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THE SIDEREAL MESSENGER
RIVINGTONS
London _Waterloo Place_
Oxford _Magdalen Street_
Cambridge _Trinity Street_
[Illustration: GALILEO’S BROKEN LENS.
EXHIBITED IN THE LOAN COLLECTION OF SCIENTIFIC APPARATUS AT THE SOUTH
KENSINGTON MUSEUM, 1876.
_From a photograph of the Science and Art Department, South
Kensington._]
THE SIDEREAL MESSENGER
OF
GALILEO GALILEI
_AND A PART OF THE PREFACE TO KEPLER’S DIOPTRICS_
CONTAINING THE ORIGINAL ACCOUNT OF GALILEO’S
ASTRONOMICAL DISCOVERIES.
A Translation with Introduction and Notes
BY
EDWARD STAFFORD CARLOS, M.A.
HEAD MATHEMATICAL MASTER IN CHRIST’S HOSPITAL.
RIVINGTONS
_WATERLOO PLACE, LONDON_
Oxford and Cambridge
MDCCCLXXX
PREFATORY NOTE.
About five years ago I was engaged in preparing a catalogue of the
ancient books which belong to Christ’s Hospital. One portion of these
books consisted of a collection of ancient mathematical works presented
at various times for the use of that part of the school which is known
as the Royal Mathematical Foundation of King Charles II. Amongst
them were some well known by name to every mathematical student, but
which few have ever seen. Perhaps the most interesting of them all
was a little volume, printed in London in 1653, containing Gassendi’s
_Explanation of the Ptolemaic and Copernican Systems of Astronomy_, as
well as that of Tycho Brahe, Galileo’s _Sidereus Nuncius_, and Kepler’s
_Dioptrics_. I found Galileo’s account of his astronomical discoveries
so interesting, both in matter and in style, that I translated it as a
recreation from school-work. I venture to think that others also will
be interested in following Galileo through the apprehension of his
famous discoveries, and in reading the language in which he announced
them.
INTRODUCTION.
In 1609, Galileo, then Professor of Mathematics at Padua, in the
service of the Venetian Republic, heard from a correspondent at Paris
of the invention of a telescope, and set to work to consider how
such an instrument could be made. The result was his invention of
the telescope known by his name, and identical in principle with the
modern opera-glass. In a maritime and warlike State, the advantages
to be expected from such an invention were immediately recognised,
and Galileo was rewarded with a confirmation of his Professorship for
life, and a handsome stipend, in recognition of his invention and
construction of the first telescope seen at Venice. In his pamphlet,
_The Sidereal Messenger_, here translated, Galileo relates how he came
to learn the value of the telescope for astronomical research; and
how his observations were rewarded by numerous discoveries in rapid
succession, and at length by that of Jupiter’s satellites. Galileo at
once saw the value of this discovery as bearing upon the establishment
of the Copernican system of astronomy, which had met with slight
acceptance, and indeed as yet had hardly any recommendation except
that of greater simplicity. Kepler had just published at Prague his
work on the planet Mars (_Commentaria de motibus Stellæ Martis_),
on which he had been engaged apparently for eight years; there he
heard of Galileo’s discoveries, and at length was invited by Galileo
himself, through a common friend, Giuliano de’ Medici, ambassador of
the Grand-Duke of Tuscany, Cosmo de’ Medici II., to the Emperor Rudolph
II., to correspond with Galileo on the subject of these discoveries.
The Emperor also requested his opinion, and Kepler accordingly examined
Galileo’s _Sidereal Messenger_ in a pamphlet, entitled _A Discussion
with the Sidereal Messenger_ (Florence, 1610).
In this _Discussion_ Kepler gives reasons for accepting Galileo’s
observations—although he was not able to verify them from want of
a telescope—and entirely supports Galileo’s views and conclusions,
adducing his own previous speculations, or pointing out, as in the case
of Galileo’s idea of earth-light on the moon, the previous conception
of the same explanation of the phenomenon. He rejects, however,
Galileo’s explanation of the copper colour of the moon in eclipses.
Kepler ends by expressing unbounded enthusiasm at the discovery of
Jupiter’s satellites, and the argument it furnishes in support of the
Copernican theory.
Soon after, in 1611, Kepler published another pamphlet, his
_Narrative_, giving an account of actual observations made in
verification of Galileo’s discoveries by himself and several friends,
whose names he gives, with a telescope made by Galileo, and belonging
to Ernest, Elector and Archbishop of Cologne. Kepler and his friends
saw the lunar mountains and three of the satellites of Jupiter, but
failed to make out any signs of the ring of Saturn corresponding to the
imperfect description of Galileo.
Kepler had previously published a treatise on Optics (Frankfort, 1604).
He now extended it to the consideration of the theory of the telescope,
and explained the principle of Galileo’s telescope; he also showed
another combination of lenses which would produce a similar effect.
