_Astronomy before Copernicus—Fracastoro—Bacon—Kepler—Galileo's
Treatise on the Sphere._
THIS period of Galileo's lectureship at Padua derives interest from its
including the first notice which we find of his having embraced the
doctrines of the Copernican astronomy. Most of our readers are aware of
the principles of the theory of the celestial motions which Copernicus
restored; but the number of those who possess much knowledge of the
cumbrous and unwieldy system which it superseded is perhaps more
limited. The present is not a fit opportunity to enter into many details
respecting it; these will find their proper place in the History of
Astronomy: but a brief sketch of its leading principles is necessary to
render what follows intelligible.
The earth was supposed to be immoveably fixed in the centre of the
universe, and immediately surrounding it the atmospheres of air and
fire, beyond which the sun, moon, and planets, were thought to be
carried round the earth, fixed each to a separate orb or heaven of solid
but transparent matter. The order of distance in which they were
supposed to be placed with regard to the central earth was as follows:
The Moon, Mercury, Venus, The Sun, Mars, Jupiter, and Saturn. It became
a question in the ages immediately preceding Copernicus, whether the Sun
was not nearer the Earth than Mercury, or at least than Venus; and this
question was one on which the astronomical theorists were then chiefly
divided.
We possess at this time a curious record of a former belief in this
arrangement of the Sun and planets, in the order in which the days of
the week have been named from them. According to the dreams of
Astrology, each planet was supposed to exert its influence in
succession, reckoning from the most distant down to the nearest, over
each hour of the twenty-four. The planet which was supposed to
predominate over the first hour, gave its name to that day.[17] The
general reader will trace this curious fact more easily with the French
or Latin names than with the English, which have been translated into
the titles of the corresponding Saxon deities. Placing the Sun and
planets in the following order, and beginning, for instance, with
Monday, or the Moon's day; Saturn ruled the second hour of that day,
Jupiter the third, and so round till we come again and again to the Moon
on the 8th, 15th, and 22d hours; Saturn ruled the 23d, Jupiter the
24th, so that the next day would be the day of Mars, or, as the Saxons
translated it, Tuisco's day, or Tuesday. In the same manner the
following days would belong respectively to Mercury or Woden, Jupiter or
Thor, Venus or Frea, Saturn or Seater, the Sun, and again the Moon. In
this manner the whole week will be found to complete the cycle of the
seven planets.
[Illustration: Cycle of the seven planets.]
The other stars were supposed to be fixed in an outer orb, beyond which
were two crystalline spheres, (as they were called,) and on the outside
of all, the _primum mobile_ or _first moveable_, which sphere was
supposed to revolve round the earth in twenty-four hours, and by its
friction, or rather, as most of the philosophers of that day chose to
term it, by the sort of heavenly influence which it exercised on the
interior orbs, to carry them round with a similar motion. Hence the
diversity of day and night. But beside this principal and general
motion, each orb was supposed to have one of its own, which was intended
to account for the apparent changes of position of the planets with
respect to the fixed stars and to each other. This supposition, however,
proving insufficient to account for all the irregularities of motion
observed, two hypotheses were introduced.—First, that to each planet
belonged several concentric spheres or heavens, casing each other like
the coats of an onion, and, secondly, that the centres of these solid
spheres, with which the planet revolved, were placed in the
circumference of a secondary revolving sphere, the centre of which
secondary sphere was situated at the earth. They thus acquired the names
of Eccentrics or Epicycles, the latter word signifying a circle upon a
circle. The whole art of astronomers was then directed towards inventing
and combining different eccentric and epicyclical motions, so as to
represent with tolerable fidelity the ever varying phenomena of the
heavens. Aristotle had lent his powerful assistance in this, as in other
branches of natural philosophy, in enabling the false system to prevail
against and obliterate the knowledge of the true, which, as we gather
from his own writings, was maintained by some philosophers before his
time. Of these ancient opinions, only a few traces now remain,
principally preserved in the works of those who were adverse to them.
