appear to have a real motion in space_. There are several _apparent_
changes of place among the stars, arising from real changes in the
earth, which, as we are not conscious of them, we refer to the stars;
but there are other motions among the stars which cannot result from any
changes in the earth, but must arise from changes in the stars
themselves. Such motions are called the _proper motions_ of the stars.
Nearly two thousand years ago, Hipparchus and Ptolemy made the most
accurate determinations in their power of the relative situations of the
stars, and their observations have been transmitted to us in Ptolemy's
'Almagest;' from which it appears that the stars retain at least _very
nearly_ the same places now as they did at that period. Still, the more
accurate methods of modern astronomers have brought to light minute
changes in the places of certain stars, which force upon us the
conclusion, _either that our solar system causes an apparent
displacement of certain stars, by a motion of its own in space, or
that they have themselves a proper motion_. Possibly, indeed, both these
causes may operate.
If the sun, and of course the earth which accompanies him, is actually
in motion, the fact may become manifest from the apparent approach of
the stars in the region which he is leaving, and the recession of those
which lie in the part of the heavens towards which he is travelling.
Were two groves of trees situated on a plain at some distance apart, and
we should go from one to the other, the trees before us would gradually
appear further and further asunder, while those we left behind would
appear to approach each other. Some years since, Sir William Herschel
supposed he had detected changes of this kind among two sets of stars in
opposite points of the heavens, and announced that the solar system was
in motion towards a point in the constellation Hercules; but other
astronomers have not found the changes in question such as would
correspond to this motion, or to any motion of the sun; and, while it is
a matter of general belief that the sun has a motion in space, the fact
is not considered as yet entirely proved.
In most cases, where a proper motion in certain stars has been
suspected, its annual amount has been so small, that many years are
required to assure us, that the effect is not owing to some other cause
than a real progressive motion in the stars themselves; but in a few
instances the fact is too obvious to admit of any doubt. Thus, the two
stars, 61 Cygni, which are nearly equal, have remained constantly at the
same or nearly at the same distance of fifteen seconds, for at least
fifty years past. Mean-while, they have shifted their local situation in
the heavens four minutes twenty-three seconds, the annual proper motion
of each star being five seconds and three tenths, by which quantity this
system is every year carried along in some unknown path, by a motion
which for many centuries must be regarded as uniform and rectilinear. A
greater proportion of the double stars than of any other indicate proper
motions, especially the binary stars, or those which have a revolution
around each other. Among stars not double, and no way differing from the
rest in any other obvious particular, a star in the constellation
Cassiopeia, (_Mu Cassiopeiæ_) has the greatest proper motion of any yet
ascertained, amounting to nearly four seconds annually.
You have doubtless heard much respecting the "immeasurable _distances_"
of the fixed stars, and will desire to learn what is known to
astronomers respecting this interesting subject.
We cannot ascertain the actual distance of any of the fixed stars, but
we can certainly determine that the nearest star is more than twenty
millions of millions of miles from the earth, (20,000,000,000,000.) For
all measurements relating to the distances of the _sun and planets_, the
radius of the earth furnishes the base line. The length of this line
being known, and the horizontal parallax of the sun or any planet, we
have the means of calculating the distance of the body from us, by
methods explained in a previous Letter. But any star, viewed from the
opposite sides of the earth, would appear from both stations to occupy
precisely the same situation in the celestial sphere, and of course it
would exhibit no horizontal parallax. But astronomers have endeavored to
find a parallax in some of the fixed stars, by taking the _diameter of
the earth's orbit_ as a base line. Yet even a change of position
amounting to one hundred and ninety millions of miles proved, until very
recently, insufficient to alter the place of a single star, so far as to
be capable of detection by very refined observations; from which it was
concluded that the stars have not even any _annual parallax_; that is,
the angle subtended by the semidiameter of the earth's orbit, at the
nearest fixed star, is insensible. The errors to which instrumental
measurements are subject, arising from the defects of instruments
themselves, from refraction, and from various other sources of
inaccuracy, are such, that the angular determinations of arcs of the
heavens cannot be relied on to less than one second, and therefore
cannot be appreciated by direct measurement. It follows, that, when
viewed from the nearest star, the diameter of the earth's orbit would be
insensible; the spider-line of the telescope would more than cover it.