This was the principle of the common astronomical telescope, often
called, from this circumstance, Kepler’s telescope, though he did not
construct it. The account of Galileo’s later astronomical discoveries
of Saturn’s ring and the phases of Venus is taken from the preface of
this work.—(Kepler’s _Dioptrics_; Augsburg, 1611.)
In 1612 Galileo published a series of observations of solar spots, and
in 1618 some observations of three comets. There exist also long series
of minute observations of Jupiter and his satellites, continued to
November 1619.—(Galileo’s _Works_; Florence, 1845.)
Further astronomical researches may have been hindered by failing
sight. One more astronomical discovery, however, that of the moon’s
librations, was made as late as 1637, and the announcement of it is
dated “dalla mia carcere di Arcetri.” Galileo died January 8, 1642.
The following editions have been used for the translation:—
Galileo’s _Works_.
1. Florence, 1718.
2. Padua, 1744.
3. Florence, 1842-56.
_Sidereus Nuncius._
1. Venice, 1610.
2. London, 1653.
Kepler’s _Works_, ed. C. Frisch. Frankfurt a. M., 1858-71.
Prodromus dissertationum mathematicarum continens Mysterium
Cosmographicum de admirabili proportione orbium cœlestium.
Tübingen, 1596.
Astronomia nova αἰτιολογητός (Commentaria de motibus stellæ
Martis). [Prague,] 1609.
THE
SIDEREAL MESSENGER
OF
GALILEO GALILEI
THE
SIDEREAL MESSENGER
_UNFOLDING GREAT AND MARVELLOUS SIGHTS, AND PROPOSING THEM TO
THE ATTENTION OF EVERY ONE, BUT ESPECIALLY PHILOSOPHERS AND
ASTRONOMERS_,
BEING SUCH AS HAVE BEEN OBSERVED BY
GALILEO GALILEI
A GENTLEMAN OF FLORENCE, PROFESSOR OF MATHEMATICS IN THE UNIVERSITY
OF PADUA,
WITH THE AID OF A
TELESCOPE
_lately invented by him_,
_Respecting the Moon’s Surface, an innumerable number of Fixed
Stars, the Milky Way, and Nebulous Stars, but especially respecting
Four Planets which revolve round the Planet Jupiter at different
distances and in different periodic times, with amazing velocity,
and which, after remaining unknown to every one up to this day, the
Author recently discovered, and determined to name the_
MEDICEAN STARS.
Venice 1610.
TO THE MOST SERENE
COSMO DE’ MEDICI, THE SECOND,
_FOURTH GRAND-DUKE OF TUSCANY_.
There is certainly something very noble and large-minded in the
intention of those who have endeavoured to protect from envy the noble
achievements of distinguished men, and to rescue their names, worthy
of immortality, from oblivion and decay. This desire has given us
the lineaments of famous men, sculptured in marble, or fashioned in
bronze, as a memorial of them to future ages; to the same feeling we
owe the erection of statues, both ordinary and equestrian; hence, as
the poet[1] says, has originated expenditure, mounting to the stars,
upon columns and pyramids; with this desire, lastly, cities have been
built, and distinguished by the names of those men, whom the gratitude
of posterity thought worthy of being handed down to all ages. For the
state of the human mind is such, that unless it be continually stirred
by the counterparts[2] of matters, obtruding themselves upon it from
without, all recollection of the matters easily passes away from it.
[1] Propertius, iii. 2. 17-22.
[2] Compare Lucretius iv. 881:
Dico animo nostro primum simulacra meandi Accidere, atque animum
pulsare.
But others, having regard for more stable and more lasting monuments,
secured the eternity of the fame of great men by placing it under the
protection, not of marble or bronze, but of the Muses’ guardianship
and the imperishable monuments of literature. But why do I mention
these things, as if human wit, content with these regions, did not
dare to advance further; whereas, since she well understood that all
human monuments do perish at last by violence, by weather, or by age,
she took a wider view, and invented more imperishable signs, over
which destroying Time and envious Age could claim no rights; so,
betaking herself to the sky, she inscribed on the well-known orbs of
the brightest stars—those everlasting orbs—the names of those who, for
eminent and god-like deeds, were accounted worthy to enjoy an eternity
in company with the stars. Wherefore the fame of Jupiter, Mars,
Mercury, Hercules, and the rest of the heroes by whose names the stars
are called, will not fade until the extinction of the splendour of the
constellations themselves.
But this invention of human shrewdness, so particularly noble and
admirable, has gone out of date ages ago, inasmuch as primeval heroes
are in possession of those bright abodes, and keep them by a sort of
right; into whose company the affection of Augustus in vain attempted
to introduce Julius Cæsar; for when he wished that the name of the
Julian constellation should be given to a star, which appeared in
his time, one of those which the Greeks and the Latins alike name,
from their hair-like tails, comets, it vanished in a short time and
mocked his too eager hope. But we are able to read the heavens for
your highness, most Serene Prince, far more truly and more happily,
for scarcely have the immortal graces of your mind begun to shine
on earth, when bright stars present themselves in the heavens, like
tongues to tell and celebrate your most surpassing virtues to all
time. Behold therefore, four stars reserved for your famous name, and
those not belonging to the common and less conspicuous multitude of
fixed stars, but in the bright ranks of the planets—four stars which,
moving differently from each other, round the planet Jupiter, the
most glorious of all the planets, as if they were his own children,
accomplish the courses of their orbits with marvellous velocity,
while all the while with one accord they complete all together mighty
revolutions every ten years round the centre of the universe, that is,
round the Sun.
