intended for everyone but mainly
philosophers and astronomers,
and making known
THE GREAT AND MARVELLOUS SIGHTS that
GALILEO GALILEI,
A FLORENTINE PATRICIAN*
and Professor of Mathematics at the University of Padua
observed with the aid of a SPYGLASS, recently discovered
by him,* on the face of the Moon, in innumerable fixed stars,
in the Milky Way, and in nebulous stars,
but above all
IN FOUR PLANETS*
that revolve around the star JUPITER with amazing speed
at different distances and in different periodic times. Unknown
to anyone up to this day, these were very recently discovered
by the Author, who decided to call them
MEDICEAN STARS*
VENICE, printed by Tommaso Baglioni, MDCX.
With permission of the authorities and copyright.
To the Most Serene Cosimo de’ Medici II, Fourth Grand Duke of Tuscany*
NOBLE and truly public-spirited was the intention of those who determined to protect from envy the great achievements of men of outstanding virtue and to rescue their names, which deserve immortality, from neglect and oblivion. Hence, as a memorial to future ages, likenesses sculptured in marble or cast in bronze; hence, statues on foot or on horseback; hence, columns and pyramids whose cost is sky-high, as the poet says;* hence also, the building of cities to bear the names of those whom posterity deemed worthy of being remembered throughout the ages. For recollection all too easily slips away from the human mind unless it is constantly reminded by outside stimuli.
There were others, however, who looked for more stable and enduring memorials. These did not entrust the eternal praise of great men to building-blocks or strips of metal but to the custody of the Muses and the imperishable monuments of literature.* But why mention this? Could it be that human ingenuity, satisfied with what happens here below, dared proceed no further? No, on the contrary, looking beyond and realizing that warfare, the weather, or the passing of time eventually raze all human monuments to the ground, less corruptible signs were sought over which devouring time and envious age could claim no rights. So betaking itself to the heavens, human ingenuity inscribed on the well-known and eternal orbs* of the brightest stars the names of those who for their eminent and godlike deeds were deemed worthy of enjoying all eternity in company of the stars. Wherefore, the fame of Jupiter, Mars, Mercury, Hercules, and the other heroes by whose names the stars are called, will not fade until the splendour of the stars themselves is extinguished. But this noble and admirable custom went out of fashion ages ago when those glorious seats were occupied by the ancient heroes who now hold them, as it were, in their own right. In vain did the affection of Augustus try to introduce Julius Caesar into their company.* The star that appeared in his day and that he wanted to name ‘Julian’ belonged to those that the Greeks call comets and we ‘hair-like’, and it vanished in a short time and mocked his too eager hope.
But, Most Serene Prince, we can augur far more genuine and happy things for your Highness, for no sooner had the immortal greatness of your mind begun to shine on Earth than bright stars presented themselves in the heavens like tongues to tell and celebrate your surpassing virtues for all time. Behold, therefore, four stars reserved for your famous name. They do not belong to the common and less distinguished multitude of fixed stars but to the illustrious rank of the planets. Moving at different rates around Jupiter, the noblest of the planets,* as if they were his own children, they trace out their orbits with marvellous speed while, at the same time, with one harmonious accord, they go round the centre of the world, namely the Sun itself,* and complete their great revolutions in twelve years.*
The Creator of the stars seems to have himself* provided me with clear reasons for dedicating these new planets to Your Highness’ famous name in preference to all others.* For just as these stars, like worthy children, never leave the side of Jupiter by any appreciable distance, so everyone knows that clemency, kindness of heart, gentleness of manners, splendour of royal blood, majesty in deportment, wide extent of influence, and authority over others have all fixed their abode and seat in your Highness. Who, I say, does not know that all these qualities emanate from the most benign planet Jupiter,* according to the will of God from Whom all good things flow? Jupiter, Jupiter I say, that at the moment of your Highness’ birth had already risen above the misty vapour on the horizon and occupied the mid-heaven. Illuminating the eastern angle* from its royal house, it looked down from its exalted throne upon your blessed birth, and poured out the brightness of its majesty in the pure air in order that your tender body and your mind, already adorned by God with the most noble ornaments, might imbibe with its first breath the whole of that strength and power.
