_Observations on the Moon—Nebulæ—Saturn—Venus—Mars._
THERE were other discoveries announced in Galileo's book of great and
unprecedented importance, and which scarcely excited less discussion
than the controverted Medicæan planets. His observations on the moon
threw additional light on the constitution of the solar system, and
cleared up the difficulties which encumbered the explanation of the
varied appearance of her surface. The different theories current at that
day, to account for these phenomena, are collected and described by
Benedetti, and also with some liveliness, in a mythological poem, by
Marini.[56] We are told, that, in the opinion of some, the dark shades
on the moon's surface arise from the interposition of opaque bodies
floating between her and the sun, which prevents his light from reaching
those parts: others thought, that on account of her vicinity to the
earth, she was partly tainted with the imperfection of our terrestrial
and elementary nature, and was not of that entirely pure and refined
substance of which the more remote heavens consist: a third party looked
on her as a vast mirror, and maintained that the dark parts of her
surface were the reflected images of our earthly forests and mountains.
Galileo's glass taught him to believe that the surface of this planet,
far from being smooth and polished, as was generally taken for granted,
really resembled our earth in its structure; he was able distinctly to
trace on it the outlines of mountains and other inequalities, the
summits of which reflected the rays of the sun before these reached the
lower parts, and the sides of which, turned from his beams, lay buried
in deep shadow. He recognised a distribution into something similar to
continents of land, and oceans of water, which reflect the sun's light
to us with greater or less vivacity, according to their constitution.
These conclusions were utterly odious to the Aristotelians; they had
formed a preconceived notion of what the moon ought to be, and they
loathed the doctrines of Galileo, who took delight, as they said, in
distorting and ruining the fairest works of nature. It was in vain he
argued, as to the imaginary perfection of the spherical form, that
although the moon, or the earth, were it absolutely smooth, would indeed
be a more perfect sphere than in its present rough state, yet touching
the perfection of the earth, considered as a natural body calculated for
a particular purpose, every one must see that absolute smoothness and
sphericity would make it not only less perfect, but as far from being
perfect as possible. "What else," he demanded, "would it be but a vast
unblessed desert, void of animals, of plants, of cities and of men; the
abode of silence and inaction; senseless, lifeless, soulless, and stript
of all those ornaments which make it now so various and so beautiful?"
He reasoned to no purpose with the slaves of the ancient schools:
nothing could console them for the destruction of their smooth
unalterable surface, and to such an absurd length was this hallucination
carried, that one opponent of Galileo, Lodovico delle Colombe,
constrained to allow the evidence of the sensible inequalities of the
moon's surface, attempted to reconcile the old doctrine with the new
observations, by asserting, that every part of the moon, which to the
terrestrial observer appeared hollow and sunken, was in fact entirely
and exactly filled up with a clear crystal substance, perfectly
imperceptible by the senses, but which restored to the moon her
accurately spherical and smooth surface. Galileo met the argument in the
manner most fitting, according to one of Aristotle's own maxims, that
"it is foolish to refute absurd opinions with too much curiosity."
"Truly," says he, "the idea is admirable, its only fault is that it is
neither demonstrated nor demonstrable; but I am perfectly ready to
believe it, provided that, with equal courtesy, I may be allowed to
raise upon your smooth surface, crystal mountains (which nobody can
perceive) ten times higher than those which I have actually seen and
measured." By threatening to proceed to such extremities, he seems to
have scared the opposite party into moderation, for we do not find that
the crystalline theory was persevered in.
In the same essay, Galileo also explained at some length the cause of
that part of the moon being visible, which is unenlightened directly by
the sun in her first and last quarter. Maestlin, and before him Leonardo
da Vinci, had already declared this to arise from what may be called
_earthshine_, or the reflection of the sun's light from the terrestrial
globe, exactly similar to that which the moon affords us when we are
similarly placed between her and the sun; but the notion had not been
favourably received, because one of the arguments against the earth
being a planet, revolving like the rest round the sun, was, that it did
not shine like them, and was therefore of a different nature; and this
argument, weak as it was in itself, the theory of terrestrial reflection
completely overturned. The more popular opinions ascribed this feeble
light, some to the fixed stars, some to Venus, some to the rays of the
sun, penetrating and shining through the moon. Even the sagacious
Benedetti adopted the notion of this light being caused by Venus, in the
same sentence in which he explains the true reason of the faint light
observed during a total eclipse of the moon, pointing out that it is
occasioned by those rays of the sun, which reach the moon, after being
bent round the sides of the earth by the action of our atmosphere.[57]
Galileo also announced the detection of innumerable stars, invisible to
the unassisted sight; and those remarkable appearances in the heavens,
generally called nebulæ, the most considerable of which is familiar to
all under the name of the milky way, when examined by his instrument,
were found to resolve themselves into a vast collection of minute stars,
too closely congregated to produce a separate impression upon the
unassisted eye.[58] Benedetti, who divined that the dark shades on the
moon's surface arose from the constitution of those parts which suffered
much of the light to pass into them, and consequently reflected a less
portion of it, had maintained that the milky way was the result of the
converse of the same phenomenon, and declared, in the language of his
astronomy, that it was a part of the eighth orb, which did not, like the
rest, allow the sun's light to traverse it freely, but reflected a small
part feebly to our sight.