Archimedes says expressly that Aristarchus of Samos, who lived about 300
B. C., taught the immobility of the sun and stars, and that the earth is
carried round the central sun.[18] Aristotle's words are: "Most of those
who assert that the whole concave is finite, say that the earth is
situated in the middle point of the universe: those who are called
Pythagoreans, who live in Italy, are of a contrary opinion. For they say
that fire is in the centre, and that the earth, which, according to
them, is one of the stars, occasions the change of day and night by its
own motion, with which it is carried about the centre." It might be
doubtful, upon this passage alone, whether the Pythagorean theory
embraced more than the diurnal motion of the earth, but a little
farther, we find the following passage: "Some, as we have said, make the
earth to be one of the stars: others say that it is placed in the centre
of the Universe, and revolves on a central axis."[19] From which, in
conjunction with the former extract, it very plainly appears that the
Pythagoreans maintained both the diurnal and annual motions of the
earth.
Some idea of the supererogatory labour entailed upon astronomers by the
adoption of the system which places the earth in the centre, may be
formed in a popular manner by observing, in passing through a thickly
planted wood, in how complicated a manner the relative positions of the
trees appear at each step to be continually changing, and by considering
the difficulty with which the laws of their apparent motions could be
traced, if we were to attempt to refer these changes to a real motion of
the trees instead of the traveller. The apparent complexity in the
heavens is still greater than in the case suggested; because, in
addition to the earth's motions, with which all the stars appear to be
impressed, each of the planets has also a real motion of its own, which
of course greatly contributes to perplex and complicate the general
appearances. Accordingly the heavens rapidly became, under this system,
"With centric and eccentric scribbled o'er,
Cycle and epicycle, orb in orb;"[20]
crossing and penetrating each other in every direction. Maestlin has
given a concise enumeration of the principal orbs which belonged to this
theory. After warning the readers that "they are not mere fictions which
have nothing to correspond with them out of the imagination, but that
they exist really, and bodily in the heavens,"[21] he describes seven
principal spheres belonging to each planet, which he classes as
Eccentrics, Epicycles, and Concentrepicycles, and explains their use in
accounting for the planet's revolutions, motions of the apogee, and
nodes, &c. &c. In what manner this multitude of solid and crystalline
orbs were secured from injuring or interfering with each other was not
very closely inquired into.
The reader will cease to expect any very intelligible explanation of
this and numberless other difficulties which belong to this unwieldy
machinery when he is introduced to the reasoning by which it was upheld.
Gerolamo Fracastoro, who lived in the sixteenth century, writes in the
following terms, in his work entitled Homocentrica, (certainly one of
the best productions of the day,) in which he endeavours to simplify the
necessary apparatus, and to explain all the phenomena (as the title of
his book implies) by concentric spheres round the earth. "There are
some, not only of the ancients but also among the moderns, who believe
that the stars move freely without any such agency; but it is difficult
to conceive in what manner they have imbued themselves with this notion,
_since not only reason, but the very senses, inform us that all the
stars are carried round fastened to solid spheres_." What ideas
Fracastoro entertained of the evidence of the "senses" it is not now
easy to guess, but he goes on to give a specimen of the "reasoning"
which appeared to him so incontrovertible. "The planets are observed to
move one while forwards, then backwards, now to the right, now to the
left, quicker and slower by turns; which variety is consistent with a
compound structure like that of an animal, which possesses in itself
various springs and principles of action, but is totally at variance
with our notion of a simple and undecaying substance like the heavens
and heavenly bodies. For that which is simple, is altogether single, and
singleness is of one only nature, and one nature can be the cause of
only one effect; and therefore it is altogether impossible that the
stars of themselves should move with such variety of motion. And
besides, if the stars move by themselves, they either move in an empty
space, or in a fluid medium like the air. But there cannot be such a
thing as empty space, and if there were such a medium, the motion of the
star would occasion condensation and rarefaction in different parts of
it, which is the property of corruptible bodies and where they exist
some violent motion is going on; but the heavens are incorruptible and
are not susceptible of violent motion, and hence, and from many other
similar reasons, any one who is not obstinate may satisfy himself that
the stars cannot have any independent motion."