But the Maker of the Stars himself seemed to direct me by clear reasons
to assign these new planets to the famous name of your highness in
preference to all others. For just as these stars, like children worthy
of their sire, never leave the side of Jupiter by any appreciable
distance, so who does not know that clemency, kindness of heart,
gentleness of manners, splendour of royal blood, nobleness in public
functions, wide extent of influence and power over others, all of which
have fixed their common abode and seat in your highness,—who, I say,
does not know that all these qualities, according to the providence of
God, from whom all good things do come, emanate from the benign star
of Jupiter? Jupiter, Jupiter, I maintain, at the instant of the birth
of your highness having at length emerged from the turbid mists of the
horizon, and being in possession of the middle quarter of the heavens,
and illuminating the eastern angle, from his own royal house, from
that exalted throne, looked out upon your most happy birth, and poured
forth into a most pure atmosphere all the brightness of his majesty,
in order that your tender body and your mind—though that was already
adorned by God with still more splendid graces—might imbibe with your
first breath the whole of that influence and power. But why should I
use only plausible arguments when I can almost absolutely demonstrate
my conclusion? It was the will of Almighty God that I should be judged
by your most serene parents not unworthy to be employed in teaching
your highness mathematics, which duty I discharged, during the four
years just passed, at that time of the year when it is customary to
take a relaxation from severer studies. Wherefore, since it evidently
fell to my lot by God’s will, to serve your highness, and so to receive
the rays of your surpassing clemency and beneficence in a position near
your person, what wonder is it if you have so warmed my heart that
it thinks about scarcely anything else day and night, but how I, who
am indeed your subject not only by inclination, but also by my very
birth and lineage, may be known to be most anxious for your glory, and
most grateful to you? And so, inasmuch as under your patronage, most
serene Cosmo, I have discovered these stars, which were unknown to
all astronomers before me, I have, with very good right, determined
to designate them with the most august name of your family. And as I
was the first to investigate them, who can rightly blame me if I give
them a name, and call them _the Medicean Stars_, hoping that as much
consideration may accrue to these stars from this title, as other stars
have brought to other heroes? For not to speak of your most serene
ancestors, to whose everlasting glory the monuments of all history bear
witness, your virtue alone, most mighty sire, can confer on those stars
an immortal name; for who can doubt that you will not only maintain and
preserve the expectations, high though they be, about yourself, which
you have aroused by the very happy beginning of your government, but
that you will also far surpass them, so that when you have conquered
others like yourself, you may still vie with yourself, and become day
by day greater than yourself and your greatness?
Accept, then, most clement Prince, this addition to the glory of your
family, reserved by the stars for you; and may you enjoy for many years
those good blessings, which are sent to you not so much from the stars
as from God, the Maker and Governor of the stars.
Your Highness’s most devoted servant,
Galileo Galilei.
Padua, _March 12, 1610_.
THE ASTRONOMICAL MESSENGER
_Containing and setting forth Observations lately made with the aid
of a newly invented_ Telescope _respecting the Moon’s Surface, the
Milky Way, Nebulous Stars, an innumerable multitude of Fixed Stars,
and also respecting Four Planets never before seen, which have been
named_
THE COSMIAN STARS.[3]
[3] The satellites of Jupiter are here called “_the Cosmian Stars_”
in honour of Cosmo de’ Medici, but elsewhere Galileo calls them
“_the Medicean Stars_.” Kepler sometimes calls them “_the Medicean
Stars_,” but more often “_satellites_.”
[Sidenote: Introduction.]
IN the present small treatise I set forth some matters of great
interest for all observers of natural phenomena to look at and
consider. They are of great interest, I think, first, from their
intrinsic excellence; secondly, from their absolute novelty; and
lastly, also on account of the instrument by the aid of which they have
been presented to my apprehension.
The number of the Fixed Stars which observers have been able to see
without artificial powers of sight up to this day can be counted. It is
therefore decidedly a great feat to add to their number, and to set
distinctly before the eyes other stars in myriads, which have never
been seen before, and which surpass the old, previously known, stars in
number more than ten times.
Again, it is a most beautiful and delightful sight to behold the body
of the Moon, which is distant from us nearly sixty _semi_-diameters[4]
of the Earth, as near as if it was at a distance of only two of the
same measures; so that the diameter of this same Moon appears about
thirty times larger, its surface about nine hundred times, and its
solid mass nearly 27,000 times larger than when it is viewed only with
the naked eye; and consequently any one may know with the certainty
that is due to the use of our senses, that the Moon certainly does not
possess a smooth and polished surface, but one rough and uneven, and,
just like the face of the Earth itself, is everywhere full of vast
protuberances, deep chasms, and sinuosities.