Taking, however, the annual parallax of a fixed star at one second, it
can be demonstrated, that the distance of the nearest fixed star _must
exceed_ 95000000 × 200000 = 190000000 × 100000, or one hundred thousand
times one hundred and ninety millions of miles. Of a distance so vast we
can form no adequate conceptions, and even seek to measure it only by
the time that light (which moves more than one hundred and ninety-two
thousand miles per second, and passes from the sun to the earth in eight
minutes and seven seconds) would take to traverse it, which is found to
be more than three and a half years.
If these conclusions are drawn with respect to the largest of the fixed
stars, which we suppose to be vastly nearer to us than those of the
smallest magnitude, the idea of distance swells upon us when we attempt
to estimate the remoteness of the latter. As it is uncertain, however,
whether the difference in the apparent magnitudes of the stars is owing
to a real difference, or merely to their being at various distances from
the eye, more or less uncertainty must attend all efforts to determine
the relative distances of the stars; but astronomers generally believe,
that the lower orders of stars are vastly more distant from us than the
higher. Of some stars it is said, that thousands of years would be
required for their light to travel down to us.
I have said that the stars have always been held, until recently, to
have no annual parallax; yet it may be observed that astronomers were
not exactly agreed on this point. Dr. Brinkley, a late eminent Irish
astronomer, supposed that he had detected an annual parallax in Alpha
Lyræ, amounting to one second and thirteen hundreths, and in Alpha
Aquilæ, of one second and forty-two hundreths. These results were
controverted by Mr. Pond, of the Royal Observatory of Greenwich; and
Mr. Struve, of Dorpat, has shown that, in a number of cases, the
supposed parallax is in a direction opposite to that which would arise
from the motion of the earth. Hence it is considered doubtful whether,
in all cases of an apparent parallax, the effect is not wholly due to
errors of observation.
But as if nothing was to be hidden from our times, the long sought for
parallax among the fixed stars has at length been found, and
consequently the distance of some of these bodies, at least, is no
longer veiled in mystery. In the year 1838, Professor Bessel, of
Köningsberg, announced the discovery of a parallax in one of the stars
of the Swan, (61 _Cygni_,) amounting to about _one third of a second_.
This seems, indeed, so small an angle, that we might have reason to
suspect the reality of the determination; but the most competent judges
who have thoroughly examined the process by which the discovery was
made, assent to its validity. What, then, do astronomers understand,
when they say that a parallax has been discovered in one of the fixed
stars, amounting to one third of a second? They mean that the star in
question apparently shifts its place in the heavens, to that amount,
when viewed at opposite extremities of the earth's orbit, namely, at
points in space distant from each other one hundred and ninety millions
of miles. On calculating the distance of the star from us from these
data, it is found to be six hundred and fifty-seven thousand seven
hundred times ninety-five millions of miles,--a distance which it would
take light more than ten years to traverse.
Indirect methods have been proposed, for ascertaining the parallax of
the fixed stars, by means of observations on the _double stars_. If the
two stars composing a double star are at different distances from us,
parallax would affect them unequally, and change their relative
positions with respect to each other; and since the ordinary sources of
error arising from the imperfection of instruments, from precession, and
from refraction, would be avoided, (as they would affect both objects
alike, and therefore would not disturb their relative positions,)
measurements taken with the micrometer of changes much less than one
second may be relied on. Sir John Herschel proposed a method, by which
changes may be determined that amount to only one fortieth of a second.
The immense distance of the fixed stars is inferred also from the fact,
that the largest telescopes do not increase their apparent magnitude.
They are still points, when viewed with glasses that magnify five
thousand times.
With respect to the NATURE OF THE STARS, it would seem fruitless to
inquire into the nature of bodies so distant, and which reveal
themselves to us only as shining points in space. Still, there are a few
very satisfactory inferences that can be made out respecting them.
First, _the fixed stars are bodies greater than our earth_. If this were
not the case, they would not be visible at such an immense distance. Dr.
