Philosophy 167: Class 11 - Part 15 - Hooke on Attractive Powers in Celestial Bodies: a Guess at Universal Gravity.
Smith, George E. (George Edwin), 1938-
This div will be replaced by the JW Player.
view transcript only
Meanwhile, we have Hooke sitting there in the royal society. In 1674 he publishes, and we're going to read this whole thing, so I want to read it off a text I know I can see. In 1674, he publishes his findings on these North stars that he sees are moving, and he says an attempt to prove the motion of the earth with the rest of the tidal is from observations made by Robert Hooke.
As I said before, Hooke is confused, as so often happens because the observed motion that he had seen was in the opposite direction from what annual stellar parallax would be. But in this monograph, which Hook certainly accused Newton of having read, there's no reason to thing he didn't.
Hooke says the following, this is all on one long page that I've broken out to make it easier to read. Quote, that all celestial bodies whatsoever, have an attraction or gravitating power toward their own centers, whereby they attract not only their own parts, and keep them from flying from them as we observe the Earth to do.
But that they do also attract all other celestial bodies that are within the sphere of their activity. And consequently, that not only the Sun and Moon, have an influence upon the body and motion of the Earth, and the Earth upon them, but that Mercury, also Venus, Mars, Saturn, and Jupiter, by their attractive powers, have a considerable influence upon its motion.
As in the same manner the corresponding attractive power of the Earth has a considerable influence upon every one of their motions also, first claim. Second claim, that all bodies whatsoever that are put into a direct and simple motion, will so continue to move forward in a straight line 'til they are, by some other effectual powers deflected and bent into a motion describing a circle, ellipse, or some other more compounded curve line.
Third, that these attractive powers are so much the more powerful in operating by how much nearer the body wrought is to their own centers. The nearer you are to the body, the stronger the attractive powers. Finish with this. Now, you can read the whole of this document, it's on supplementary material.
Now, what these several degrees are, degrees of variation, of the strength of the attractive powers, what they are, I have not yet experimentally verified. But it is a notion which, if fully prosecuted, as it ought to be, will mightily assist the astronomer to reduce all the celestial motions to a certain rule, which I doubt will never be done true without it.
He that understands the nature of the circular pendulum and circular motion, will easily understand the whole ground of this principle and will know where to find direction in nature for the true stating there of. This I only hint at at present to such as have ability and opportunity of prosecuting this inquiry, and are not wanting an industry for observing and calculating.
Wishing hardly such may be found, having myself many other things in hand which I would first complete and therefore cannot so well attend it. But this I durst promise the undertaker, that he will find all the great motions of the world to be influenced by this principle and that the true understanding thereof will be the true perfection of astronomy.
That was published in 1674 by Hooke. The 1678 Cometa, he's learned from Wren about the inverse square, and he's now announcing that the variation is an inverse square variation. It shouldn't shock you that when Newton published this, the theory of gravity, Hooke said, I said it. I said it years before he ever touched it.
And the worst part is the question that gets Newton started is Hooke's asking Newton, what's the trajectory described under an inverse square force directed towards the center. Okay, so Newton did not just come along and suddenly create the theory of gravity etcetera. You can picture Lisa Jardine pulling this out as she does very beautifully in her book and say, in what sense didn't Newton steal this?
And of course, Newton's own answer was Hooke merely guessed Newton actually proved it, and that was the context in which Newton said, Kepler only guessed it was an ellipse. I proved it in exactly the same way,Hooke only guessed about the mutual attraction, I proved it. But the thing I'm trying to drive home is, Newton is in a very, very rich context of speculation.
A speculation that, as you go back and read that, is fundamentally what Newton ends up saying, except Newton does it fully mathematically, and Hooke was not a decent mathematician. In fact I'll end tonight with just a remark. When Hooke first claimed Newton was plagiarizing him in 1686, Newton rewrote book three of the Prigipia to be as mathematical as possible, announcing that only people properly training in mathematics could then read it.
And it was absolutely clear who he was referring to. Okay? So this is the context in which we're gonna turn to Newton. We'll see Newton first having a falling-out with Hooke next week. Then we'll see Hook initiating and giving him the key idea. Then we'll see Newton start to run with it, with Hooke sort of out of the picture.
Fair enough? We're now ready to start with Newton.