primary, being seen like the entrance of the moon into the earth's
shadow at the same moment of absolute time, at all places where the
planet is visible, and being wholly independent of parallax, that is,
presenting the same phenomenon to places remote from each other; being,
moreover, predicted beforehand, with great accuracy, for the instant of
its occurrence at Greenwich, and given in the Nautical Almanac; this
would seem to be one of those events which are peculiarly adapted for
finding the longitude. For you will recollect, that "any instantaneous
appearance in the heavens, visible at the same moment of absolute time
at any two places, may be employed for determining the difference of
longitude between those places; for the difference in their local times,
as indicated by clocks or chronometers, allowing fifteen degrees for
every hour, will show their difference of longitude."
With respect to the method by the eclipses of Jupiter's satellites, it
must be remarked, that the extinction of light in the satellite, at its
immersion, and the recovery of its light at its emersion, are not
instantaneous, but gradual; for the satellite, like the moon, occupies
some time in entering into the shadow, or in emerging from it, which
occasions a progressive diminution or increase of light. Two observers
in the same room, observing with different telescopes the same eclipse,
will frequently disagree, in noting its time, to the amount of fifteen
or twenty seconds. Better methods, therefore, of finding the longitude,
are now employed, although the facility with which the necessary
observations can be made, and the little calculation required, still
render this method eligible in many cases where extreme accuracy is not
important. As a telescope is essential for observing an eclipse of one
of the satellites, it is obvious that this method cannot be practised at
sea, since the telescope cannot be used on board of ship, for want of
the requisite steadiness.
The grand discovery of the _progressive motion of light_ was first made
by observations on the eclipses of Jupiter's satellites. In the year
1675, it was remarked by Roemer, a Danish astronomer, on comparing
together observations of these eclipses during many successive years,
that they take place sooner by about sixteen minutes, when the earth is
on the same side of the sun with the planet, than when she is on the
opposite side. The difference he ascribes to the progressive motion of
light, which takes that time to pass through the diameter of the earth's
orbit, making the velocity of light about one hundred and ninety-two
thousand miles per second. So great a velocity startled astronomers at
first, and produced some degree of distrust of this explanation of the
phenomenon; but the subsequent discovery of what is called the
aberration of light, led to an independent estimation of the velocity of
light, with almost precisely the same result.
Few greater feats have ever been performed by the human mind, than to
measure the speed of light,--a speed so great, as would carry it across
the Atlantic Ocean in the sixty-fourth part of a second, and around the
globe in less than the seventh part of a second! Thus has man applied
his scale to the motions of an element, that literally leaps from world
to world in the twinkling of an eye. This is one example of the great
power which the invention of the telescope conferred on man.
Could we plant ourselves on the surface of this vast planet, we should
see the same starry firmament expanding over our heads as we see now;
and the same would be true if we could fly from one planetary world to
another, until we made the circuit of them all; but the sun and the
planetary system would present themselves to us under new and strange
aspects. The sun himself would dwindle to one twenty-seventh of his
present surface, (Fig. 53, facing page 236,) and afford a degree of
light and heat proportionally diminished; Mercury, Venus, and even the
Earth, would all disappear, being too near the sun to be visible; Mars
would be as seldom seen as Mercury is by us, and constitute the only
inferior planet. On the other hand, Saturn would shine with greatly
augmented size and splendor. When in opposition to the sun, (at which
time it comes nearest to Jupiter,) it would be a grand object, appearing
larger than either Venus or Jupiter does to us. When, however, passing
to the other side of the sun, through its superior conjunction, it would
gradually diminish in size and brightness, and at length become much
less than it ever appears to us, since it would then be four hundred
millions of miles further from Jupiter than it ever is from us.
