Galileo and His Judges

by F. R. Wegg-Prosser

“I imagine a movable body projected in a horizontal plane, all impediments [to motion] being removed; it is then manifest from what has been said more fully elsewhere, that its (the body’s) motion will be uniform and perpetual upon the plane, if the plane be extended to infinity.”

This of course involves the principle of the first of the three laws of motion, the Newtonian laws, as they are frequently called, because the man whose name they bear was the one who used them clearly and consistently as the basis of a great astronomical theory. The law, as now usually stated, is fuller and more explicit than that given by Galileo, and may be enunciated thus: “Every body perseveres in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed on it.”

It is, however, greatly to the scientific credit of Galileo that before the close of his life he should have emancipated himself from the erroneous idea that circular motion alone is naturally uniform, and should have stated in the language just quoted the true mechanical doctrine, unknown to his predecessors, unknown even to Kepler, a doctrine which involved nothing less than a revolution in the conception of the laws of motion. Nor was this his only contribution to the science of mechanics; he it was who first understood the law that regulates the velocity of falling bodies; he perceived that they were acted upon by an uniformly accelerating force, that of terrestrial gravity, and that the velocity at any given point is proportional to the time of descent.

The principle of virtual velocities is said by some persons to have been discovered by Galileo, and it appears that he stated it fully and clearly; but he can scarcely be said to be the discoverer of it, as it had been known to others, and had even—at least as exemplified in the case of the lever—been noticed by Aristotle. There is, however, no doubt that Galileo was the greatest man of his day in mechanical knowledge, whether we attribute more or less weight to the light he threw on particular details.

In astronomy he was necessarily a discoverer, for the all-important reason that, as already stated, he was the first man that ever used the telescope for investigating the phenomena of the heavens. He thus saw what no one previously had seen,[15] the satellites of Jupiter, the spots on the Sun, and the moon-like phases of the planet Venus, besides the greatly increased number of stars, so many of which are invisible to the naked eye.

The first-mentioned of these discoveries, that of the satellites of Jupiter, seems to have created an immense sensation among the savants of that day. It suggested that the theories of Ptolemy were anything but complete or correct, and yet it proved nothing, excepting against those à priori reasoners, who would not believe that a body round which a moon circulated could itself be in motion; but the phases of Venus were simply conclusive against the Ptolemaic system, and for this reason: According to that system Venus was a planet revolving round the Earth in an orbit outside that of Mercury, but within that of the Sun. Now the phases of Venus did not correspond with any supposed period of her revolution round the Earth, as the phases of the Moon obviously do, nor did any one ever imagine that the Earth went round Venus. They did, however, correspond with the time of a probable orbit in which either Venus revolved round the Sun or the Sun round Venus; and here again this latter alternative was inadmissible. There remained, therefore, the one only reasonable solution of the phenomenon, namely, that Venus travelled in an orbit round the Sun. This was further confirmed when, in December, 1639, our own countryman, Horrox, at that time a young curate residing in the north of England, but gifted with a knowledge of astronomy which would have done credit to a man of double his age and experience, observed a transit of the planet across the Sun’s disc. This occurred some few years after Galileo’s condemnation; but it may be remarked that Gassendi had already, in November, 1631, witnessed a transit of Mercury. Thus it appeared that these two planets revolved round the Sun, contrary to what Ptolemy had supposed. And yet this was not conclusive in favour of Copernicanism, for the theory of Tycho Brahé was precisely to this effect: that the planets revolved round the Sun, and that the Sun in his turn circulated round the Earth. This hypothesis was of the nature of a compromise, and it has been said that Tycho was led to it by his interpretation of Scripture rather than of Nature; yet he was one of the best astronomers and best observers of his age, and had Kepler for one of his pupils. He had a reason, too, for rejecting Copernicanism which in his time seemed to have considerable weight, namely, the incredible distances at which the fixed stars must be supposed to be placed if the theory were true, since no sensible motion could be detected among them—apparent motion, that is—such as would result from the annual motion of the Earth if the stars were at any distance approaching to that of the planets. We know now how futile this objection is, but in that age there was an idea that Nature could never allow of such a waste of space as is implied in these vast distances. If Tycho had lived longer, we may well doubt whether he would have adhered to his system. Kepler saw its weakness, and was the first to discover the true nature of the curves which both the Earth and the planets describe in their respective orbits; and this, although he did not know the first law of motion. His books, published in 1619 and 1622, stated not only the elliptic form of the orbits, which no one previously had found out, but also the important law connecting the distances of the planets with their periods of revolution.

It is necessary to bear in mind how gradually these various items of knowledge dawned upon the scientific world, and how imperfect was the state in which the study of astronomy remained until the discovery of that great law of gravitation, which binds together and regulates the physical universe. Men of mature years had not then learnt the lesson now taught to youths at college, that in natural science we must discard à priori arguments, and trust to the experimental method for guidance. It has been said contemptuously that the Cardinals who condemned Galileo and the Copernican system were not only ignorant of the science of the present day (which was inevitable), but even of that of their own day. If that means merely that they were deficient in that far-reaching intelligence which enables some gifted men to foresee the future effect of recent discoveries and hypotheses scarcely emerged from a state of embryo, we may readily grant it.

We may allow also that some of the recent discoveries of Galileo, as, for instance, that of the phases of Venus, were not at first fully appreciated, nor their bearing on the controversy perfectly understood, excepting by professed astronomers. It required careful observation to perceive that this planet’s phases were only to be explained on the theory of her revolving round the Sun.

On the other hand, if these ecclesiastics were wise enough to see the futility of Galileo’s argument drawn from the tides, it is certainly not for us to blame them; the tides have nothing to do with the questions then at issue.

And it is only fair to remember that supposing Ptolemy completely overthrown, as in reality he assuredly was, by the observations on Venus and Mercury, there remained the system of Tycho Brahé, as has been remarked already, and this system partly met the case of those phenomena that Ptolemy failed in accounting for; and although we can easily see now that it was something of the nature of a makeshift, at that time there was no clear or conclusive evidence against it.