The Anti-Copernicans would probably have been well pleased, if by these
eternally renewed discussions and disputes, they could have occupied
Galileo's time sufficiently to detain his attention from his telescope
and astronomical observations; but he knew too well where his real
strength lay, and they had scarcely time to compound any thing like an
argument against him and his theories, before they found him in
possession of some new facts, which they were unprepared to meet,
otherwise than by the never-failing resource of abuse and affected
contempt. The year had not expired before Galileo had new intelligence
to communicate of the highest importance. Perhaps he had been taught
caution from the numerous piracies which had been committed upon his
discoveries, and he first announced his new discoveries enigmatically,
veiling their real import by transpositions of the letters in the words
which described them, (a practice then common, and not disused even at a
much later date,) and inviting all astronomers to declare, within a
certain time, if they had noted any thing new in the heavens worthy of
observation. The transposed letters which he published were—
"_Smaismrmilme poeta leumi bvne nugttaviras._"
Kepler, in the true spirit of his riddling philosophy, endeavoured to
decypher the meaning, and fancied he had succeeded when he formed a
barbarous Latin verse,
"_Salve umbistineum geminatum Martia proles_,"
conceiving that the discovery, whatever it might be, related to the
planet Mars, to which Kepler's attention had before been particularly
directed. The reader, however, need not weary himself in seeking a
translation of this solution, for at the request of the Emperor Rodolph,
Galileo speedily sent to him the real reading—
_Altissimum planetam tergeminum observavi_;
that is, "I have observed that the most distant planet is triple," or,
as he further explains the matter, "I have with great admiration
observed that Saturn is not a single star, but three together, which as
it were touch each other; they have no relative motion, and are
constituted in this form [Symbol: oOo] the middle being somewhat larger
than the lateral ones. If we examine them with an eye-glass which
magnifies the surface less than 1000 times, the three stars do not
appear very distinctly, but Saturn has an oblong appearance, like the
appearance of an olive, thus [Symbol: horizontal 0]. Now I have
discovered a court for Jupiter, and two servants for this old man, who
aid his steps and never quit his side." Galileo was, however, no match
in this style of writing for Kepler, who disapproved his friend's
metaphor, and, in his usual fanciful and amusing strain,—"I will not,"
said he, "make an old man of Saturn, nor slaves of his attendant globes,
but rather let this tricorporate form be Geryon, so shall Galileo be
Hercules, and the telescope his club; armed with which, he has conquered
that distant planet, and dragged him from the remotest depths of nature,
and exposed him to the view of all." Galileo's glass was not of
sufficient power to shew him the real constitution of this extraordinary
planet; it was reserved for Huyghens, about the year 1656, to declare to
the world that these supposed attendant stars are in fact part of a ring
which surrounds, and yet is completely distinct from the body of
Saturn;[59] and the still more accurate observations of Herschel have
ascertained that it consists of two concentric rings revolving round the
planet, and separated from each other by a space which our most powerful
telescopes scarcely enable us to measure.
Galileo's second statement concluded with the remark, that "in the other
planets nothing new was to be observed;" but a month had scarcely
elapsed, before he communicated to the world another enigma,
_Hæc immatura à me jam frustra leguntur oy_,
which, as he said, contained the announcement of a new phenomenon, in
the highest degree important to the truth of the Copernican system. The
interpretation of this is,
_Cynthiæ figuras æmulatur mater amorum_,
that is to say,—Venus rivals the appearances of the moon—for Venus
being now arrived at that part of her orbit in which she is placed
between the earth and the sun, and consequently, with only a part of her
enlightened surface turned towards the earth, the telescope shewed her
in a crescent form, like the moon in a similar position, and tracing her
through the whole of her orbit round the sun, or at least so long as she
was not invisible from his overpowering light, Galileo had the
satisfaction of seeing the enlightened portion in each position assume
the form appropriate to that hypothesis. It was with reason, therefore,
that he laid stress on the importance of this observation, which also
established another doctrine scarcely less obnoxious to the
Anti-Copernicans, namely, that a new point of resemblance was here found
between the earth and one of the principal planets; and as the
reflection from the earth upon the moon had shewn it to be luminous like
the planets when subjected to the rays of the sun, so this change of
apparent figure demonstrated that one of the planets not near the earth,
and therefore probably all, were in their own nature not luminous, and
only reflected the sun's light which fell upon them; an inference, of
which the probability was still farther increased a few years later by
the observation of the transit of Mercury over the sun's disc.