Although Jupiter comes four hundred millions of miles nearer to Uranus
than the earth does, yet it is still thirteen hundred millions of miles
distant from that planet. Hence the augmentation of the magnitude and
light of Uranus would be barely sufficient to render it distinguishable
by the naked eye. It appears, therefore, that Saturn is the peculiar
ornament of the firmament of Jupiter, and would present to the telescope
most interesting and sublime phenomena. As we owe the revelation of the
system of Jupiter and his attendant worlds wholly to the telescope, and
as the discovery and observation of them constituted a large portion of
the glory of Galileo, I am now forcibly reminded of his labors, and will
recur to his history, and finish the sketch which I commenced in a
previous Letter.
LETTER XXII.
COPERNICUS.--GALILEO.
"They leave at length the nether gloom, and stand
Before the portals of a better land;
To happier plains they come, and fairer groves,
The seats of those whom Heaven, benignant, loves;
A brighter day, a bluer ether, spreads
Its lucid depths above their favored heads;
And, purged from mists that veil our earthly skies,
Shine suns and stars unseen by mortal eyes."--_Virgil._
IN order to appreciate the value of the contributions which Galileo made
to astronomy, soon after the invention of the telescope, it is necessary
to glance at the state of the science when he commenced his discoveries
For many centuries, during the middle ages, a dark night had hung over
astronomy, through which hardly a ray of light penetrated, when, in the
eastern part of civilized Europe, a luminary appeared, that proved the
harbinger of a bright and glorious day. This was Copernicus, a native of
Thorn, in Prussia. He was born in 1473. Though destined for the
profession of medicine, from his earliest years he displayed a great
fondness and genius for mathematical studies, and pursued them with
distinguished success in the University of Cracow. At the age of
twenty-five years, he resorted to Italy, for the purpose of studying
astronomy, where he resided a number of years. Thus prepared, he
returned to his native country, and, having acquired an ecclesiastical
living that was adequate to his support in his frugal mode of life, he
established himself at Frauenberg, a small town near the mouth of the
Vistula, where he spent nearly forty years in observing the heavens, and
meditating on the celestial motions. He occupied the upper part of a
humble farm-house, through the roof of which he could find access to an
unobstructed sky, and there he carried on his observations. His
instruments, however, were few and imperfect, and it does not appear
that he added any thing to the art of practical astronomy. This was
reserved for Tycho Brahe, who came a half a century after him. Nor did
Copernicus enrich the science with any important discoveries. It was not
so much his genius or taste to search for new bodies, or new phenomena
among the stars, as it was to explain the reasons of the most obvious
and well-known appearances and motions of the heavenly bodies. With this
view, he gave his mind to long-continued and profound meditation.
Copernicus tells us that he was first led to think that the apparent
motions of the heavenly bodies, in their diurnal revolution, were owing
to the real motion of the earth in the opposite direction, from
observing instances of the same kind among terrestrial objects; as when
the shore seems to the mariner to recede, as he rapidly sails from it;
and as trees and other objects seem to glide by us, when, on riding
swiftly past them, we lose the consciousness of our own motion. He was
also smitten with the _simplicity_ prevalent in all the works and
operations of Nature, which is more and more conspicuous the more they
are understood; and he hence concluded that the planets do not move in
the complicated paths which most preceding astronomers assigned to them.
I shall explain to you, hereafter, the details of his system. I need
only at present remind you that the hypothesis which he espoused and
defended, (being substantially the same as that proposed by Pythagoras,
five hundred years before the Christian era,) supposes, first, that the
apparent movements of the sun by day, and of the moon and stars by
night, from east to west, result from the actual revolution of the earth
on its own axis from west to east; and, secondly, that the earth and all
the planets revolve about the sun in circular orbits. This hypothesis,
when he first assumed it, was with him, as it had been with Pythagoras,
little more than mere conjecture. The arguments by which its truth was
to be finally established were not yet developed, and could not be,
without the aid of the telescope, which was not yet invented. Upon this
hypothesis, however, he set out to explain all the phenomena of the
visible heavens,--as the diurnal revolutions of the sun, moon, and
stars, the slow progress of the planets through the signs of the zodiac,
and the numerous irregularities to which the planetary motions are
subject. These last are apparently so capricious,--being for some time
forward, then stationary, then backward, then stationary again, and
finally direct, a second time, in the order of the signs, and constantly
varying in the velocity of their movements,--that nothing but
long-continued and severe meditation could have solved all these
appearances, in conformity with the idea that each planet is pursuing
its simple way all the while in a circle around the sun. Although,
therefore, Pythagoras fathomed the profound doctrine that the sun is the
centre around which the earth and all the planets revolve, yet we have
no evidence that he ever solved the irregular motions of the planets in
conformity with his hypothesis, although the explanation of the diurnal
revolution of the heavens, by that hypothesis, involved no difficulty.