[4] Galileo says, “per sex denas fere terrestres _diametros_ a
nobis remotum” by mistake for _semi-diametros_, and the same
mistake occurs in p. 11.
Then to have got rid of disputes about the Galaxy or Milky Way, and
to have made its nature clear to the very senses, not to say to the
understanding, seems by no means a matter which ought to be considered
of slight importance. In addition to this, to point out, as with one’s
finger, the nature of those stars which every one of the astronomers
up to this time has called _nebulous_, and to demonstrate that it is
very different from what has hitherto been believed, will be pleasant,
and very fine. But that which will excite the greatest astonishment by
far, and which indeed especially moved me to call the attention of all
astronomers and philosophers, is this, namely, that I have discovered
four planets, neither known nor observed by any one of the astronomers
before my time, which have their orbits round a certain bright star,
one of those previously known, like Venus and Mercury round the Sun,
and are sometimes in front of it, sometimes behind it, though they
never depart from it beyond certain limits. All which facts were
discovered and observed a few days ago by the help of a telescope[5]
devised by me, through God’s grace first enlightening my mind.
[5] The words used by Galileo for “telescope” are _perspicillum_,
_specillum instrumentum_, _organum_, and _occhiale_ (Ital.).
Kepler uses also _oculare tubus_, _arundo dioptrica_. The word
“_telescopium_” is used by Gassendi, 1647.
Perchance other discoveries still more excellent will be made from time
to time by me or by other observers, with the assistance of a similar
instrument, so I will first briefly record its shape and preparation,
as well as the occasion of its being devised, and then I will give an
account of the observations made by me.
[Sidenote: Galileo’s account of the invention of his telescope.]
About ten months ago a report reached my ears that a Dutchman had
constructed a telescope, by the aid of which visible objects, although
at a great distance from the eye of the observer, were seen distinctly
as if near; and some proofs of its most wonderful performances were
reported, which some gave credence to, but others contradicted. A few
days after, I received confirmation of the report in a letter written
from Paris by a noble Frenchman, Jaques Badovere, which finally
determined me to give myself up first to inquire into the principle of
the telescope, and then to consider the means by which I might compass
the invention of a similar instrument, which a little while after I
succeeded in doing, through deep study of the theory of Refraction;
and I prepared a tube, at first of lead, in the ends of which I fitted
two glass lenses, both plane on one side, but on the other side one
spherically convex, and the other concave. Then bringing my eye to the
concave lens I saw objects satisfactorily large and near, for they
appeared one-third of the distance off and nine times larger than
when they are seen with the natural eye alone. I shortly afterwards
constructed another telescope with more nicety, which magnified
objects more than sixty times. At length, by sparing neither labour
nor expense, I succeeded in constructing for myself an instrument
so superior that objects seen through it appear magnified nearly a
thousand times, and more than thirty times nearer than if viewed by the
natural powers of sight alone.
[Sidenote: Galileo’s first observations with his telescope.]
It would be altogether a waste of time to enumerate the number and
importance of the benefits which this instrument may be expected to
confer, when used by land or sea. But without paying attention to its
use for terrestrial objects, I betook myself to observations of the
heavenly bodies; and first of all, I viewed the Moon as near as if it
was scarcely two _semi_-diameters[6] of the Earth distant. After the
Moon, I frequently observed other heavenly bodies, both fixed stars
and planets, with incredible delight; and, when I saw their very great
number, I began to consider about a method by which I might be able to
measure their distances apart, and at length I found one. And here it
is fitting that all who intend to turn their attention to observations
of this kind should receive certain cautions. For, in the first
place, it is absolutely necessary for them to prepare a most perfect
telescope, one which will show very bright objects distinct and free
from any mistiness, and will magnify them at least 400 times, for then
it will show them as if only one-twentieth of their distance off. For
unless the instrument be of such power, it will be in vain to attempt
to view all the things which have been seen by me in the heavens, or
which will be enumerated hereafter.
[6] “Vix per duas Telluris _diametros_,” by mistake for
“semi-diametros.”
[Sidenote: Method of determining the magnifying power of the telescope.]
But in order that any one may be a little more certain about the
magnifying power of his instrument, he shall fashion two circles, or
two square pieces of paper, one of which is 400 times greater than the
other, but that will be when the diameter of the greater is twenty
times the length of the diameter of the other. Then he shall view from
a distance simultaneously both surfaces, fixed on the same wall, the
smaller with one eye applied to the telescope, and the larger with the
other eye unassisted; for that may be done without inconvenience at one
and the same instant with both eyes open. Then both figures will appear
of the same size, if the instrument magnifies objects in the desired
proportion.
[Sidenote: Method of measuring small angular distances between heavenly
bodies by the size of the aperture of the telescope.]
