native of Samos, an island in the Ægean sea, and flourished about five
hundred years before the Christian era. After travelling more than
thirty years in Egypt and Chaldea, and spending several years more at
Sparta, to learn the laws and institutions of Lycurgus, he returned to
his native island to dispense the riches he had acquired to his
countrymen. But they, probably fearful of incurring the displeasure of
the gods by the freedom with which he inquired into the secrets of the
skies, gave him so unwelcome a reception, that he retired from them, in
disgust, and established his school at Crotona, on the southeastern
coast of Italy. Hither, as to an oracle, the fame of his wisdom
attracted hundreds of admiring pupils, whom he instructed in every
species of knowledge. From the visionary notions which are generally
understood to have been entertained on the subject of astronomy, by the
ancients, we are apt to imagine that they knew less than they actually
did of the truths of this science. But Pythagoras was acquainted with
many important facts in astronomy, and entertained many opinions
respecting the system of the world, which are now held to be true. Among
other things well known to Pythagoras, either derived from his own
investigations, or received from his predecessors, were the following;
and we may note them as a synopsis of the state of astronomical
knowledge at that age of the world. First, the principal
_constellations_. These had begun to be formed in the earliest ages of
the world. Several of them, bearing the same name as at present, are
mentioned in the writings of Hesiod and Homer; and the "sweet influences
of the Pleiades," and the "bands of Orion," are beautifully alluded to
in the book of Job. Secondly, _eclipses_. Pythagoras knew both the
causes of eclipses and how to predict them; not, indeed, in the accurate
manner now practised, but by means of the Saros. Thirdly, Pythagoras had
divined the true _system of the world_, holding that the sun, and not
the earth, (as was generally held by the ancients, even for many ages
after Pythagoras,) is the centre around which all the planets revolve;
and that the stars are so many suns, each the centre of a system like
our own. Among lesser things, he knew that the earth is round; that its
surface is naturally divided into five zones; and that the ecliptic is
inclined to the equator. He also held that the earth revolves daily on
its axis, and yearly around the sun; that the galaxy is an assemblage of
small stars; and that it is the same luminary, namely, Venus, that
constitutes both the morning and evening star; whereas all the ancients
before him had supposed that each was a separate planet, and accordingly
the morning star was called Lucifer, and the evening star, Hesperus. He
held, also, that the planets were inhabited, and even went so far as to
calculate the size of some of the animals in the moon. Pythagoras was
also so great an enthusiast in music, that he not only assigned to it a
conspicuous place in his system of education, but even supposed that the
heavenly bodies themselves were arranged at distances corresponding to
the intervals of the diatonic scale, and imagined them to pursue their
sublime march to notes created by their own harmonious movements, called
the 'music of the spheres;' but he maintained that this celestial
concert, though loud and grand, is not audible to the feeble organs of
man, but only to the gods. With few exceptions, however, the opinions of
Pythagoras on the system of the world were founded in truth. Yet they
were rejected by Aristotle, and by most succeeding astronomers, down to
the time of Copernicus; and in their place was substituted the doctrine
of _crystalline spheres_, first taught by Eudoxus, who lived about three
hundred and seventy years before Christ. According to this system, the
heavenly bodies are set like gems in hollow solid orbs, composed of
crystal so transparent, that no anterior orb obstructs in the least the
view of any of the orbs that lie behind it. The sun and the planets have
each its separate orb; but the fixed stars are all set in the same
grand orb; and beyond this is another still, the _primum mobile_, which
revolves daily, from east to west, and carries along with it all the
other orbs. Above the whole spreads the _grand empyrean_, or third
heavens, the abode of perpetual serenity.
To account for the planetary motions, it was supposed that each of the
planetary orbs, as well as that of the sun, has a motion of its own,
eastward, while it partakes of the common diurnal motion of the starry
sphere. Aristotle taught that these motions are effected by a tutelary
genius of each planet, residing in it, and directing its motions, as the
mind of man directs his movements.
Two hundred years after Pythagoras, arose the famous school of
Alexandria, under the Ptolemies. These were a succession of Egyptian
kings, and are not to be confounded with Ptolemy, the astronomer. By the
munificent patronage of this enlightened family, for the space of three
hundred years, beginning at the death of Alexander the Great, from whom
the eldest of the Ptolemies had received his kingdom, the school of
Alexandria concentrated in its vast library and princely halls, erected
for the accommodation of the philosophers, nearly all the science and
learning of the world. In wandering over the immense territories of
ignorance and barbarism which covered, at that time, almost the entire
face of the earth, the eye reposes upon this little spot, as upon a
verdant island in the midst of the desert. Among the choice fruits that
grew in this garden of astronomy were several of the most distinguished
ornaments of ancient science, of whom the most eminent were Hipparchus
and Ptolemy. Hipparchus is justly considered as the Newton of antiquity.