Ignorant as Copernicus was of the principle of gravitation, and of most
of the laws of motion, he could go but little way in following out the
consequences of his own hypothesis; and all that can be claimed for him
is, that he solved, by means of it, most of the common phenomena of the
celestial motions. He indeed got upon the road to truth, and advanced
some way in its sure path; but he was able to adduce but few independent
proofs, to show that it was the truth. It was only quite at the close of
his life that he published his system to the world, and that only at the
urgent request of his friends; anticipating, perhaps, the opposition of
a bigoted priesthood, whose fury was afterwards poured upon the head of
Galileo, for maintaining the same doctrines.
Although, therefore, the system of Copernicus afforded an explanation of
the celestial motions, far more simple and rational than the previous
systems which made the earth the centre of those motions, yet this fact
alone was not sufficient to compel the assent of astronomers; for the
greater part, to say the least, of the same phenomena, could be
explained on either hypothesis. With the old doctrine astronomers were
already familiar, a circumstance which made it seem easier; while the
new doctrines would seem more difficult, from their being imperfectly
understood. Accordingly, for nearly a century after the publication of
the system of Copernicus, he gained few disciples. Tycho Brahe rejected
it, and proposed one of his own, of which I shall hereafter give you
some account; and it would probably have fallen quite into oblivion, had
not the observations of Galileo, with his newly-invented telescope,
brought to light innumerable proofs of its truth, far more cogent than
any which Copernicus himself had been able to devise.
Galileo no sooner had completed his telescope, and directed it to the
heavens, than a world of wonders suddenly burst upon his enraptured
sight. Pointing it to the moon, he was presented with a sight of her
mottled disk, and of her mountains and valleys. The sun exhibited his
spots; Venus, her phases; and Jupiter, his expanded orb, and his retinue
of moons. These last he named, in honor of his patron, Cosmo d'Medici,
_Medicean stars_. So great was this honor deemed of associating one's
name with the stars, that express application was made to Galileo, by
the court of France, to award this distinction to the reigning Monarch,
Henry the Fourth, with plain intimations, that by so doing he would
render himself and his family rich and powerful for ever.
Galileo published the result of his discoveries in a paper, denominated
'_Nuncius Sidereus_,' the 'Messenger of the Stars.' In that ignorant and
marvellous age, this publication produced a wonderful excitement. "Many
doubted, many positively refused to believe, so novel an announcement;
all were struck with the greatest astonishment, according to their
respective opinions, either at the new view of the universe thus offered
to them, or at the high audacity of Galileo, in inventing such fables."
Even Kepler, the great German astronomer, of whom I shall tell you more
by and by, wrote to Galileo, and desired him to supply him with
arguments, by which he might answer the objections to these pretended
discoveries with which he was continually assailed. Galileo answered him
as follows: "In the first place, I return you my thanks that you first,
and almost alone, before the question had been sifted, (such is your
candor, and the loftiness of your mind,) put faith in my assertions. You
tell me you have some telescopes, but not sufficiently good to magnify
distant objects with clearness, and that you anxiously expect a sight of
mine, which magnifies images more than a thousand times. It is mine no
longer, for the Grand Duke of Tuscany has asked it of me, and intends to
lay it up in his museum, among his most rare and precious curiosities,
in eternal remembrance of the invention.
