Vistas in Astronomy, 1976, Vol. 20, pp. 11-15. Pergamon Press. Printed in Great Britain
SESS ION 2- THE ROYAL OBSERVATORY GREENWICH : SETT ING THE SCENE
2. SOME ASPECTS OF POST-RENAISSANCE ASTRONOMY
WILLIAM R. SHEA
McGill University, Montreal
In 1610, Galileo published his Sidereus Nuncius in which he revealed the existence of unknown stars, the nature of the Milky Way, the rugged surface of the Moon, and the presence of satellites around Jupiter. The striking similarities between the Moon and the Earth made the already dubious division between the sublunary and the celestial worlds even less plausible, and the fact that Jupiter could orbit with a train of four satellites provided Galileo with a reply to those who asked how the Earth could rush through space without losing its Moon. The telescope continued to bear witness to the truth of Copernicus' vision, and within a few months Galileo was able to announce that Venus had phases, and that the variations of the apparent diameters of Mars and Venus were in agreement with the heliocentric theory. By the end of 1610, the Copernican hypothesis had crossed the threshold of respectability. Even Clavius, the scholarly and prudent astronomer of the Collegio Romano went so far as to commend Galileo's discoveries in no uncertain terms. 1
But the battle was by no means won. A mere looking-glass could not dispel a theory about the structure of the world. The Aristotelians felt that Galileo would have to focus the eye of his mind on the real problems before he could persuade them to alter their convictions about the nature of the universe.
Francesco Sizzi, a Florentine who was later to anticipate Galileo's discovery that the path of the sunspots is oblique, wrote a strongly-worded tract in which he pointed out the manifest incon- veniences of increasing the number of planets from seven to eleven by the addition of the four alleged satellites of Jupiter. He offered a variety of arguments, drawn from the microcosm-macrocosm analogy, of which the following is perhaps the most cogent: since we have seven openings in our head (two eyes, two ears, two nostrils and one mouth) it is therefore "natural" that there should be only seven planets in the face of the heavens. 2
Sizzi was not taken too seriously even in his own day, but his preoccupation with the number of planets is symptomatic of his age and was shared by more prominent figures of the Scientific Revolution. The young Kepler incessantly asked himself, "Why six planets?" and we know that he never abandoned the Pythagorean solution based on the four regular solids that he first advocated in his Mysterium Cosmographicum of 1597.
Some 200 years later, when the planets had, with the addition of Uranus, grown to seven, an interest in their numbers waxed again. What sparked the revival was Bode's Law, a numerical series that begins with four and then increases step by step according to a simple rule in such a way that the numbers coincide very closely with the distances of the planets from the Sun. When Uranus was discovered it was found to fit this law and hence vastly increased its plausibility. But Bode's Law also gave a value for a planetary orbit between Mars and Jupiter, and on the Continent German astro- nomers banded themselves into groups to hunt for the missing planet much to the dismay of the eminent philosopher, G.W.F. Hegel, who felt that the sequence in Bode's Law was purely accidental and did not warrant such a flurry of scientific activity, Hegel does not actually say that he was worried about the taxpayers' or the patrons' money, but there can be no doubt that he believed in the rationalization of research and that he wished that precious observation time should not be wasted. Scanning the sky for a planet whose existence was surmised from Bode's Law appeared to Hegel to be an act of intellectual and administrative irresponsibility. A little sound reasoning, he argued in his Habilitationsschrift of 1801, would show that the seven known planets are suitably spaced and that
12 WILLIAM R. SHEA
there is no gap crying out to be filled between Mars and Jupiter. Like Kepler he found his solution in Plato but this time in the so-called Pythagorean series rather than in the regular solids. This series (1, 2, 3, 4, (22), 9 (32), 8 (23), 27 (33) not only conveniently limits the number of planets to 7, but it also retains between the fourth and the fifth planet the large distance whose incongruity caused astronomers all over Europe to look for a planet between Mars and Jupiter. On Hegel's neo- Pythagorean explanation there is no incongruity, unless it is to be found in his strange and unexplain- ed substitution of 16 for 8 as the relative distance of the sixth planet Saturn. 3
Astronomers may experience difficulties in justifying their use of precious observation time but armchair philosophers are subject to even greater perils. Before Hegel had delivered his inaugural lecture at Iena in 1801, Piazzi had sighted the asteroid Ceres and, within 7 years, Pallas, Juno, and Vesta had been detected by enthusiasts of Bode's Law.