He sought his knowledge of the heavenly bodies not in the illusory
suggestions of a fervid imagination, but in the vigorous application of
an intellect of the first order. Previous to this period, celestial
observations were made chiefly with the naked eye: but Hipparchus was in
possession of instruments for measuring angles, and knew how to resolve
spherical triangles. These were great steps beyond all his predecessors.
He ascertained the length of the year within six minutes of the truth.
He discovered the eccentricity, or elliptical figure, of the solar
orbit, although he supposed the sun actually to move uniformly in a
circle, but the earth to be placed out of the centre. He also determined
the positions of the points among the stars where the earth is nearest
to the sun, and where it is most remote from it. He formed very accurate
estimates of the obliquity of the ecliptic and of the precession of the
equinoxes. He computed the exact period of the synodic revolution of the
moon, and the inclination of the lunar orbit; discovered the backward
motion of her node and of her line of apsides; and made the first
attempts to ascertain the horizontal parallaxes of the sun and moon.
Upon the appearance of a new star in the firmament, he undertook, as
already mentioned, to number the stars, and to assign to each its true
place in the heavens, in order that posterity might have the means of
judging what changes, if any, were going forward among these apparently
unalterable bodies.
Although Hipparchus is generally considered as belonging to the
Alexandrian school, yet he lived at Rhodes, and there made his
astronomical observations, about one hundred and forty years before the
Christian era. One of his treatises has come down to us; but his
principal discoveries have been transmitted through the 'Almagest' of
Ptolemy. Ptolemy flourished at Alexandria nearly three centuries after
Hipparchus, in the second century after Christ. His great work, the
'Almagest,' which has conveyed to us most that we know respecting the
astronomical knowledge of the ancients, was the universal text-book of
astronomers for fourteen centuries.
[Illustration Fig. 77.]
The name of this celebrated astronomer has also descended to us,
associated with the system of the world which prevailed from Ptolemy to
Copernicus, called the _Ptolemaic System_. The doctrines of the
Ptolemaic system did not originate with Ptolemy, but, being digested by
him out of materials furnished by various hands, it has come down to us
under the sanction of his name. According to this system, the earth is
the centre of the universe, and all the heavenly bodies daily revolve
around it, from east to west. But although this hypothesis would account
for the apparent diurnal motion of the firmament, yet it would not
account for the apparent annual motion of the sun, nor for the slow
motions of the planets from west to east. In order to explain these
phenomena, recourse was had to _deferents_ and _epicycles_,--an
explanation devised by Apollonius, one of the greatest geometers of
antiquity. He conceived that, in the circumference of a circle, having
the earth for its centre, there moves the centre of a smaller circle in
the circumference of which the planet revolves. The circle surrounding
the earth was called the deferent, while the smaller circle, whose
centre was always in the circumference of the deferent, was called the
epicycle. Thus, if E, Fig. 77, represents the earth, ABC will be the
deferent, and DFG, the epicycle; and it is obvious that the motion of a
body from west to east, in this small circle, would be alternately
direct, stationary, and retrograde, as was explained, in a previous
Letter, to be actually the case with the apparent motions of the
planets. The hypothesis, however, is inconsistent with the _phases_ of
Mercury and Venus, which, being between us and the sun, on both sides of
the epicycle, would present their dark sides towards us at both
conjunctions with the sun, whereas, at one of the conjunctions, it is
known that they exhibit their disks illuminated. It is, moreover, absurd
to speak of a geometrical centre, which has no bodily existence, moving
round the earth on the circumference of another circle. In addition to
these absurdities, the whole Ptolemaic system is encumbered with the
following difficulties: First, it is a mere hypothesis, having no
evidence in its favor except that it explains the phenomena. This
evidence is insufficient of itself, since it frequently happens that
each of two hypotheses, which are directly opposite to each other, will
explain all the known phenomena. But the Ptolemaic system does not even
do this, as it is inconsistent with the phases of Mercury and Venus, as
already observed. Secondly, now that we are acquainted with the
distances of the remoter planets, and especially the fixed stars, the
swiftness of motion, implied in a daily revolution of the starry
firmament around the earth, renders such a motion wholly incredible.
Thirdly, the centrifugal force which would be generated in these bodies,
especially in the sun, renders it impossible that they can continue to
revolve around the earth as a centre. Absurd, however, as the system of
Ptolemy was, for many centuries no great philosophic genius appeared to
expose its fallacies, and it therefore guided the faith of astronomers
of all countries down to the time of Copernicus.