But why should I use merely plausible arguments when I can demonstrate my conclusion with practically absolute certainly? It pleased God Almighty that I should be considered worthy by your Most Serene Parents to teach your Highness mathematics* during the last four years at that time of year when it is customary to rest from more exacting studies. It was clearly God’s will that I should serve your Highness and be exposed at close quarters to the rays of your incredible kindness and gentleness. What wonder is it, therefore, if you have so warmed my heart that I, who am your subject not only by choice but by birth and lineage, should night and day scarcely think of anything else but how to make known how grateful I am to you, and how desirous to promote your glory. And so, since it was under your auspices, most serene Cosimo, that I discovered those stars that were unknown to all astronomers before me, I have the right to call them by the most august name of your family. Since I was the first to discover them, who can rightly reproach me if I also give them a name and call them MEDICEAN STARS, hoping that perhaps as much honour may accrue to the stars from this title, as other stars have brought the other heroes? For to say nothing of your most serene ancestors whose everlasting glory is attested by the historical record, your virtue alone, Great Hero, can confer on those stars a name that is immortal. For who can doubt that you will not only maintain and preserve the high expectations that you have aroused at the beginning of your reign, but far surpass them? So when you have conquered your equals, you will nonetheless still vie with yourself and become greater day by day.
Receive then, most clement Prince, this honour that was reserved for you by the stars, and may you enjoy for many years those blessings that descended upon you not so much from the stars, as from God, their Creator and Governor.
Padua, on the fourth day before the Ides of March MDCX. Your Highness’ most devoted servant,
Galileo Galilei
THE undersigned, their Excellencies the Heads of the Council of Ten,* having been informed by the Overseers* of the University of Padua that the two persons appointed for this task, namely the Reverend Father Inquisitor and the circumspect Secretary of the Senate, Giovanni Maraviglia, declared under oath that in the book entitled Sidereal Message, etc.* of Galileo Galilei there is nothing that is contrary to the Holy Catholic Faith, principles or good customs, and that it is worthy of being printed, authorize its publication in this city.
that contains and explains
recent observations
made with the aid of a new spyglass of the face of the Moon,
the Milky Way, the nebulous stars, and
an innumerable number of fixed ones, as well as in
four planets,
called cosmic stars*
that have never been seen before.
GREAT indeed are the things that this small book offers to the consideration and study of anyone interested in nature. I call them great because of the excellence of the subject matter and their absolute and unheard-of novelty, but also on account of the instrument whereby they became known to our senses.
It is surely a great thing to add countless stars to the large number that have already been observed with the naked eye, and to render them clearly visible when they had never been seen before, and are more than ten times as numerous as the old with which we are familiar.*
It is a most beautiful and a very pleasing sight to look at the body of the Moon, which is removed from us by almost sixty terrestrial diameters,* and to see it as if it were only two diameters away.* This means that the diameter of the Moon looks almost thirty times larger, its surface nine hundred times bigger, and its whole body close to twenty-seven thousand times more voluminous than when seen with the naked eye.* The observational evidence is so compelling that anyone can grasp for himself that the Moon’s surface is not smooth and polished but rough and uneven. Like the face of the Earth, it is covered all over with huge bumps, deep holes, and chasms.
Furthermore, it is no small matter to have put an end to disputes about the Galaxy, namely the Milky Way, and to have made its nature clear to the senses, let alone the understanding. It is also a fine and a pleasant thing to be able to point out, as with one’s finger, the nature of those stars that all astronomers have hitherto called nebulous, and to show that it is very different from what was believed until now.