In his Naturpbilosopbie of 1816, Hegel graciously acknowledged the existence of these asteroids and, since he had by then become Europe's leading philosopher, he even admitted some merit in Bode's Law. He would not, however, abandon his deep conviction that the planetary system was "governed by constructs of the mind". The Pythagorean series was not, he regretfully confessed, the one that the Creator had used in ordering the planets. All that could now be adduced was that the planets fell into three groups. The four planets with no satellites, or only one such as the Earth, made up the first, the asteroids the second, and the third was composed of the outer planets with several satellites or rings such as Saturn. 4 Fortunately, Hegel died in 1831 and was spared the shock of the discovery of two satellites of Mars by Asaph Hall in 1877!
For the purpose of this paper I consider Post-Renaissance astronomy as covering the period between the hermetic justification of the number seven by Sizzi and the Pythagorean vindication of the same number by Hegel.
Of course, during this period much was going on outside the minds of philosophers, and the most exciting element was not the quest for the "true" mathematical series but the increase in size of the universe.
Copernicus had been fully aware of the consequence of asserting that the Earth moved around the Sun although the fixed stars appear immobile. This could be reconciled with the experimental error of 6 to 10 minutes of arc present in the determination of the position of stars only if they were a thousand times farther than the radius of the Earth's orbit. This meant that a vast tract of empty space must separate Saturn from the sphere of fixed stars and Copernicus seems to have positively welcomed this as evidence of the perfection of "the godlike work of the Best and Greatest Artist" who clearly set apart "the moved from the unmoved", s But many saw in this enormous distance both a lack of proportion and a meaningless void. Simplicio, in Galileo's Dialogue on tbe Great World Systems, expresses a view common to many astronomers including Tycho Brahe: "Now when we see the beautiful order of the planets, arranged around the Earth at distances commensurate with their producing upon it their effects for our benefit, why go on to place between the highest orb, namely that of Saturn, and the stellar sphere an enormous, superfluous and vain space without any star whatsoever? To what end? For the use and convenience of whom? ''6
Yet Copernicus had a radical precursor in Cardinal Cusanus who, in his Docta Ignorantia, spoke of an infinite world, a view that was championed in somewhat more strident tones by Giordano Bruno in the sixteenth century. For Bruno's followers there were as many worlds as there are fixed stars and they declared that the nova of 1604 heralded the birth of a new world. This, for Kepler, was sleepwalking of the worst kind: the notion of an infinite universe made him shudder as at the mention of something occult. 7
But the telescope went on disclosing how similar other planets are to the Earth. The most spectacular revelation was the discovery that the apparently smooth and perfectly spherical surface of the Moon was really "uneven, rough, and full of cavities and prominences". 8 The Moon appeared so much like the Earth that Galileo described a large circular spot in the centre as offering "the same appearance as would a region like Bohemia if that were enclosed by very lofty mountains arranged exactly in a circle". 9
Such a comparison was fraught with danger in the home of the Counter-Reformation and Monsignor Ciampoli, Galileo's friend in Rome, found it necessary to warn Galileo: "Your opinion regarding the phenomena of light and shadow in the bright and clark spots of the Moon creates some analogy between the lunar globe and the Earth; somebody expands on this, and says that you place human inhabitants on the Moon; the next fellow starts to dispute how these can be descended from
Some aspects of post-Renaissance astronomy 13
Adam, or how they can have come off Noah's ark, and many other extravagances you never dreamed Of" . 1 0
But the tendency to people the planets proved irresistible. On reading the Sidereus Nuncius, Kepler immediately bethought himself of co-operating with Galileo in preparing for an eventual trip to the Moon and Jupiter. These planets probably had inhabitants and it was likely that explorers from the Earth would reach them as soon as the art of flying was masteredJ
In 1638, the Bishop of Chester, John Wilkins, writing under somewhat less severe theological constraints than Galileo, affirmed the plausibility of extraterrestrials but still felt it necessary to de- vote half his book to showing that such an opinion was neither impious nor irrational. A few years later, Pierre Borel thought he could prove that the Moon was inhabited, 12 and Fontenelle, in his widely read Entretiens sur la pluralit'e des mondes, simply took it for granted. By the end of the century, as is evidenced in J ohn Ray's The Wisdom of God Manifested in the Works of Creation, it had achieved the status of a theological commonplace.13