After the age of Ptolemy, the science made little progress. With the
decline of Grecian liberty, the arts and sciences declined also; and the
Romans, then masters of the world, were ever more ambitious to gain
conquests over man than over matter; and they accordingly never produced
a single great astronomer. During the middle ages, the Arabians were
almost the only astronomers, and they cultivated this noble study
chiefly as subsidiary to astrology.
At length, in the fifteenth century, Copernicus arose, and after forty
years of intense study and meditation, divined the true system of the
world. You will recollect that the Copernican system maintains, 1. That
the _apparent_ diurnal motions of the heavenly bodies, from east to
west, is owing to the _real_ revolution of the earth on its own axis
from west to east; and, 2. That the sun is the centre around which the
earth and planets all revolve from west to east. It rests on the
following arguments: In the first place, _the earth revolves on its own
axis_. First, because this supposition is vastly more _simple_.
Secondly, it is agreeable to _analogy_, since all the other planets that
afford any means of determining the question, are seen to revolve on
their axes. Thirdly, the _spheroidal figure_ of the earth is the figure
of equilibrium, that results from a revolution on its axis. Fourthly,
the _diminished weight_ of bodies at the equator indicates a centrifugal
force arising from such a revolution. Fifthly, bodies let fall from a
high eminence, fall _eastward of their base_, indicating that when
further from the centre of the earth they were subject to a greater
velocity, which, in consequence of their inertia, they do not entirely
lose in descending to the lower level.
In the second place, _the planets, including the earth, revolve about
the sun_. First, the _phases_ of Mercury and Venus are precisely such,
as would result from their circulating around the sun in orbits within
that of the earth; but they are never seen in opposition, as they would
be, if they circulate around the earth. Secondly, the superior planets
do indeed revolve around the earth; but they also revolve around the
sun, as is evident from their phases, and from the known dimensions of
their orbits; and that the sun, and not the earth, is the _centre_ of
their motions, is inferred from the greater symmetry of their motions,
as referred to the sun, than as referred to the earth; and especially
from the laws of gravitation, which forbid our supposing that bodies so
much larger than the earth, as some of these bodies are, can circulate
permanently around the earth, the latter remaining all the while at
rest.
In the third place, the annual motion of _the earth_ itself is indicated
also by the most conclusive arguments. For, first, since all the
planets, with their satellites and the comets, revolve about the sun,
analogy leads us to infer the same respecting the earth and its
satellite, as those of Jupiter and Saturn, and indicates that it is a
law of the solar system that the smaller bodies revolve about the
larger. Secondly, on the supposition that the earth performs an annual
revolution around the sun, it is embraced along with the planets, in
Kepler's law, that the squares of the times are as the cubes of the
distances; otherwise, it forms an exception, and the only known
exception, to this law.
Such are the leading arguments upon which rests the Copernican system of
astronomy. They were, however, only very partially known to Copernicus
himself, as the state both of mechanical science, and of astronomical
observation, was not then sufficiently matured to show him the strength
of his own doctrine, since he knew nothing of the telescope, and nothing
of the principle of universal gravitation. The evidence of this
beautiful system being left by Copernicus in so imperfect a state, and
indeed his own reasonings in support of it being tinctured with some
errors, we need not so much wonder that Tycho Brahe, who immediately
followed Copernicus, did not give it his assent, but, influenced by
certain passages of Scripture, he still maintained, with Ptolemy, that
the earth is in the centre of the universe; and he accounted for the
diurnal motions in the same manner as Ptolemy had done, namely, by an
actual revolution of the whole host of heaven around the earth every
twenty-four hours. But he rejected the scheme of deferents and
epicycles, and held that the moon revolves about the earth as the centre
of her motions; but that the sun and not the earth is the centre of the
planetary motions; and that the sun, accompanied by the planets, moves
around the earth once a year, somewhat in the manner in which we now
conceive of Jupiter and his satellites as revolving around the sun. This
system is liable to most of the objections that lie against the
Ptolemaic system, with the disadvantage of being more complex.
Kepler and Galileo, however, as appeared in the sketch of their lives,
embraced the theory of Copernicus with great avidity, and all their
labors contributed to swell the evidence of its truth. When we see with
what immense labor and difficulty the disciples of Ptolemy sought to
reconcile every new phenomenon of the heavens with their system, and
then see how easily and naturally all the successive discoveries of
Galileo and Kepler fall in with the theory of Copernicus, we feel the
full force of those beautiful lines of Cowper which I have chosen for
the motto of this Letter.
