moon's motion, according to her Synodical Revolution, or different Aspects (of New-moon, Full Moon, &c.) beside what concerns her own Periodical motion.
For which, this consideration of the Common Center of Gravity of the Earth and Moon, is so proper a remedy (especially if it shall be found precisely to answer those Phænomena, which I have not Examined, but am very apt to believe) 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 Center 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 according to the Complication of their several motions, will somewhat change the position of Jupiter, as to that Common center of Gravity of all these Bodies; which yet, because of their smallness, may chance to be so little, as that, at this distance, the change of his apparent place may not be discernable. And what is said of Jupiter, is in the like manner to be understood of Saturne and his Satelles, discovered by Hugenius: For all these Satellites are to their Principals, as so many Moons to the Earth. And I do very well remember, in the Letters forecited, Mr. Horrocks expresseth some such little inequalities in Saturnes motion, of which he could not imagine what account to give, as if (to use his Expression) this crabbed Old Saturn had despised his Youth. Which, for ought I know, might well enough have been accounted for, if at that time the Satelles of Saturn had been discovered, and that Mr. Horrocks had thought of such a notion as the Common Center of Gravity of Saturn and his Companion, to be considerable, as to the guiding of his motion.
You have now, in obedience to your Commands, an Account of my thoughts, as to this matter, though yet immature and unpolished: What use you will please to make of them, I shall leave to your prudence, &c.
An APPENDIX, written by way of Letter to the Publisher; Being an answer to some Objections, made by several Persons, to the precedent Discourse.
I Received yours; and am very well contented, that objections be made against my Hypothesis concerning Tydes: being
proposed but as a conjecture to be examined; and, upon that Examination, rectified, if there be occasion; or rejected, if it will not hold water.
1. To the first objection of those you mention; That it appears not how two Bodies, that have no tye, can have one common Center of Gravity: that is (for so I understand the intendment of the objection) can act or be acted in the same manner, as if they were connected: I shall onely answer, that it is harder to shew How they have, than That they have it. That the Load-stone and Iron have somewhat equivalent to a Tye; though we see it not, yet by the effects we know. And it would be easy to shew, that two Load-stones, at once applyed, in different positions, to the same Needle, at some convenient distance, will draw it, not to point directly to either of them, but to some point between both; which point is, as to those two, the common Center of Attraction; and it is the same, as if some one Load-stone were in that point. Yet have these two Load stones no connection or tye, though a Common Center of Virtue according to which they joyntly act. And as to the present case, How the Earth and Moon are connected; I will not now undertake to shew (nor is it necessary to my purpose;) but, That there is somewhat, that doth connect them, (as much as what connects the Load-stone, and the Iron, which it draws,) is past doubt to those, who allow them to be carryed about by the Sun, as one Aggregate or Body, whose parts keep a respective position to one another: Like as Jupiter with his four Satellites, and Saturn with his one. Some Tye there is, that makes those Satellites attend their Lords, and move in a Body; though we do not See that Tye; nor Hear the Words of Command. And so here.