But what is even more admirable, and what we mainly want to let astronomers and philosophers know, is that we have found four wandering stars* that no one before us had heard about or observed, and that these revolve around one of the conspicuous planets. Like Venus and Mercury, which go around the Sun, they have their own periods of revolution so that they sometimes precede, sometimes follow their planet but in such a way that they never stray beyond certain limits.* All this was found and observed a few days ago with a spyglass that I devised after having been enlightened by divine grace.
Other and perhaps greater things will be discovered in the days to come by me or by others with the aid of a similar instrument, so before giving an account of my observations I will briefly say something about its shape and its construction, as well as how I came to think of it.
About ten months ago* the rumour reached us that a Dutchman had made a spyglass by the aid of which visible objects, although at a great distance from the eye of the observer, were seen distinctly as if near. News of this truly wonderful result spread, and if it was believed by some, it was denied by others. But confirmation arrived a few days later in a letter written from Paris by a noble Frenchman, Jacques Badouère,* and this is what made me concentrate all my energy on finding how this was achieved and by what means I could make a similar instrument. Basing myself on the theory of refraction, I achieved my goal in little time. I first got a tube of lead, and fitted the ends with two lenses that were both plane on one side but, on the other side, convex in one case, and concave in the other. I then applied my eye to the concave lens, and saw objects as fairly large and near at hand. They appeared to be three times closer and nine times larger than when seen with the naked eye alone. I then constructed a better instrument that made objects appear sixty times bigger.* Sparing neither time nor expense, I managed to construct an instrument that was so good that objects seen through it appeared a thousand times bigger, and more that thirty times closer than when viewed by the natural power of sight alone. It would be superfluous to list the number and the importance of the advantages of this instrument on land and at sea.
But leaving terrestrial observation, I turned to the study of the heavens and saw, first of all, the Moon as near as if it were hardly two terrestrial diameters away.* Next I repeatedly observed, with the greatest pleasure, the fixed and the wandering stars. When I realized their huge number, I began to ponder how to measure the intervals between them, and I eventually discovered a way of doing so. It is fitting to warn those who may want to undertake this kind of observation that they must, in the first place, acquire an excellent spyglass that shows objects clearly, distinctly, and free from any haziness. It should magnify at least four hundred times so that the objects appear twenty times closer. Without such an instrument, everything we saw in the heavens and that we list below will be sought for in vain.
In order to determine without great trouble the magnifying power of the instrument, trace on paper the outline of two circles or two squares such that one is four hundred times as large as the other, as will be the case when its diameter is twenty times that of the other. Then, having attached these two figures to the same wall, observe them both simultaneously from a distance, looking at the smaller one through the spyglass, and at the larger with the other, naked eye. This can easily be done by keeping both eyes opened at the same time. The two figures will appear to be of the same size if the instrument magnifies the objects in the said ratio. With such an instrument, we can determine distances, and we proceed in the following way. In the interest of clarity, let ABCD be the tube, and E the eye of the observer. When there are no lenses in the tube, the rays are carried to the object FG along the straight lines ECF and EDG,* but when the lenses are inserted they are carried along the refracted lines ECH and EDI. The rays, which before were directed without constraint towards FG, are now squeezed together and only include the part HI. Next we determine the ratio of the distance EH to the line HI and, with the help of a table of sines, we find that the size of the angle subtended at the eye by object HI is only a few minutes of an arc.* Now if we perforate thin sheets of metal, some with larger and others with smaller holes, and place over the lens CD one size or another, as need may be, we can obtain any number of angles subtending a few minutes of an arc. By this means we shall be able to conveniently measure the intervals between stars a few minutes apart within an error of one or two minutes.
Let it suffice for the present, however, to have touched upon this rapidly and to have given a foretaste of what is to come, for on some further occasion we shall provide a complete theory of this instrument.* But let us now review the observations that we made over the last two months, and here I call upon all those who are eager for true philosophy to witness the beginnings of important considerations.