It is curious that only twenty-five years before this discovery of the
phases (or appearances) of Venus, a commentator of Aristotle, under the
name of Lucillus Philalthæus, had advanced the doctrine that all the
planets except the moon are luminous of themselves, and in proof of his
assertion had urged, "that if the other planets and fixed stars received
their light from the sun, they would, as they approached and receded
from him, or as he approached and receded from them, assume the same
phases as the moon, which, he adds, we have never yet observed."—He
further remarks, "that Mercury and Venus would, in the supposed case of
their being nearer the earth than the sun, eclipse it occasionally, just
as eclipses are occasioned by the moon." Perhaps it is still more
remarkable, that these very passages, in which the reasoning is so
correct, though the facts are too hastily taken for granted, (the common
error of that school,) are quoted by Benedetti, expressly to shew the
ignorance and presumption of the author. Copernicus, whose want of
instruments had prevented him from observing the horned appearance of
Venus when between the earth and sun, had perceived how formidable an
obstacle the non-appearance of this phenomenon presented to his system;
he endeavoured, though unsatisfactorily, to account for it by supposing
that the rays of the sun passed freely through the body of the planet,
and Galileo takes occasion to praise him for not being deterred from
adopting the system, which, on the whole, appeared to agree best with
the phenomena, by meeting with some which it did not enable him to
explain. Milton, whose poem is filled with allusions to Galileo and his
astronomy, has not suffered this beautiful phenomenon to pass unnoticed.
After describing the creation of the Sun, he adds:—
Hither, as to their fountain, other stars
Repairing, in their golden urns draw light,
And hence the morning planet gilds her horns.[60]
Galileo also assured himself, at the same time, that the fixed stars did
not receive their light from the sun. This he ascertained by comparing
the vividness of their light, in all positions, with the feebleness of
that of the distant planets, and by observing the different degrees of
brightness with which all the planets shone at different distances from
the sun. The more remote planets did not, of course, afford equal
facilities with Venus for so decisive an observation; but Galileo
thought he observed, that when Mars was in quadratures, (or in the
quarters, the middle points of his path on either side,) his figure
varied slightly from a perfect circle. Galileo concludes the letter, in
which he announces these last observations to his pupil Castelli, with
the following expressions, shewing how justly he estimated the
opposition they encountered:—"You almost make me laugh by saying that
these clear observations are sufficient to convince the most obstinate:
it seems you have yet to learn that long ago the observations were
enough to convince those who are capable of reasoning, and those who
wish to learn the truth; but that to convince the obstinate, and those
who care for nothing beyond the vain applause of the stupid and
senseless vulgar, not even the testimony of the stars would suffice,
were they to descend on earth to speak for themselves. Let us then
endeavour to procure some knowledge for ourselves, and rest contented
with this sole satisfaction; but of advancing in popular opinion, or
gaining the assent of the book-philosophers, let us abandon both the
hope and the desire."
FOOTNOTES:
[56] Adone di Marini, Venetiis, 1623, Cant. x.
[57] Speculat. Lib Venetiis, 1585, Epistolæ.
[58] This opinion, with respect to the milky way, had been held by some
of the ancient astronomers. _See_ Manilius. Lib. i. v. 753.
"_Anne magis densâ stellarum turba coronâ_
"_Contexit flammas, et crasso lumine candet,_
"_Et fulgore nitet collato clarior orbis._"
[59] Huyghens announced his discovery in this form: _a a a a a a a c c c
c c d e e e e e g h i i i i i i i l l l l m m n n n n n n n n n o o o o
p p q r r s t t t t t u u u u u_, which he afterwards recomposed into
the sentence. _Annulo cingitur, tenui, plano, nusquam cohærente, ad
eclipticam inclinato_. De Saturni Lunâ. Hagæ, 1656.
[60] B. vii. v. 364. Other passages may be examined in B. i. 286; iii.
565-590, 722-733; iv. 589; v. 261, 414; vii. 577; viii. 1-178.