After such an instrument has been prepared, the method of measuring
distances remains for inquiry, and this we shall accomplish by the
following contrivance:—
[Illustration]
For the sake of being more easily understood, I will suppose a tube A
B C D.[7] Let E be the eye of the observer; then, when there are no
lenses in the tube rays from the eye to the object F G would be drawn
in the straight lines E C F, E D G, but when the lenses have been
inserted, let the rays go in the bent lines E C H, E D I,—for they are
contracted, and those which originally, when unaffected by the lenses,
were directed to the object F G, will include only the part H I. Hence
the ratio of the distance E H to the line H I being known, we shall
be able to find, by means of a table of sines, the magnitude of the
angle subtended at the eye by the object H I, which we shall find to
contain only some minutes. But if we fit on the lens C D thin plates
of metal, pierced, some with larger, others with smaller apertures, by
putting on over the lens sometimes one plate, sometimes another, as may
be necessary, we shall construct at our pleasure different subtending
angles of more or fewer minutes, by the help of which we shall be
able to measure conveniently the intervals between stars separated by
an angular distance of some minutes, within an error of one or two
minutes. But let it suffice for the present to have thus slightly
touched, and as it were just put our lips to these matters, for on
some other opportunity I will publish the theory of this instrument in
completeness.
[7] [Illustration]
The line C H in Galileo’s figure represents the small pencil of
rays from H which, after refraction through the telescope, reach
the eye E. The enlarged figure shows that if O P be the radius of
the aperture employed, the point H of the object would be just
outside the field of view. The method, however, is at best only
a very rough one, as the boundary of the field of view in this
telescope is unavoidably indistinct.
Now let me review the observations made by me during the two months
just past, again inviting the attention of all who are eager for true
philosophy to the beginnings which led to the sight of most important
phenomena.
[Sidenote: The Moon. Ruggedness of its surface. Existence of lunar
mountains and valleys.]
Let me speak first of the surface of the Moon, which is turned towards
us. For the sake of being understood more easily, I distinguish two
parts in it, which I call respectively the brighter and the darker. The
brighter part seems to surround and pervade the whole hemisphere; but
the darker part, like a sort of cloud, discolours the Moon’s surface
and makes it appear covered with spots. Now these spots, as they
are somewhat dark and of considerable size, are plain to every one,
and every age has seen them, wherefore I shall call them _great_ or
_ancient_ spots, to distinguish them from other spots, smaller in size,
but so thickly scattered that they sprinkle the whole surface of the
Moon, but especially the brighter portion of it. These spots have never
been observed by any one before me; and from my observations of them,
often repeated, I have been led to that opinion which I have expressed,
namely, that I feel sure that the surface of the Moon is not perfectly
smooth, free from inequalities and exactly spherical, as a large
school of philosophers considers with regard to the Moon and the other
heavenly bodies, but that, on the contrary, it is full of inequalities,
uneven, full of hollows and protuberances, just like the surface of the
Earth itself, which is varied everywhere by lofty mountains and deep
valleys.
Sketches by Galileo to shew:—
[Illustration: the indentation of the terminator and illuminated
summits of mountains in the dark part of the moon;]
[Illustration: the shape of a lunar mountain and of a walled plain.
Galileo: ‘Sidereus Nuncius,’ Venice 1610.]
The appearances from which we may gather these conclusions are of the
following nature:—On the fourth or fifth day after new-moon, when
the Moon presents itself to us with bright horns, the boundary which
divides the part in shadow from the enlightened part does not extend
continuously in an ellipse, as would happen in the case of a perfectly
spherical body, but it is marked out by an irregular, uneven, and very
wavy line, as represented in the figure given, for several bright
excrescences, as they may be called, extend beyond the boundary of
light and shadow into the dark part, and on the other hand pieces of
shadow encroach upon the light:—nay, even a great quantity of small
blackish spots, altogether separated from the dark part, sprinkle
everywhere almost the whole space which is at the time flooded with the
Sun’s light, with the exception of that part alone which is occupied by
the great and ancient spots. I have noticed that the small spots just
mentioned have this common characteristic always and in every case,
that they have the dark part towards the Sun’s position, and on the
side away from the Sun they have brighter boundaries, as if they were
crowned with shining summits. Now we have an appearance quite similar
on the Earth about sunrise, when we behold the valleys, not yet flooded
with light, but the mountains surrounding them on the side opposite to
the Sun already ablaze with the splendour of his beams; and just as
the shadows in the hollows of the Earth diminish in size as the Sun
rises higher, so also these spots on the Moon lose their blackness
as the illuminated part grows larger and larger. Again, not only are
the boundaries of light and shadow in the Moon seen to be uneven
and sinuous, but—and this produces still greater astonishment—there
appear very many bright points within the darkened portion of the
Moon, altogether divided and broken off from the illuminated tract,
and separated from it by no inconsiderable interval, which, after a
little while, gradually increase in size and brightness, and after
an hour or two become joined on to the rest of the bright portion,
now become somewhat larger; but in the meantime others, one here and
another there, shooting up as if growing, are lighted up within the
shaded portion, increase in size, and at last are linked on to the
same luminous surface, now still more extended. An example of this
is given in the same figure. Now, is it not the case on the Earth
before sunrise, that while the level plain is still in shadow, the
peaks of the most lofty mountains are illuminated by the Sun’s rays?