Let us first consider the face that the Moon turns towards us and, to make things easier, I distinguish two parts, the brighter and the darker one. The brighter seems to surround and illuminate the whole hemisphere of the Moon whereas the darker one, like a cloud, spreads over the face of the Moon and makes it appear covered with spots that are somewhat dark and of considerable size. These have always been observed and are obvious to everyone. We shall call them great or ancient spots to distinguish them from others of smaller size that are so thickly scattered that they cover the whole surface of the Moon but mainly the brighter part. These were never observed by anyone before us. After examining them repeatedly we were led to a conclusion about which we are certain. The surface of the Moon is not even, smooth, and perfectly spherical, as the majority of philosophers have conjectured that it and the other celestial bodies are, but on the contrary, rough and uneven, and covered with cavities and protuberances just like the face of the Earth, which is rendered diverse by lofty mountains and deep valleys. The appearances that enabled me to reach this conclusion are the following.
On the fourth or the fifth day after new moon, when the Moon presents itself with bright horns, the boundary-line that separates the darker from the brighter side does not follow a regular oval line, as would be the case on a perfectly spherical solid, but traces out an uneven, rough, and altogether sinuous line, as the figure below shows. Several bright excrescences, as it were, extend beyond the boundary of light and darkness and penetrate into the darker part, while on the other hand, patches from the dark side enter the brighter one. Indeed, a great number of small blackish spots, completely separated from the dark part, are scattered over most of the area that is already flooded by sunlight, with the exception of the part occupied by the great and ancient spots. We note furthermore that the small spots just mentioned always have this feature in common, that their darker side faces the Sun while on the side opposite the Sun their contours are brighter, as if they were crowned with shining peaks. We see exactly the same thing on Earth at sunrise when the sunlight has not yet spread over the valleys although the mountains surrounding them on the side away from the Sun are already shining brightly. And just as shadows in hollows on Earth decrease in size as the Sun rises higher, so these lunar spots shed their darkness as their illuminated parts grow larger.
But not only is the boundary of light and shadow on the Moon seen to be uneven and sinuous, what causes even greater astonishment is that very bright points appear inside the darker portion of the Moon. They are divided and separated from the illuminated part, and removed from it by a considerable distance. After some time, they gradually increase in size and brightness until, after two or three hours, they become joined with the rest of the bright portion, which has now increased in size. In the meantime, more and more bright points light up inside the dark portion, swell in size, and eventually embrace the brighter surface that has extended still further. This is illustrated in the same figure.
On Earth, before sunrise, are not the peaks of the highest mountains illuminated by the Sun’s rays while the plains are still in shadow? In a little while, does not the light spread further until the middle and larger parts of these mountains become illuminated and, in the end, when the Sun has risen, the illuminated parts of the plains and the hills are joined? On the Moon, however, the difference between high peaks and depressions appears to be much greater than the one caused by ruggedness on the surface of the Earth, as we shall show below. In the meantime, I cannot pass over in silence something worthy of consideration that I observed when the Moon was hasting towards first-quarter, as can be seen in the same figure above. Near the lower cusp, a great dark gulf extends into the illuminated side. I observed it for a while and saw that it was dark throughout, but after a couple of hours a bright peak began to emerge a little below the centre of the depression. It gradually grew in size and assumed a triangular shape that was still completely detached and separated from the illuminated area. Around it three other small points soon began to shine until, when the Moon was just about to set, this triangular shape, which had become more extended and larger, joined the rest of the illuminated part and penetrated into the dark gulf like a vast promontory, still surrounded by the three bright peaks that we have just mentioned. At the ends of the top and bottom cusps some bright points emerged, completely separated from the rest of the illuminated part, as can be seen in the same figure.