Wollaston, a distinguished English philosopher, attempted to estimate
the magnitudes of certain of the fixed stars from the light which they
afford. By means of an accurate photometer, (an instrument for measuring
the relative intensities of light,) he compared the light of Sirius with
that of the sun. He next inquired how far the sun must be removed from
us, in order to appear no brighter than Sirius. He found the distance to
be one hundred and forty-one thousand times its present distance. But
Sirius is more than two hundred thousand times as far off as the sun;
hence he inferred that, upon the lowest computation, it must actually
give out twice as much light as the sun; or that, in point of splendor,
Sirius must be at least equal to two suns. Indeed, he has rendered it
probable, that its light is equal to that of fourteen suns. There is
reason, however, to believe that the stars are actually of various
magnitudes, and that their apparent difference is not owing merely to
their different distances. Bessel estimates the quantity of matter in
the two members of a double star in the Swan, as less than half that of
the sun.
Secondly, _the fixed stars are suns_. We have already seen that they are
large bodies; that they are immensely further off than the furthest
planet; that they shine by their own light; in short, that their
appearance is, in all respects, the same as the sun would exhibit if
removed to the region of the stars. Hence we infer that they are bodies
of the same kind with the sun. We are justified, therefore, by a sound
analogy, in concluding that the stars were made for the same end as the
sun, namely, as the centres of attraction to other planetary worlds, to
which they severally dispense light and heat. Although the starry
heavens present, in a clear night, a spectacle of unrivalled grandeur
and beauty, yet it must be admitted that the chief purpose of the stars
could not have been to adorn the night, since by far the greater part of
them are invisible to the naked eye; nor as landmarks to the navigator,
for only a very small proportion of them are adapted to this purpose;
nor, finally, to influence the earth by their attractions, since their
distance renders such an effect entirely insensible. If they are suns,
and if they exert no important agencies upon our world, but are bodies
evidently adapted to the same purpose as our sun, then it is as rational
to suppose that they were made to give light and heat, as that the eye
was made for seeing and the ear for hearing. It is obvious to inquire,
next, to what they dispense these gifts, if not to planetary worlds; and
why to planetary worlds, if not for the use of percipient beings? We are
thus led, almost inevitably, to the idea of a _plurality of worlds_; and
the conclusion is forced upon us, that the spot which the Creator has
assigned to us is but a humble province in his boundless empire.
LETTER XXX.
SYSTEM OF THE WORLD
"O how unlike the complex works of man,
Heaven's easy, artless, unincumbered, plan."--_Cowper._
HAVING now explained to you, as far as I am able to do it in so short a
space, the leading phenomena of the heavenly bodies, it only remains to
inform you of the different systems of the world which have prevailed in
different ages,--a subject which will necessarily involve a sketch of
the history of astronomy.
By a system of the world, I understand an explanation of _the
arrangement of all the bodies that compose the material universe, and of
their relations to each other_. It is otherwise called the 'Mechanism of
the Heavens;' and indeed, in the system of the world, we figure to
ourselves a machine, all parts of which have a mutual dependence, and
conspire to one great end. "The machines that were first invented," says
Adam Smith, "to perform any particular movement, are always the most
complex; and succeeding artists generally discover that, with fewer
wheels, and with fewer principles of motion, than had originally been
employed, the same effects may be more easily produced. The first
systems, in the same manner, are always the most complex; and a
particular connecting chain or principle is generally thought necessary,
to unite every two seemingly disjointed appearances; but it often
happens, that _one great connecting principle_ is afterwards found to be
sufficient to bind together all the discordant phenomena that occur in a
whole species of things!" This remark is strikingly applicable to the
origin and progress of systems of astronomy. It is a remarkable fact in
the history of the human mind, that astronomy is the oldest of the
sciences, having been cultivated, with no small success, long before any
attention was paid to the causes of the common terrestrial phenomena.
The opinion has always prevailed among those who were unenlightened by
science, that very extraordinary appearances in the sky, as comets,
fiery meteors, and eclipses, are omens of the wrath of heaven. They
have, therefore, in all ages, been watched with the greatest attention:
and their appearances have been minutely recorded by the historians of
the times. The idea, moreover, that the aspects of the stars are
connected with the destinies of individuals and of empires, has been
remarkably prevalent from the earliest records of history down to a very
late period, and, indeed, still lingers among the uneducated and
credulous. This notion gave rise to ASTROLOGY,--an art which professed
to be able, by a knowledge of the varying aspects of the planets and
stars, to penetrate the veil of futurity, and to foretel approaching
irregularities of Nature herself, and the fortunes of kingdoms and of
individuals. That department of astrology which took cognizance of
extraordinary occurrences in the natural world, as tempests,
earthquakes, eclipses, and volcanoes, both to predict their approach and
to interpret their meaning, was called _natural astrology_: that which
related to the fortunes of men and of empires, _judicial astrology_.