Newton received the torch of truth from Galileo, and transmitted it to
his successors, with its light enlarged and purified; and since that
period, every new discovery, whether the fruit of refined instrumental
observation or of profound mathematical analysis, has only added lustre
to the glory of Copernicus.
With Newton commenced a new and wonderful era in astronomy,
distinguished above all others, not merely for the production of the
greatest of men, but also for the establishment of those most important
auxiliaries to our science, the Royal Society of London, the Academy of
Sciences at Paris, and the Observatory of Greenwich. I may add the
commencement of the Transactions of the Royal Society, and the Memoirs
of the Academy of Sciences, which have been continued to the present
time,--both precious storehouses of astronomical riches. The Observatory
of Greenwich, moreover, has been under the direction of an extraordinary
succession of great astronomers. Their names are Flamstead, Halley,
Bradley, Maskeleyne, Pond, and Airy,--the last being still at his post,
and worthy of continuing a line so truly illustrious. The observations
accumulated at this celebrated Observatory are so numerous, and so much
superior to those of any other institution in the world, that it has
been said that astronomy would suffer little, if all other contemporary
observations of the same kind were annihilated. Sir William Herschel,
however, labored chiefly in a different sphere. The Astronomers Royal
devoted themselves not so much to the discovery of new objects among
the heavenly bodies, as to the exact determination of the places of the
bodies already known, and to the developement of new laws or facts among
the celestial motions. But Herschel, having constructed telescopes of
far greater reach than any ever used before, employed them to sound new
and untried depths in the profundities of space. We have already seen
what interesting and amazing discoveries he made of double stars,
clusters, and nebulæ.
The English have done most for astronomy in observation and discovery;
but the French and Germans, in developing, by the most profound
mathematical investigation, the great laws of physical astronomy.
It only remains to inquire, whether the Copernican system is now to be
regarded as a full exposition of the 'Mechanism of the Heavens,' or
whether there subsist higher orders of relations between the fixed stars
themselves.
The revolutions of the _binary stars_ afford conclusive evidence of at
least subordinate systems of suns, governed by the same laws as those
which regulate the motions of the solar system. The _nebulæ_ also
compose peculiar systems, in which the members are evidently bound
together by some common relation.
In these marks of organization,--of stars associated together in
clusters; of sun revolving around sun; and of nebulæ disposed in regular
figures,--we recognise different members of some grand system, links in
one great chain that binds together all parts of the universe; as we see
Jupiter and his satellites combined in one subordinate system, and
Saturn and his satellites in another,--each a vast kingdom, and both
uniting with a number of other individual parts, to compose an empire
still more vast.
This fact being now established, that the stars are immense bodies, like
the sun, and that they are subject to the laws of gravitation, we cannot
conceive how they can be preserved from falling into final disorder and
ruin, unless they move in harmonious concert, like the members of the
solar system. Otherwise, those that are situated on the confines of
creation, being retained by no forces from without, while they are
subject to the attraction of all the bodies within, must leave their
stations, and move inward with accelerated velocity; and thus all the
bodies in the universe would at length fall together in the common
centre of gravity. The immense distance at which the stars are placed
from each other would indeed delay such a catastrophe; but this must be
the ultimate tendency of the material world, unless sustained in one
harmonious system by nicely-adjusted motions. To leave entirely out of
view our confidence in the wisdom and preserving goodness of the
Creator, and reasoning merely from what we know of the stability of the
solar system, we should be justified in inferring, that other worlds are
not subject to forces which operate only to hasten their decay, and to
involve them in final ruin.
We conclude, therefore, that the material universe is one great system;
that the combination of planets with their satellites constitutes the
first or lowest order of worlds; that next to these, planets are linked
to suns; that these are bound to other suns, composing a still higher
order in the scale of being; and finally, that all the different systems
of worlds move around their common centre of gravity.
LETTER XXXI.
NATURAL THEOLOGY.
----"Philosophy, baptized
In the pure fountain of Eternal Love,
Has eyes indeed; and, viewing all she sees
As meant to indicate a God to man,
Gives Him the praise, and forfeits not her own."--_Cowper._
I INTENDED, my dear Friend, to comply with your request "that I would
discuss the arguments which astronomy affords to natural theology;" but
these Letters have been already extended so much further than I
anticipated, that I shall conclude with suggesting a few of those moral
and religious reflections, which ought always to follow in the train of
such a survey of the heavenly bodies as we have now taken.
