Galileo and His Judges

by F. R. Wegg-Prosser

As a sequel to the story of Galileo, I think it may be interesting to inquire what the evidence, as we now have it, proves with regard to the truth of the Copernican theory, there being two opposite and contradictory errors on this subject, and these not merely popular errors, but shared to some extent by educated and otherwise learned men. But I must, before proceeding, remind my readers that I use the word Copernican simply to signify the system of modern astronomy, that in which the Sun is the centre round which the Earth and the other planets revolve, and not as meaning the precise theory of Copernicus, which (as I have said) was overthrown by Kepler, when he discovered that the planetary orbits were not circular but elliptical, the Sun, moreover, not being strictly in the centre, but in one of the foci of the orbit.

Now it is a plain fact, which all persons must perceive, that either the Earth revolves on its axis in twenty-four hours (more accurately 23 hours 56 mins. 5 secs.), or else that the whole of the celestial bodies are carried round the Earth in that same time. It is also a fact no less perceptible to careful observers, that either the Sun goes round the Earth in the course of a year, or else that the Earth goes round the Sun. The question is how these facts are to be accounted for.

The first of the two errors I have just mentioned is that which supposes the Copernican theory to have been directly and conclusively proved. This I imagine to be very common, and to arise from the elementary books learnt by schoolboys, which state (naturally enough) the modern theory of astronomy without the reasons that support it.

We need not dwell long on this point. Persons who have got this erroneous impression misunderstand the nature of the evidence. Some things in astronomy can be positively proved from observation, as, for instance, the existence of sun-spots. Many things in mechanics, chemistry, optics, and other branches of physical study can be demonstrated by experiment. The motion of the Earth round the Sun cannot, however, be so treated. It is inferred, and very rightly so, from the fact that it explains completely and easily all the observed phenomena, while, on the other hand, there are certain things which, as far as our present knowledge goes, cannot be explained in any other way; and the same argument applies to the rotation of the Earth on its axis. But though all this is perfectly clear so far, who can possibly say that as science progresses some explanation may not be hereafter found consistent with the antagonistic theory—consistent, let us say, with the system of Tycho Brahé, or some modification of it? I need not add that I consider the future discovery of such explanation as so improbable, that one may practically dismiss the idea, but I should be sorry to deny it as being conceivably possible.

The other, and opposite, error is that of certain well-meaning but ill-informed persons, who imagine that the Copernican theory is even now doubtful and liable to be overthrown—liable, I mean, in a real and practical sense, and not by distant contingencies, such as those at which I have just hinted, and which may be considered as shadowy and intangible. I do not suppose that amongst educated men there are many such scientific recusants; but at any rate it may be useful to give a short summary of the evidence on which the Copernican conclusion is based. In doing this I fear I shall tire the patience of my readers by partly repeating Galileo’s own arguments, which I have already quoted in discussing the Dialogue. This cannot easily be avoided, for much of his reasoning is so sound and so forcible, that after the lapse of more than two centuries we can add but little to it. On the other hand, there are grave mistakes that must be shunned; and, moreover, there have been discoveries made since the day when the Dialogue was written, of inestimable importance.

The best way of treating the question is to resume the history of astronomical research from the point where we dropped it; that is, at the time when Galileo first made known to the world the result of his observations.

It ought to be clearly understood that from the moment the telescope was turned on the heavens, the old system of astronomy was doomed, and nothing could finally have saved it. For a time prejudice and other more creditable feelings kept it floating on the sea of speculation, but such a state of things could not last; and the startling information that men like Galileo, Fabricius, and Scheiner imparted to the scientific world, could not fail to expel the old theory of the universe from the minds of men—at least, men of intellectual capacity—gradually and slowly, but yet most surely.

Now we have seen what the revelations were which the telescope at once displayed, even in its comparatively rude and imperfect state. There were the spots on the Sun, the satellites of Jupiter, the phases of Venus, the greater apparent size of the superior planets (Mars and the rest) when on the opposite side of the Earth from the Sun, this last phenomenon being quite inconsistent with the system of Ptolemy.

One consequence of all this was that the less enlightened men of the old school indulged in a violent antipathy to the new-fangled instrument, which threatened to overthrow their time-honoured traditions, and simply refused to believe in the telescope and its results. Thus the principal professor of philosophy at Padua, when invited by Galileo to look through his glass at the Moon and the planets, pertinaciously refused to do so. Simplicio, who, of course, represents in the Dialogue the prejudices of men of this stamp, admits (as we have seen) his feelings on this subject, and his suspicions that the new discoveries were to be attributed to optical errors. He was willing to be corrected if mistaken, but such had hitherto been his opinion.

It was not, however, to be expected that men of sound sense would allow themselves to be misled for any length of time by fallacies such as these. Continued observations carefully made are sure to correct mere optical errors, and after a reasonable interval it must have been evident that the phenomena discerned through the telescope were facts that had to be dealt with—not phantoms to be ignored.