Galileo and His Judges

by F. R. Wegg-Prosser

It being clear then that Venus and Mercury revolve round the Sun, Galileo shows what strong ground there is for inferring that the superior planets, Mars, Jupiter, and Saturn (the others not being then known), do so also; this he judges from the greater size of these latter, and particularly of Mars, when in opposition than when in conjunction; whence we may conclude that the Earth, which as well as the Sun is contained within their orbits, is not in the centre of them, or nearly so. It is remarkable that Galileo treats all the planets as revolving in circles, though one would think he must at that time have been aware of Kepler’s discovery—that they move in ellipses. He makes Simplicio grant these last-mentioned points, which is curious; and he also explains how the telescope showed phenomena, such as the phases of Venus, which were unknown to Copernicus. Simplicio has hitherto had no confidence in this new instrument, and following in the footsteps of his friends the Peripatetic philosophers, has supposed the appearances in question to be optical illusions arising from the lenses used; he will, however, gladly be corrected if in error. Simplicio’s mathematical acquirements are not very great, and it is necessary to explain to him that the areas of circles vary in proportion, not to their diameters simply but to the squares of the diameters, a point which arises in reference to the false judgment formed by the naked eye as to the size of the celestial bodies, an error which is corrected by the telescope. Then to those who made it a difficulty that the Earth should move round the Sun, not alone, but accompanied by the Moon, Salviati is made to reply that Jupiter revolves round the Sun accompanied by four moons.

Again the greater simplicity of the Copernican theory, in accounting for the planetary motions, as they appear to us, is expounded by the same personage.

Galileo occasionally makes the interlocutors allude to himself as “il nostro amico comune,” “il nostro Accademico Linceo,” etc., and thus claims credit for having been the first to discover the solar spots, a credit which ought not to belong exclusively to him, as Fabricius and the Jesuit Father Scheiner saw the spots at about the same time.

An argument is here attempted to be drawn in favour of the Earth’s annual motion from the apparent course of the Sun-spots, and the curves they sometimes describe (as viewed from hence), owing to the inclination of the Sun’s axis to an axis perpendicular to the plane of the ecliptic—an inclination of about 7°; there is nothing, however, at all conclusive in such argument, because the appearances in question result from the different relative positions of the Earth and Sun at different seasons of the year, and would be the same whichever of the two bodies were in motion.

There follows some conversation arising from one of the anti-Copernican books of that day; one of the difficulties suggested, being the vast distance at which you must suppose the fixed stars to be placed, if Copernicus be right. We who are accustomed to the idea of these immense distances, can scarcely understand the prejudices of the philosophers of that age against admitting them. And it is worth noting that Galileo takes for granted, while answering these theoretical objections, the calculation of his predecessors—that the distance of the Sun is that of 1,208 semi-diameters of the Earth, that is something more than 4,800,000 miles, about one-nineteenth part of what we now know it to be. So also he supposes the size of the Sun to be much less than what is really the case. He was also under the erroneous impression, arising doubtless from the imperfection of the instruments he used, that the stars really had an apparent diameter, though less than Tycho Brahé and other astronomers had supposed, and estimates the angular diameter of a star of the first magnitude at about 5″; consequently he imagined the stars to be much nearer than is actually the fact. It is well known to modern observers, that the apparent size of a star is the effect of an optical illusion, and that greatly as the stars vary in brightness, they present no appreciable diameter at all to the eye; not even those classed as being of the first magnitude.

Another and more weighty objection to Copernicus is, however, urged by the mouth of Simplicio, and it is this—if the Earth really makes an annual revolution round the Sun, why do not the fixed stars, viewed as they must be at different seasons of the year from points so widely distant, change their apparent positions in the heavens? We have just seen that the true distance of the Sun was not known at that time;—if it had been known, and if the men of that age had been aware that the diameter of the Earth’s orbit was about 184,000,000 miles in length, the objection would have been still more forcible. But the modern answer to it is conclusive: the stars, or rather a certain number of them, do actually undergo a small displacement in their apparent position every year, or in the technical language of astronomy, they have an annual parallax, a fact which not merely disposes of the objection, but actually confirms the truth of the Copernican theory.

Galileo’s reply (by the mouth of Salviati) is to the effect that the followers of Ptolemy admit that it takes 36,000 years to effect a complete revolution of the starry sphere; then, judging from the planets, the length of time required for the orbit is in proportion to the distance, and we suppose the distance of the starry sphere to be, on such assumption, 10,800 semi-diameters of the Earth’s orbit (or Sun’s orbit, as they called it). At so great a distance as that, the change of position caused by the Earth’s annual motion round the Sun would not be appreciable.

The principle of this reply is of course quite sound, and we, who know the stars to be considerably farther from us than the above estimate supposes, can well understand that the vast majority of them have no annual parallax whatever, that the finest instruments can discover.

To further objections drawn from the enormous distances of the stars, and the difficulty of perceiving the use which such remote bodies can be to the Earth, it is replied that such speculations are useless and presumptuous, and also that words like small, very small, immense, etc., are relative rather than absolute.

Some pains are taken in the course of the dialogue to explain how the stars, according to their different positions, would be affected by annual parallax, supposing such to be discoverable, and assuming the motion of the Earth. And a minute explanation is also given, on this latter assumption, of the length of day and night varying in different latitudes according to the seasons; illustrating the fact that details which appear to us elementary and are taught to schoolboys, were strange to the minds even of educated and learned men in those days.