"You ask, my dear Kepler, for other testimonies. I produce, for one, the
Grand Duke, who, after observing the Medicean planets several times with
me at Pisa, during the last months, made me a present, at parting, of
more than a thousand florins, and has now invited me to attach myself to
him, with the annual salary of one thousand florins, and with the title
of 'Philosopher and Principal Mathematician to His Highness;' without
the duties of any office to perform, but with the most complete leisure.
I produce, for another witness, myself, who, although already endowed in
this College with the noble salary of one thousand florins, such as no
professor of mathematics ever before received, and which I might
securely enjoy during my life, even if these planets should deceive me
and should disappear, yet quit this situation, and take me where want
and disgrace will be my punishment, should I prove to have been
mistaken."
The learned professors in the universities, who, in those days, were
unaccustomed to employ their senses in inquiring into the phenomena of
Nature, but satisfied themselves with the authority of Aristotle, on all
subjects, were among the most incredulous with respect to the
discoveries of Galileo. "Oh, my dear Kepler," says Galileo, "how I wish
that we could have one hearty laugh together. Here, at Padua, is the
principal Professor of Philosophy, whom I have repeatedly and urgently
requested to look at the moon and planets through my glass, which he
pertinaciously refuses to do. Why are you not here? What shouts of
laughter we should have at this glorious folly, and to hear the
Professor of Philosophy at Pisa laboring before the Grand Duke, with
logical arguments, as if with magical incantations, to charm the new
planets out of the sky."
The following argument by Sizzi, a contemporary astronomer of some note,
to prove that there can be only seven planets, is a specimen of the
logic with which Galileo was assailed. "There are seven windows given
to animals in the domicile of the head, through which the air is
admitted to the tabernacle of the body, to enlighten, to warm, and to
nourish it; which windows are the principal parts of the microcosm, or
little world,--two nostrils, two eyes, two ears, and one mouth. So in
the heavens, as in a macrocosm, or great world, there are two favorable
stars, Jupiter and Venus; two unpropitious, Mars and Saturn; two
luminaries, the Sun and Moon; and Mercury alone, undecided and
indifferent. From which, and from many other phenomena of Nature, such
as the seven metals, &c., which it were tedious to enumerate, we gather
that the number of planets is necessarily seven. Moreover, the
satellites are invisible to the naked eye, and therefore can exercise no
influence over the earth, and therefore would be useless, and therefore
do not exist. Besides, as well the Jews and other ancient nations, as
modern Europeans, have adopted the division of the week into seven days,
and have named them from the seven planets. Now, if we increase the
number of planets, this whole system falls to the ground."
When, at length, the astronomers of the schools found it useless to deny
the fact that Jupiter is attended by smaller bodies, which revolve
around him, they shifted their ground of warfare, and asserted that
Galileo had not told the whole truth; that there were not merely _four_
satellites, but a still greater number; one said five; another, nine;
and another, twelve; but, in a little time, Jupiter moved forward in his
orbit, and left all behind him, save the four Medicean stars.
It had been objected to the Copernican system, that were Venus a body
which revolved around the sun in an orbit interior to that of the earth,
she would undergo changes similar to those of the moon. As no such
changes could be detected by the naked eye, no satisfactory answer could
be given to this objection; but the telescope set all right, by showing,
in fact, the phases of Venus. The same instrument, disclosed, also, in
the system of Jupiter and his moons, a miniature exhibition of the solar
system itself. It showed the actual existence of the motion of a number
of bodies around one central orb, exactly similar to that which was
predicated of the sun and planets. Every one, therefore, of these new
and interesting discoveries, helped to confirm the truth of the system
of Copernicus.