Some persons may perhaps think that arguments of this force are
unnecessarily dragged from the obscurity to which they are now for the
most part happily consigned; but it is essential, in order to set
Galileo's character and merits in their true light, to show how low at
this time philosophy had fallen. For we shall form a very inadequate
notion of his powers and deserts if we do not contemplate him in the
midst of men who, though of undoubted talent and ingenuity, could so far
bewilder themselves as to mistake such a string of unmeaning phrases for
argument: we must reflect on the difficulty every one experiences in
delivering himself from the erroneous impressions of infancy, which will
remain stamped upon the imagination in spite of all the efforts of
matured reason to erase them, and consider every step of Galileo's
course as a triumph over difficulties of a like nature. We ought to be
fully penetrated with this feeling before we sit down to the perusal of
his works, every line of which will then increase our admiration of the
penetrating acuteness of his invention and unswerving accuracy of his
judgment. In almost every page we discover an allusion to some new
experiment, or the germ of some new theory; and amid all this wonderful
fertility it is rarely indeed that we find the exuberance of his
imagination seducing him from the rigid path of philosophical induction.
This is the more remarkable as he was surrounded by friends and
contemporaries of a different temperament and much less cautious
disposition. A disadvantageous contrast is occasionally furnished even
by the sagacious Bacon, who could so far deviate from the sound
principles of inductive philosophy, as to write, for instance, in the
following strain, bordering upon the worst manner of the
Aristotelians:—"Motion in a circle has no limit, and seems to emanate
from the appetite of the body, which moves only for the sake of moving,
and that it may follow itself and seek its own embraces, and put in
action and enjoy its own nature, and exercise its peculiar operation: on
the contrary, motion in a straight line seems transitory, and to move
towards a limit of cessation or rest, and that it may reach some point,
and then put off its motion."[22] Bacon rejected all the machinery of
the _primum mobile_ and the solid spheres, the eccentrics and the
epicycles, and carried his dislike of these doctrines so far as to
assert that nothing short of their gross absurdity could have driven
theorists to the extravagant supposition of the motion of the earth,
which, said he, "we know to be most false."[23] Instances of extravagant
suppositions and premature generalizations are to be found in almost
every page of his other great contemporary, Kepler.
It is with pain that we observe Delambre taking every opportunity, in
his admirable History of Astronomy, to undervalue and sneer at Galileo,
seemingly for the sake of elevating the character of Kepler, who appears
his principal favourite, but whose merit as a philosopher cannot safely
be brought into competition with that of his illustrious contemporary.
Delambre is especially dissatisfied with Galileo, for taking no notice,
in his "System of the World," of the celebrated laws of the planetary
motions which Kepler discovered, and which are now inseparably connected
with his name. The analysis of Newton and his successors has now
identified those apparently mysterious laws with the general phenomena
of motion, and has thus entitled them to an attention of which, before
that time, they were scarcely worthy; at any rate not more than is at
present the empirical law which includes the distances of all the
planets from the sun (roughly taken) in one algebraical formula. The
observations of Kepler's day were scarcely accurate enough to prove that
the relations which he discovered between the distances of the planets
from the sun and the periods of their revolutions around him were
necessarily to be received as demonstrated truths; and Galileo surely
acted most prudently and philosophically in holding himself altogether
aloof from Kepler's fanciful devices and numeral concinnities, although,
with all the extravagance, they possessed much of the genius of the
Platonic reveries, and although it did happen that Galileo, by
systematically avoiding them, failed to recognise some important truths.
Galileo probably was thinking of those very laws, when he said of
Kepler, "He possesses a bold and free genius, perhaps too much so; but
his mode of philosophizing is widely different from mine." We shall have
further occasion in the sequel to recognise the justice of this remark.
In the treatise on the Sphere which bears Galileo's name, and which, if
he be indeed the author of it, was composed during the early part of his
residence at Padua, he also adopts the Ptolemaic system, placing the
earth immoveable in the centre, and adducing against its motion the
usual arguments, which in his subsequent writings he ridicules and
refutes. Some doubts have been expressed of its authenticity; but,
however this may be, we have it under Galileo's own hand that he taught
the Ptolemaic system, in compliance with popular prejudices, for some
time after he had privately become a convert to the contrary opinions.