There was also a great number of dark spots in both cusps, but mainly in the lower one. Those nearer the boundary of light and shadow appeared larger and darker while those further removed were fainter and not so dark. But as we mentioned above, the dark portion of each spot was always turned towards the incoming rays of the Sun while the bright rim that surrounded it on the other side always faced the dark region of the Moon. This part of the lunar surface, which is spotted as a peacock’s tail is decked with azure eyes, resembles glass vases that are plunged while still hot into cold water and acquire that crackled and wavy surface from which they receive the common name of frosted glass. But the great spots on the Moon do not appear to be cracked or crowded with depressions and prominences in this manner, but rather to be even and uniform for only here and there do some bright patches emerge.
So if someone wanted to revive the ancient Pythagorean theory, namely that the Moon is like another Earth, its land surface would be more fittingly represented by the brighter region, and the expanse of water by the darker one. I have never doubted that if the terrestrial globe were observed from afar, bathed in sunlight, the land surface would appear brighter and the expanse of water darker. Furthermore, whether the Moon is waxing or waning, the great spots on the Moon appear to be more depressed than the brighter tracts that appear here and there in the vicinity of the great spots, and always along the boundary of light and shadow, as we noticed when drawing the figures. The edges of the large spots are not only lower but more even, and free from creases and ruggedness. The bright part stands out particularly near the spots and, before first quarter and approaching last quarter, huge stretches arise above and below a certain spot in the higher and northerly region of the Moon,* as can be seen in the figures reproduced here below.
Before the last quarter this same spot is seen to be surrounded by darker contours that, like the highest ridges of mountains, appear darker on the side that is facing away from the Sun, and brighter on the side that is turned towards the Sun. Now just the opposite happens in the case of the cavities where the side that is facing away from the Sun appears brilliant, while the side that is turned towards the Sun appears dark and shadowy. When the illuminated portion of the Moon has decreased in size, and the spot we have mentioned is nearly all covered in darkness, the brighter mountain ridges climb above these shadows. This twofold appearance of the spot is illustrated in the following figures:
There is another thing that I must not omit, because I found it very striking: near the centre of the Moon there is a cavity that is larger than all the other ones and perfectly round in shape. I observed it near both the first and the last quarter, and I have portrayed it as well as possible in the second of the two figures above. As to light and shade, this cavity offers the same appearance as would a region like Bohemia, if it were enclosed on all sides by very high mountains arranged along the circumference of a perfect circle. For on the Moon this area is walled in by such high peaks that the side adjacent to the dark portion of the Moon is seen to be bathed in sunlight before the boundary between light and shadow reaches halfway across the cavity. Just like other spots, its shaded portion faces the Sun while its lighted part faces the dark region of the Moon. It gives me pleasure, for the third time, to draw attention to this very cogent argument that ruggedness and unevenness are spread over the entire brighter region of the Moon. Of these spots, moreover, the darker ones are always next to the boundary between light and shadow, and those further away appear smaller in size and less dark, so that when the Moon, at its opposition to the Sun, becomes full there remains only a slight and faint difference between the darkness of the cavities and the brightness of the peaks.
The things we have enumerated were observed in the brighter parts of the Moon. In the great spots we see no such differences of depressions and heights as those that we are compelled to recognize in the brighter regions on account of the changes in their shape that result from the different ways they are illuminated by rays of the Sun that arrive from a variety of angles. Inside the great spots we also find small zones that are less dark, as we have indicated in the illustrations, but they always have the same appearance, and their darkness neither increases nor decreases, although they sometimes appear a little darker or a little brighter according as the rays of the Sun fall upon them more or less obliquely. Furthermore, they are joined to their neighbouring parts by a very gradual connection so that their boundaries run together and blend. But matters are quite different in the case of the spots that occupy the brighter part of the Moon’s surface.