Among many ancient nations, astrologers were held in the highest
estimation, and were kept near the persons of monarchs; and the practice
of the art constituted a lucrative profession throughout the middle
ages. Nor were the ignorant and uneducated portions of society alone the
dupes of its pretensions. Hippocrates, the 'Father of Medicine,' ranks
astrology among the most important branches of knowledge to the
physician; and Tycho Brahe, and Lord Bacon, were firm believers in its
mysteries. Astrology, fallacious as it was, must be acknowledged to have
rendered the greatest services to astronomy, by leading to the accurate
observation and diligent study of the stars.
At a period of very remote antiquity, astronomy was cultivated in China,
India, Chaldea, and Egypt. The Chaldeans were particularly
distinguished for the accuracy and extent of their astronomical
observations. Calisthenes, the Greek philosopher who accompanied
Alexander the Great in his Eastern conquests, transmitted to Aristotle a
series of observations made at Babylon nineteen centuries before the
capture of that city by Alexander; and the wise men of Babylon and the
Chaldean astrologers are referred to in the Sacred Writings. They
enjoyed a clear sky and a mild climate, and their pursuits as shepherds
favored long-continued observations; while the admiration and respect
accorded to the profession, rendered it an object of still higher
ambition.
In the seventh century before the Christian era, astronomy began to be
cultivated in Greece; and there arose successively three celebrated
astronomical schools,--the school of Miletus, the school of Crotona, and
the school of Alexandria. The first was established by Thales, six
hundred and forty years before Christ; the second, by Pythagoras, one
hundred and forty years afterwards; and the third, by the Ptolemies of
Egypt, about three hundred years before the Christian era. As Egypt and
Babylon were renowned among the most ancient nations, for their
knowledge of the sciences, long before they were cultivated in Greece,
it was the practice of the Greeks, when they aspired to the character of
philosophers and sages, to resort to these countries to imbibe wisdom at
its fountains. Thales, after extensive travels in Crete and Egypt,
returned to his native place, Miletus, a town on the coast of Asia
Minor, where he established the first school of astronomy in Greece.
Although the minds of these ancient astronomers were beclouded with much
error, yet Thales taught a few truths which do honor to his sagacity. He
held that the stars are formed of fire; that the moon receives her light
from the sun, and is invisible at her conjunctions because she is hid in
the sun's rays. He taught the sphericity of the earth, but adopted the
common error of placing it in the centre of the world. He introduced
the division of the sphere into five zones, and taught the obliquity of
the ecliptic. He was acquainted with the Saros, or sacred period of the
Chaldeans, (see page 192,) and employed it in calculating eclipses. It
was Thales that predicted the famous eclipse of the sun which terminated
the war between the Lydians and the Medes, as mentioned in a former
Letter. Indeed, Thales is universally regarded as a bright but solitary
star, glimmering through mists on the distant horizon.
To Thales succeeded, in the school of Miletus, two other astronomers of
much celebrity, Anaximander and Anaxagoras. Among many absurd things
held by Anaximander, he first taught the sublime doctrine that the
planets are inhabited, and that the stars are suns of other systems.
Anaxagoras attempted to explain all the secrets of the skies by natural
causes. His reasonings, indeed, were alloyed with many absurd notions;
but still he alone, among the astronomers, maintained the existence of
one God. His doctrines alarmed his countrymen, by their audacity and
impiety to their gods, whose prerogatives he was thought to invade; and,
to deprecate their wrath, sentence of death was pronounced on the
philosopher and all his family,--a sentence which was commuted only for
the sad alternative of perpetual banishment. The very genius of the
heathen mythology was at war with the truth. False in itself, it trained
the mind to the love of what was false in the interpretation of nature;
it arrayed itself against the simplicity of truth, and persecuted and
put to death its most ardent votaries. The religion of the Bible, on the
other hand, lends all its aid to truth in nature as well as in morals
and religion. In its very genius it inculcates and inspires the love of
truth; it suggests, by its analogies, the existence of established laws
in the system of the world; and holds out the moon and the stars, which
the Creator has ordained, as fit objects to give us exalted views of his
glory and wisdom.