In a letter, apparently the first which he wrote to Kepler, dated from
Padua, 1597, he says, acknowledging the receipt of Kepler's Mysterium
Cosmographicum, "I have as yet read nothing beyond the preface of your
book, from which however I catch a glimpse of your meaning, and feel
great joy on meeting with so powerful an associate in the pursuit of
truth, and consequently such a friend to truth itself, for it is
deplorable that there should be so few who care about truth, and who do
not persist in their perverse mode of philosophizing; but as this is not
the fit time for lamenting the melancholy condition of our times, but
for congratulating you on your elegant discoveries in confirmation of
the truth, I shall only add a promise to peruse your book
dispassionately, and with a conviction that I shall find in it much to
admire. _This I shall do the more willingly because many years ago I
became a convert to the opinions of Copernicus_,[24] and by that theory
have succeeded in fully explaining many phenomena, which on the contrary
hypothesis are altogether inexplicable. I have arranged many arguments
and confutations of the opposite opinions, _which however I have not yet
dared to publish_, fearing the fate of our master Copernicus, who,
although he has earned immortal fame among a few, yet by an infinite
number (for so only can the number of fools be measured) is exploded and
derided. If there were many such as you, I would venture to publish my
speculations; but, since that is not so, I shall take time to consider
of it." This interesting letter was the beginning of the friendship of
these two great men, which lasted uninterruptedly till 1630, the date of
Kepler's death. That extraordinary genius never omitted an opportunity
of testifying his admiration of Galileo, although there were not wanting
persons envious of their good understanding, who exerted themselves to
provoke coolness and quarrel between them. Thus Brutius writes to Kepler
in 1602:[25] "Galileo tells me he has written to you, and has got your
book, which however he denied to Magini, and I abused him for praising
you with too many qualifications. I know it to be a fact that, both in
his lectures, and elsewhere, he is publishing your inventions as his
own; but I have taken care, and shall continue to do so, that all this
shall redound not to his credit but to yours." The only notice which
Kepler took of these repeated insinuations, which appear to have been
utterly groundless, was, by renewed expressions of respect and
admiration, to testify the value he set upon his friend and
fellow-labourer in philosophy.
FOOTNOTES:
[17] Dion Cassius, lib. 37.
[18] The pretended translation by Roberval of an Arabic version of
Aristarchus, "De Systemate Mundi," in which the Copernican system is
fully developed, is spurious. Menage asserts this in his observations on
Diogen. Laert. lib. 8, sec. 85, tom. ii., p. 389. (Ed. Amst. 1692.) The
commentary contains many authorities well worth consulting. Delambre,
Histoire de l'Astronomie, infers it from its not containing some
opinions which Archimedes tells us were held by Aristarchus. A more
direct proof may be gathered from the following blunder of the supposed
translator. Astronomers had been long aware that the earth in different
parts of her orbit is at different distances from the sun. Roberval
wished to claim for Aristarchus the credit of having known this, and
introduced into his book, not only the mention of the fact, but an
explanation of its cause. Accordingly he makes Aristarchus give a reason
"why the sun's apogee (or place of greatest distance from the earth)
must always be at the north summer solstice." In fact, it was there, or
nearly so, in Roberval's time, and he knew not but that it had always
been there. It is however moveable, and, when Aristarchus lived, was
nearly half way between the solstices and equinoxes. He therefore would
hardly have given a reason for the necessity of a phenomenon of which,
if he observed anything on the subject, he must have observed the
contrary. The change in the obliquity of the earth's axis to the
ecliptic was known in the time of Roberval, and he accordingly has
introduced the proper value which it had in Aristarchus's time.
[19] De Cœlo. lib. 2.
[20] Paradise Lost, b. viii. v. 83.
[21] Itaque tam circulos primi motus quam orbes secundorum mobilium
reverâ in cœlesti corpore esse concludimus, &c. Non ergo sunt mera
figmenta, quibus extra mentem nihil correspondeat. M. Maestlini, De
Astronomiæ Hypothesibus disputatio. Heidelbergæ, 1582.
[22] Opuscula Philosophica, Thema Cœli.
[23] "Nobis constat falsissimum esse." De Aug. Scient. lib. iii. c. 3,
1623.
[24] Id autum eò libentius faciam, quod in Copernici sententiam multis
abhinc annis venerim.—Kepl. Epistolæ.
[25] Kepleri Epistolæ.