Although there is evidence enough in the structure, arrangement, and
laws, which prevail among the heavenly bodies, to prove the _existence_
of God, yet I think there are many subordinate parts of His works far
better adapted to this purpose than these, being more fully within our
comprehension. It was intended, no doubt, that the evidence of His being
should be accessible to all His creatures, and should not depend on a
kind of knowledge possessed by comparatively few. The mechanism of the
eye is probably not more perfect than that of the universe; but we can
analyze it better, and more fully understand the design of each part.
But the existence of God being once proved, and it being admitted that
He is the Creator and Governor of the world, then the discoveries of
astronomy are admirably adapted to perform just that office in relation
to the Great First Cause, which is assigned to them in the Bible,
namely, "to declare the glory of God, and to show His handiwork." In
other words, the discoveries of astronomy are peculiarly fitted,--more
so, perhaps, than any other department of creation,--to exhibit the
unity, power, and wisdom, of the Creator.
The most modern discoveries have multiplied the proofs of the _unity_ of
God. It has usually been offered as sufficient evidence of the truth of
this doctrine, that the laws of Nature are found to be uniform when
applied to the utmost bounds of the _solar system_; that the law of
gravitation controls alike the motions of Mercury, and those of Uranus;
and that its operation is one and the same upon the moon and upon the
satellites of Saturn. It was, however, impossible, until recently, to
predicate the same uniformity in the great laws of the universe
respecting the starry worlds, except by a feeble analogy. However
improbable, it was still possible, that in these distant worlds other
laws might prevail, and other Lords exercise dominion. But the discovery
of the revolutions of the binary stars, in exact accordance with the law
of gravitation, not merely in a single instance, but in many instances,
in all cases, indeed, wherever those revolutions have advanced so far as
to determine their law of action, gives us demonstration, instead of
analogy, of the prevalence of the same law among the other systems as
that which rules in ours.
The marks of a still higher organization in the structure of clusters
and nebulæ, all bearing that same characteristic union of resemblance
and variety which belongs to all the other works of creation that fall
under our notice, speak loudly of one, and only one, grand design. Every
new discovery of the telescope, therefore, has added new proofs to the
great truth that God is one: nor, so far as I know, has a single fact
appeared, that is not entirely consonant with it. Light, moreover, which
brings us intelligence, and, in most cases, the only intelligence we
have, of these remote orbs, testifies to the same truth, being similar
in its properties and uniform in its motions, from whatever star it
emanates.
In displays of the _power_ of Jehovah, nothing can compare with the
starry heavens. The magnitudes, distances, and velocities, of the
heavenly bodies are so much beyond every thing of this kind which
belongs to things around us, from which we borrowed our first ideas of
these qualities, that we can scarcely avoid looking with incredulity at
the numerical results to which the unerring principles of mathematics
have conducted us. And when we attempt to apply our measures to the
fixed stars, and especially to the nebulæ, the result is absolutely
overwhelming: the mind refuses its aid in our attempts to grasp the
great ideas. Nor less conspicuous, among the phenomena of the heavenly
bodies, is the _wisdom_ of the Creator. In the first place, this
attribute is every where exhibited _in the happy adaptation of means to
their ends_. No principle can be imagined more simple, and at the same
time more effectual to answer the purposes which it serves, than
gravitation. No position can be given to the sun and planets so fitted,
as far as we can judge, to fulfil their mutual relations, as that which
the Creator has given them. I say, as far as we can judge; for we find
this to be the case in respect to our own planet and its attendant
satellite, and hence have reason to infer that the same is the case in
the other planets, evidently holding, as they do, a similar relation to
the sun. Thus the position of the earth at just such a distance from the
sun as suits the nature of its animal and vegetable kingdoms, and
confining the range of solar heat, vast as it might easily become,
within such narrow bounds; the inclination of the earth's axis to the
plane of its orbit, so as to produce the agreeable vicissitudes of the
seasons, and increase the varieties of animal and vegetable life, still
confining the degree of inclination so exactly within the bounds of
safety, that, were it much to transcend its present limits, the changes
of temperature of the different seasons would be too sudden and violent
for the existence of either animals or vegetables; the revolution of the
earth on its axis, so happily dividing time into hours of business and
of repose; the adaptation of the moon to the earth, so as to afford to
us her greatest amount of light just at the times when it is needed
most, and giving to the moon just such a quantity of matter, and placing
her at just such a distance from the earth, as serves to raise a tide
productive of every conceivable advantage, without the evils which would
result from a stagnation of the waters on the one hand, or from their
overflow on the other;--these are a few examples of the wisdom displayed
in the mutual relations instituted between the sun, the earth, and the
moon.