Galileo and His Judges

by F. R. Wegg-Prosser

Thus, when it was found that the planet Venus presented to the eye phases such as the Moon does, instead of always appearing like a round body, it became evident that she revolved, not as Ptolemy supposed, round the Earth, but round the Sun, an inference subsequently confirmed by the observation of her transits over the Sun’s disc.

This being so, the adherents of Ptolemy had to meet this difficulty: here was a planet much nearer to the Earth than to the Sun,[24] and yet revolving round the latter in preference to the former. There was clearly, then, some attractive force belonging to the Sun (whatever its nature might be), greater than that of the Earth, which Venus obeyed; the same was true of Mercury, with the difference that this planet was much nearer to the Sun. Then as regards the superior planets, Mars, Jupiter, Saturn, the probability that the Sun was the great central power that controlled their movements was a very strong one. There is but little to add on these topics to Galileo’s own forcible argument in the third day’s dialogue; he is, however, inaccurate in his figures, and states that Mars appears sixty times as large when in opposition to the Sun, as at conjunction. More recent observations have shown that he appears rather more than thirty times as large when at his nearest point to the Earth, than he does when near his conjunction with the Sun, and consequently at his farthest point from the Earth; but this variation is quite sufficient for the argument, and proves incontestably that if Mars revolves round the Earth as in any way the centre of his orbit, it must be in an ellipse of so great eccentricity as no one could reasonably imagine him to do; indeed, the anti-Copernicans of Galileo’s day knew nothing of the elliptic motions of the planets; neither, as we have seen, did Galileo himself.

The same argument, drawn from the apparent size of the planet at different periods, applies also to Jupiter and Saturn—the other exterior planets were discovered much later—only not so strikingly as in the case of Mars. The improbability, if we once admit that all the planets revolve round the Sun, that the Earth, occupying the position it does, should be at rest, while the Sun, controlling the motions of the planets (vast bodies, some of them), circled, nevertheless, round the Earth; the improbability, I say, of this is so great as to be almost overwhelming; at any rate, unless the difficulties of the counter hypothesis were shown to be insurmountable, which, as we know, is far from being the case. It was of course possible, without going the lengths of the Paduan professor, and setting oneself against the telescope altogether, to admit the facts but deny the inferences; to grant, for instance, that Mars appeared to have a diameter more than six times as great in one position as in another, and to attribute it, as I hinted just now, to some extraordinary eccentricity in his orbit round the Earth; but it is not wise to look through a telescope with the eyes of the body open and the eyes of the mind closed; and generally it is but right to be guided by clear and distinct probabilities when discussing questions of natural philosophy on scientific grounds—and it is of these alone that I am at the present moment speaking.

It must be borne in mind distinctly that the discovery of the moon-like phases of Venus, showing her to revolve round the Sun, was simply conclusive as against the old system of Ptolemy, which had so long been the received system of astronomy. The theory of Tycho Brahé, or some modification of it, was the only one that could henceforth be adopted. But when you dethrone an ancient theory which has for centuries held an almost undisputed sway, you have to reconsider your whole position, and compromises such as that of Tycho are not always adequate to the emergency.

But these considerations formed only a part of this complicated controversy. The anti-Copernicans of the seventeenth century would not even admit the revolution of the Earth on its own axis, and were consequently forced to hold that the whole of the heavenly bodies were carried round this our globe in twenty-four hours. In ancient times, when men knew little or nothing of the sizes and distances of the Sun, the planets, or the stars, such a belief was quite reasonable and natural; they thought the stars were set as if they were jewels in a hollow sphere, which was turned round its poles each day. But the astronomers of Galileo’s day knew something far more accurate than this; he himself, as we observed in the Dialogue, greatly under-estimated the distance and the size of the Sun, and had but a very imperfect idea of the enormous interval that separates us from the stars; yet he evidently perceived the improbability of all these vast and remote bodies revolving with an almost inconceivable velocity round the Earth every twenty-four hours. And what must be our judgment on such a subject, seeing that we know the Sun’s mean distance to be about 92,000,000 miles, more than nineteen times as much as Galileo’s estimate? And yet some of the planets are farther and much farther from us than the Sun. Then as regards the stars, α Centauri, the nearest of them, is calculated to be more than 20,000,000,000,000 miles distant; but this calculation supposes the truth of the Copernican theory, and that we may not seem to argue in a circle, we will not use it, but content ourselves with saying that, from certain reasons about which there can be no mistake, we are sure that the distance of the stars is very considerably greater than even the remotest planet in our own system, which is Neptune. Now, this planet’s distance from the Sun is computed at 2,775,000,000 miles, and if, indeed, he is carried daily round the Earth in a circle, it must be with a velocity exceeding that of light; the stars, therefore, with a velocity far greater still. Now, nothing with which we are acquainted moves with so great a speed as light—or, as some men call it, radiant energy, meaning thereby to include heat as well as light in the term—a speed estimated at 186,000 miles in a second of time. Are we then to believe that the stars are carried in a circle round the Earth every day at a velocity much exceeding even this? It seems almost enough to ask such a question without pausing for the answer. The simple rotation of the Earth on its own axis explains all the phenomena without resorting to such extreme suppositions as those just mentioned.

It is remarkable that no one of any note—at least, in modern times, for I am not so sure about the ancients—ever appears to have suggested the intermediate theory of the Earth revolving on its axis, and yet remaining stationary as regards any motion of translation. With our present knowledge of astronomy we could not entertain such an opinion, though in the early part of the seventeenth century it might have been considered plausible. Since, however, it has not been maintained by any noteworthy author, we need not further discuss it.

The reader will bear in mind what has already been said on this branch of the subject in the second day’s dialogue,[25] and it is not necessary to repeat it in detail. It may, however, be useful to mention a few experiments of a later date, which have tended to confirm the truth of the Earth’s diurnal revolution.

Before the close of the seventeenth century it was observed that a diminution of gravity occurred at, and near, the equator. This was proved by the vibration of the pendulum, an experiment associated chiefly with the name of Richer; and it has, if I mistake not, been since then carefully tested by spring balances. This phenomenon is owing partly to the spheroidal figure of the Earth—itself the result of the rotation on the axis—but principally to the centrifugal tendency being greater at the equator, from the higher velocity of rotation.

I have already alluded to the trade winds, and the argument to be drawn from them, which I think a sound and strong one; but I need not dwell on it further.

It is, however, well worth remembering that in our own day another proof has been given, which has been generally allowed to be an important one. It is the result of an experiment of Foucault, and is simply this: if a pendulum, with a heavy weight attached to it, be made to oscillate in a plane due north and south, say in the latitude of Paris, the pendulum, after a time, and supposing it to continue in movement long enough for the purpose of observation, will oscillate in a direction slightly north-east and south-west. Now the pendulum moves naturally always in the same direction, backwards and forwards, as originally started, and if the Earth were shaped like a cylinder no change would be detected; but the spherical form of the Earth, as it rotates on its axis, here makes the whole difference; the floor of the room where the pendulum vibrates is carried round the axis of rotation, as everything else is, but the plane of oscillation remaining the same—or parallel to the original one—it no longer points north and south. At the equator this phenomenon would disappear, and in the southern hemisphere it would be the other way: that is, the pendulum would vibrate north-west and south-east.