But a fearful cloud was now rising over Galileo, which spread itself,
and grew darker every hour. The Church of Rome had taken alarm at the
new doctrines respecting the earth's motion, as contrary to the
declarations of the Bible, and a formidable difficulty presented itself,
namely, how to publish and defend these doctrines, without invoking the
terrible punishments inflicted by the Inquisition on heretics. No work
could be printed without license from the court of Rome; and any
opinions supposed to be held and much more known to be taught by any
one, which, by an ignorant and superstitious priesthood, could be
interpreted as contrary to Scripture, would expose the offender to the
severest punishments, even to imprisonment, scourging, and death. We,
who live in an age so distinguished for freedom of thought and opinion,
can form but a very inadequate conception of the bondage in which the
minds of men were held by the chains of the Inquisition. It was
necessary, therefore, for Galileo to proceed with the greatest caution
in promulgating truths which his own discoveries had confirmed. He did
not, like the Christian martyrs, proclaim the truth in the face of
persecutions and tortures; but while he sought to give currency to the
Copernican doctrines, he labored, at the same time, by cunning
artifices, to blind the ecclesiastics to his real designs, and thus to
escape the effects of their hostility.
Before Galileo published his doctrines in form, he had expressed himself
so freely, as to have excited much alarm among the ecclesiastics. One of
them preached publicly against him, taking for his text, the passage,
"Ye men of Galilee, why stand ye here gazing up into heaven?" He
therefore thought it prudent to resort to Rome, and confront his enemies
face to face. A contemporary describes his appearance there in the
following terms, in a letter addressed to a Romish Cardinal: "Your
Eminence would be delighted with Galileo, if you heard him holding
forth, as he often does, in the midst of fifteen or twenty, all
violently attacking him, sometimes in one house, sometimes in another.
But he is armed after such fashion, that he laughs all of them to scorn;
and even if the novelty of his opinions prevents entire persuasion, at
least he convicts of emptiness most of the arguments with which his
adversaries endeavor to overwhelm him."
In 1616, Galileo, as he himself states, had a most gracious audience of
the Pope, Paul the Fifth, which lasted for nearly an hour, at the end of
which his Holiness assured him, that the Congregation were no longer in
a humor to listen lightly to calumnies against him, and that so long as
he occupied the Papal chair, Galileo might think himself out of all
danger. Nevertheless, he was not allowed to return home, without
receiving formal notice not to teach the opinions of Copernicus, "that
the sun is in the centre of the system, and that the earth moves about
it," from that time forward, in any manner.
Galileo had a most sarcastic vein, and often rallied his persecutors
with the keenest irony. This he exhibited, some time after quitting
Rome, in an epistle which he sent to the Arch Duke Leopold, accompanying
his 'Theory of the Tides.' "This theory," says he, "occurred to me when
in Rome, whilst the theologians were debating on the prohibition of
Copernicus's book, and of the opinion maintained in it of the motion of
the earth, which I at that time believed; until it pleased those
gentlemen to suspend the book, and to declare the opinion false and
repugnant to the Holy Scriptures. Now, as I know how well it becomes me
to obey and believe the decisions of my superiors, which proceed out of
more profound knowledge than the weakness of my intellect can attain
to, this theory, which I send you, which is founded on the motion of the
earth, I now look upon as a fiction and a dream, and beg your Highness
to receive it as such. But, as poets often learn to prize the creations
of their fancy, so, in like manner, do I set some value on this
absurdity of mine. It is true, that when I sketched this little work, I
did hope that Copernicus would not, after eighty years, be convicted of
error; and I had intended to develope and amplify it further; but a
voice from heaven suddenly awakened me, and at once annihilated all my
confused and entangled fancies."
It is difficult, however, sometimes to decide whether the language of
Galileo is ironical, or whether he uses it with subtlety, with the hope
of evading the anathemas of the Inquisition. Thus he ends one of his
writings with the following passage: "In conclusion, since the motion
attributed to the earth, which I, as a pious and Catholic person,
consider most false, and not to exist, accommodates itself so well to
explain so many and such different phenomena, I shall not feel sure
that, false as it is, it may not just as deludingly correspond with the
phenomena of comets."