After a little while does not the light spread further, while the
middle and larger parts of those mountains are becoming illuminated;
and at length, when the Sun has risen, do not the illuminated parts
of the plains and hills join together? The grandeur, however, of
such prominences and depressions in the Moon seems to surpass both
in magnitude and extent the ruggedness of the Earth’s surface, as I
shall hereafter show. And here I cannot refrain from mentioning what a
remarkable spectacle I observed while the Moon was rapidly approaching
her first quarter, a representation of which is given in the same
illustration, placed opposite page 16. A protuberance of the shadow, of
great size, indented the illuminated part in the neighbourhood of the
lower cusp; and when I had observed this indentation longer, and had
seen that it was dark throughout, at length, after about two hours, a
bright peak began to arise a little below the middle of the depression;
this by degrees increased, and presented a triangular shape, but was
as yet quite detached and separated from the illuminated surface. Soon
around it three other small points began to shine, until, when the Moon
was just about to set, that triangular figure, having now extended and
widened, began to be connected with the rest of the illuminated part,
and, still girt with the three bright peaks already mentioned, suddenly
burst into the indentation of shadow like a vast promontory of light.
At the ends of the upper and lower cusps also certain bright points,
quite away from the rest of the bright part, began to rise out of the
shadow, as is seen depicted in the same illustration.
[Sidenote: The lunar spots are suggested to be possibly seas bordered
by ranges of mountains.]
In both horns also, but especially in the lower one, there was a great
quantity of dark spots, of which those which are nearer the boundary
of light and shadow appear larger and darker, but those which are more
remote less dark and more indistinct. In all cases, however, just as I
have mentioned before, the dark portion of the spot faces the position
of the Sun’s illumination, and a brighter edge surrounds the darkened
spot on the side away from the Sun, and towards the region of the Moon
in shadow. This part of the surface of the Moon, where it is marked
with spots like a peacock’s tail with its azure eyes, is rendered like
those glass vases which, through being plunged while still hot from the
kiln into cold water, acquire a crackled and wavy surface, from which
circumstance they are commonly called _frosted glasses_.[8] Now the
great spots of the Moon observed at the same time are not seen to be
at all similarly broken, or full of depressions and prominences, but
rather to be even and uniform; for only here and there some spaces,
rather brighter than the rest, crop up; so that if any one wishes to
revive the old opinion of the Pythagoreans, that the Moon is another
Earth, so to say, the brighter portion may very fitly represent the
surface of the land, and the darker the expanse of water. Indeed, I
have never doubted that if the sphere of the Earth were seen from a
distance, when flooded with the Sun’s rays, that part of the surface
which is land would present itself to view as brighter, and that which
is water as darker in comparison. Moreover, the great spots in the
Moon are seen to be more depressed than the brighter tracts; for in
the Moon, both when crescent and when waning, on the boundary between
the light and shadow, which projects in some places round the great
spots, the adjacent regions are always brighter, as I have noticed in
drawing my illustrations, and the edges of the spots referred to are
not only more depressed than the brighter parts, but are more even,
and are not broken by ridges or ruggednesses. But the brighter part
stands out most near the spots, so that both before the first quarter
and about the third quarter also, around a certain spot in the upper
part of the figure, that is, occupying the northern region of the Moon,
some vast prominences on the upper and lower sides of it rise to an
enormous elevation, as the illustrations show. This same spot before
the third quarter is seen to be walled round with boundaries of a
deeper shade, which just like very lofty mountain summits appear darker
on the side away from the Sun, and brighter on the side where they face
the Sun; but in the case of the cavities the opposite happens, for the
part of them away from the Sun appears brilliant, and that part which
lies nearer to the Sun dark and in shadow. After a time, when the
enlightened portion of the Moon’s surface has diminished in size, as
soon as the whole or nearly so of the spot already mentioned is covered
with shadow, the brighter ridges of the mountains mount high above the
shade. These two appearances are shown in the illustrations which are
given.
[8] Specimens of _frosted or crackled Venetian glass_ are to be
seen in the Slade Collection, British Museum, and fully justify
Galileo’s comparison.
[Sidenote: Description of a lunar crater, perhaps Tycho.][9]
[9] Webb, _Celestial Objects for Common Telescopes_, p. 104,
suggests this identification.