Fontenelle indulged in a description of the physical and psychological traits of the inhabitants of the various planets. The Marquise, his pupil, is made to understand that the people on Venus must resemble the Moors of Granada, a small black people full of spirit and fire always ready to make love, busy writing poetry and eager to invent fiestas, dances and parades every dayJ a Fontenelle's Entretiens ran into thirty-three editions during his lifetime and scientists do not appear to have looked askance at them. Huygens' objection to such phantasies was not that they were wrong-headed but that they lacked a sound scientific footing, which he proceeded to supply in his own Cosmotbeoreos. Since the same laws apply throughout the universe, Huygens described in considerable detail both the populations and the eco-systems of our neighboring planets. He was also able to enlighten his readers on engineering techniques and on the practice of shipbuilding especially on Jupiter and Saturn. 1 s
The eighteenth century may be said to bask in the light of these new certitudes. Berkeley asserts on the grounds of "common sense" that "there are innumerable orders of intelligent being more happy and more perfect than man" J 6 J.H. Lambert, in his Cosmologiscbe Briefe of 1751, also felt that the existence of extraterrestrials was obvious.~ 7 When Bode translated Fontenelle's Entretiens into German, he left no doubt that he undertook the task in a scientific rather than a literary frame of mind. The passage in which Fontenelle describes the extreme liveliness of the inhabitants of Venus called forth the following comment: "Peculiar[ One finds rather with us here [in Berlin] that too much heat makes the mind more sleepy and lazy rather than l ively") s
Kant was convinced that the planets were populated and he described the characteristics of the various planetary races as conditioned by their relative position. As the various races of mankind have their physical and mental characteristics moulded by their geographical environment so the moral and intellectual characteristics of the various planetary beings are determined by their distance from the Sun.
Kant was inclined to seek the most perfect classes of rational beings far away from the centre of the solar system since it is likely that the matter of the outer planets is lighter and finer and would prove less of a hindrance to the activities of the soul. In the third part of his Universal Natural History and Theory oftbe Heavens (unaccountably omitted from the English translation) he even suggests that Newton would be looked upon as an ape on Jupiter or worse still on Saturn. 19
But while a community of nations, albeit of varying moral and intellectual aptitudes, was being postulated, the planets themselves were moving apart at a tremendous pace as each successively more precise determination increased the size of the solar system.
One of the advantages of the Copernican system was the remarkably accurate determination of the relative planetary distances as can be seen when they are compared with the modern data given in parentheses: 0.37 (0.38), 0.72 (0.72), 1.52 (1.52), 5.22 (5.20), 9.17 (9.54). The importance of such relative spacing was brought out in Kepler's Third Law connecting the periods and mean distances of the planets. Kepler was eager to point out how suitable it was to use the Earth-Sun distance as the measuring rod of the Universe but, however appropriate, the measuring-rod could not begin to convey the grandiose dimensions of the solar system until its length had been determined. This was not achieved with reasonable accuracy until about 40 years after Kepler's death when, in 1672, Richer and Picard measured the parallax of Mars. The value of the parallax of Mars yielded only its distance, but by Kepler's Third Law the distances of all the other planets followed readily, and so the richly endowed inhabitants of Saturn at the outer limits of the solar system, turned out to be not at 20,000 but at 200,000 Earth-radii.
14 WILLIAM R. SHEA
If the solar system had increased tenfold, the stars had receded even more. But by how much? Kepler had offered 60 million Earth-radii for the distance of the fixed stars, a value that he obtained from geometry rather than observation, namely by assuming that the orbit of Saturn was a geo - metrical mean between the diameter of the Sun and the diameter of the spheres of fixed stars. 20 Bold as this conjecture of 60 million Earth-radii was, it represents only 1% of the correct value for the nearest stars.
Huygens arrived at a much larger value by assuming that the Sun and Sirius were similar stars