There is one other point which I must on no account forget, which I
have noticed and rather wondered at. It is this:—The middle of the
Moon, as it seems, is occupied by a certain cavity larger than all the
rest, and in shape perfectly round. I have looked at this depression
near both the first and third quarters, and I have represented it as
well as I can in the second illustration already given. It produces
the same appearance as to effects of light and shade as a tract like
Bohemia would produce on the Earth, if it were shut in on all sides by
very lofty mountains arranged on the circumference of a perfect circle;
for the tract in the Moon is walled in with peaks of such enormous
height that the furthest side adjacent to the dark portion of the
Moon is seen bathed in sunlight before the boundary between light and
shade reaches half-way across the circular space. But according to the
characteristic property of the rest of the spots, the shaded portion of
this too faces the Sun, and the bright part is towards the dark side of
the Moon, which for the third time I advise to be carefully noticed as
a most solid proof of the ruggednesses and unevennesses spread over the
whole of the bright region of the Moon. Of these spots, moreover, the
darkest are always those which are near to the boundary-line between
the light and the shadow, but those further off appear both smaller
in size and less decidedly dark; so that at length, when the Moon at
opposition becomes full, the darkness of the cavities differs from the
brightness of the prominences with a subdued and very slight difference.
[Sidenote: Reasons for believing that there is a difference of
constitution in various parts of the Moon’s surface.]
These phenomena which we have reviewed are observed in the bright
tracts of the Moon. In the great spots we do not see such differences
of depressions and prominences as we are compelled to recognise in the
brighter parts, owing to the change of their shapes under different
degrees of illumination by the Sun’s rays according to the manifold
variety of the Sun’s position with regard to the Moon. Still, in the
great spots there do exist some spaces rather less dark than the rest,
as I have noted in the illustrations, but these spaces always have the
same appearance, and the depth of their shadow is neither intensified
nor diminished; they do appear indeed sometimes a little more shaded,
sometimes a little less, but the change of colour is very slight,
according as the Sun’s rays fall upon them more or less obliquely;
and besides, they are joined to the adjacent parts of the spots with
a very gradual connection, so that their boundaries mingle and melt
into the surrounding region. But it is quite different with the spots
which occupy the brighter parts of the Moon’s surface, for, just as
if they were precipitous crags with numerous rugged and jagged peaks,
they have well-defined boundaries through the sharp contrast of light
and shade. Moreover, inside those great spots certain other tracts are
seen brighter than the surrounding region, and some of them very bright
indeed, but the appearance of these, as well as of the darker parts,
is always the same; there is no change of shape or brightness or depth
of shadow, so that it becomes a matter of certainty and beyond doubt
that their appearance is owing to real dissimilarity of parts, and
not to unevennesses only in their configuration, changing in different
ways the shadows of the same parts according to the variations of their
illumination by the Sun, which really happens in the case of the other
smaller spots occupying the brighter portion of the Moon, for day by
day they change, increase, decrease, or disappear, inasmuch as they
derive their origin only from the shadows of prominences.
[Sidenote: Explanation of the evenness of the illuminated part of
the circumference of the Moon’s orb by the analogy of terrestrial
phenomena, or by a possible lunar atmosphere.]
But here I feel that some people may be troubled with grave doubt, and
perhaps seized with a difficulty so serious as to compel them to feel
uncertain about the conclusion just explained and supported by so many
phenomena. For if that part of the Moon’s surface which reflects the
Sun’s rays most brightly is full of sinuosities, protuberances, and
cavities innumerable, why, when the Moon is increasing, does the outer
edge which looks toward the west, when the Moon is waning, the other
half-circumference towards the east, and at full-moon the whole circle,
appear not uneven, rugged, and irregular, but perfectly round and
circular, as sharply defined as if marked out with a pair of compasses,
and without the indentations of any protuberances or cavities? And most
remarkably so, because the whole unbroken edge belongs to that part of
the Moon’s surface which possesses the property of appearing brighter
than the rest, which I have said to be throughout full of protuberances
and cavities. For not one of the Great Spots extends quite to the
circumference, but all of them are seen to be together away from the
edge. Of this phenomenon, which affords a handle for such serious
doubt, I produce two causes, and so two solutions of the difficulty.
The first solution which I offer is this:—If the protuberances and
cavities in the body of the Moon existed only on the edge of the
circle that bounds the hemisphere which we see, then the Moon might,
or rather must, show itself to us with the appearance of a toothed
wheel, being bounded with an irregular and uneven circumference; but
if, instead of a single set of prominences arranged along the actual
circumference only, very many ranges of mountains with their cavities
and ruggednesses are set one behind the other along the extreme edge
of the Moon, and that too not only in the hemisphere which we see, but
also in that which is turned away from us, but still near the boundary
of the hemisphere, then the eye, viewing them afar off, will not at
all be able to detect the differences of prominences and cavities, for
the intervals between the mountains situated in the same circle, or in
the same chain, are hidden by the jutting forward of other prominences
situated in other ranges, and especially if the eye of the observer
is placed in the same line with the tops of the prominences mentioned.
So on the Earth, the summits of a number of mountains close together
appear situated in one plane, if the spectator is a long way off and
standing at the same elevation. So when the sea is rough, the tops of
the waves seem to form one plane, although between the billows there is
many a gulf and chasm, so deep that not only the hulls, but even the
bulwarks, masts, and sails of stately ships are hidden amongst them.
Therefore, as within the Moon, as well as round her circumference,
there is a manifold arrangement of prominences and cavities, and the
eye, regarding them from a great distance, is placed in nearly the
same plane with their summits, no one need think it strange that they
present themselves to the visual ray which just grazes them as an
unbroken line quite free from unevennesses. To this explanation may be
added another, namely, that there is round the body of the Moon, just
as round the Earth, an envelope of some substance denser than the rest
of the ether, which is sufficient to receive and reflect the Sun’s
rays, although it does not possess so much opaqueness as to be able to
prevent our seeing through it—especially when it is not illuminated.
That envelope, when illuminated by the Sun’s rays, renders the body
of the Moon apparently larger than it really is, and would be able
to stop our sight from penetrating to the solid body of the Moon, if
its thickness were greater; now, it is of greater thickness about the
circumference of the Moon, greater, I mean, not in actual thickness,
but with reference to our sight-rays, which cut it obliquely; and so it
may stop our vision, especially when it is in a state of brightness,
and may conceal the true circumference of the Moon on the side towards
the Sun.
[Illustration]
This may be understood more clearly from the adjoining figure, in which
the body of the Moon, A B C, is surrounded by an enveloping atmosphere,
D E G. An eye at F penetrates to the middle parts of the Moon, as
at A, through a thickness, D A, of the atmosphere; but towards the
extreme parts a mass of atmosphere of greater depth, E B, shuts out its
boundary from our sight. An argument in favour of this is, that the
illuminated portion of the Moon appears of larger circumference than
the rest of the orb which is in shadow.
Perhaps also some will think that this same cause affords a very
reasonable explanation why the greater spots on the Moon are not seen
to reach to the edge of the circumference on any side, although it
might be expected that some would be found about the edge as well as
elsewhere; and it seems credible that there are spots there, but that
they cannot be seen because they are hidden by a mass of atmosphere too
thick and too bright for the sight to penetrate.
[Illustration]
[Sidenote: Calculation to show that the height of some lunar mountains
exceeds four Italian miles[10] (22,000 British feet).]
[10] In the list of the heights of lunar mountains determined by
Beer and Maedler, given in their work _Der Mond_ (Berlin, 1837),
there are six which exceed 3000 toises, or 19,000 British feet.
I think that it has been sufficiently made clear, from the explanation
of phenomena which have been given, that the brighter part of the
Moon’s surface is dotted everywhere with protuberances and cavities;
it only remains for me to speak about their size, and to show that the
ruggednesses of the Earth’s surface are far smaller than those of the
Moon’s; smaller, I mean, absolutely, so to say, and not only smaller
in proportion to the size of the orbs on which they are. And this is
plainly shown thus:—As I often observed in various positions of the
Moon with reference to the Sun, that some summits within the portion
of the Moon in shadow appeared illumined, although at some distance
from the boundary of the light (the terminator), by comparing their
distance with the complete diameter of the Moon, I learnt that it
sometimes exceeded the one-twentieth (1/20th) part of the diameter.
Suppose the distance to be exactly 1/20th part of the diameter, and
let the diagram represent the Moon’s orb, of which C A F is a great
circle, E its centre, and C F a diameter, which consequently bears
to the diameter of the Earth the ratio 2:7; and since the diameter
of the Earth, according to the most exact observations, contains
7000 Italian miles, C F will be 2000, and C E 1000, and the 1/20th
part of the whole, C F, 100 miles. Also let C F be a diameter of the
great circle which divides the bright part of the Moon from the dark
part (for, owing to the very great distance of the Sun from the Moon
this circle does not differ sensibly from a great one), and let the
distance of A from the point C be 1/20th part of that diameter; let
the radius E A be drawn, and let it be produced to cut the tangent
line G C D, which represents the ray that illumines the summit, in
the point D. Then the arc C A or the straight line C D will be 100 of
such units, as C E contains 1000. The sum of the squares of D C, C E is
therefore 1,010,000, and the square of D E is equal to this; therefore
the whole E D will be more than 1004; and A D will be more than 4 of
such units, as C E contained 1000. Therefore the height of A D in the
Moon, which represents a summit reaching up to the Sun’s ray, G C D,
and separated from the extremity C by the distance C D, is more than
4 Italian miles; but in the Earth there are no mountains which reach
to the perpendicular height even of one mile. We are therefore left to
conclude that it is clear that the prominences of the Moon are loftier
than those of the Earth.
[Sidenote: The faint illumination of the Moon’s disc about new-moon
explained to be due to earth-light.]
I wish in this place to assign the cause of another lunar phenomenon
well worthy of notice, and although this phenomenon was observed by me
not lately, but many years ago, and has been pointed out to some of my
intimate friends and pupils, explained, and assigned to its true cause,
yet as the observation of it is rendered easier and more vivid by the
help of a telescope, I have considered that it would not be unsuitably
introduced in this place, but I wish to introduce it chiefly in order
that the connection and resemblance between the Moon and the Earth may
appear more plainly.