The sharp contrast of light and shadow gives them well defined boundaries as if they were steep walls covered with jagged and projecting rocks. Moreover inside these great spots certain small zones are observed to be brighter than the surrounding region, and some are very bright indeed. But their appearance, as well as that of the darker parts, is always the same because there is never any change in their shape, brightness, or darkness. Hence, it is proved beyond doubt that their appearance results from a real dissimilarity of parts and not merely from the different ways the rays of the Sun alter their shape or the size of their shadow, as happens for other smaller spots found in the brighter part of the Moon. These change from day to day, grow, decrease, and vanish because they are only produced by the shadows that are cast by the peaks.
Nevertheless, I am told that many have serious reservations on this point and are so concerned that they feel compelled to doubt a conclusion already explained and confirmed by so many observations. If that part of the lunar surface that reflects the sunlight more brightly is full of chasms, that is, countless bumps and depressions, why is it that the western edge of the waxing Moon, the eastern edge of the waning Moon, and the whole periphery of the full Moon are not seen to be uneven, rough, and sinuous? On the contrary, they appear as precisely round as if they had been drawn with a compass, and undamaged by bumps or cavities. The more so as the whole border is made of the brighter lunar material which, as we have said, is full of bumps and holes. None of the great spots extend as far as the circumference, but are always seen gathered together far from the rim.
Of this fact, which has given rise to such serious doubt, let me propose a twofold explanation and, thereby, a twofold solution. First, if the bumps and the hollows on the lunar body extended only along the circumference at the very end of the hemisphere that we see, then the Moon could—indeed, should—show itself somewhat as a toothed wheel, terminated by a bumpy and sinuous edge. But if there is not only one range of peaks, situated just along the circumference, but many ranges of mountains with their hollows and canyons disposed in ranks near the Moon’s edge, and if these are not only on the hemisphere that is visible, but also on the other side (close however to the boundary between the hemispheres), then, from afar, the eye will not be able to distinguish the peaks from the depressions because the intervals between the mountains lying along one circle, namely along the same range, will be hidden by the interposition of yet other peaks on yet other ranges. This will be especially true if the observer is placed along the same straight line as the summits of the peaks. This happens on Earth when the summits of several mountains are so close together that they appear to lie on the same plane surface for an observer located at some distance and at the same altitude. Likewise, when the sea is rough the crests of the waves appear to lie along the same plane, although between the billows there are chasms and hollows of such depth that they hide not only the hulls but also the sterns, the masts, and the rigging of tall ships. Now because there are intricate arrangements of prominences and cavities on the Moon and along its circumference, and because the eye, regarding them from afar, lies in nearly the same plane as their summits, it should come as no surprise that they present themselves to the visual ray that grazes them as a regular and unbroken line.*
To this explanation another may be added, namely that there is around the body of the Moon, just as around the Earth, a substance denser than the rest of the aether* and enclosed in an orb that can receive and reflect the Sun’s rays, but without being so opaque as to prevent seeing through it (especially when it is not illuminated). The orb, when illuminated by the rays of the Sun, makes the body of the Moon appear as a larger sphere than it is, and, were it thicker, it would stop our sight from penetrating to the actual body of the Moon. But it is indeed thicker near the rim of the Moon. I do not mean thicker in an absolute sense but relative to our visual rays that cut it obliquely. Therefore, it can obstruct our vision and, especially when illuminated, conceal the circumference of the Moon that is exposed to the Sun. This can be understood more clearly in the figure below in which the body of the Moon, ABC, is surrounded by a vaporous orb, DEG:
Our vision, coming from F, penetrates to the central region of the Moon, such as at A, through the lesser thickness of vapours, DA, whereas towards the extreme edge a mass of thicker vapours, EB, shuts out its boundary from our sight. Evidence for this is that the illuminated part of the Moon appears to have a larger rim than the rest of the orb that lies in shadow. It can reasonably be conjectured that this is also the cause why the great spots on the Moon are not seen to reach the edge of the circumference on any side, although it can be supposed that some should be found there. If they are not visible, it is probably because they are hidden under a deeper and brighter vaporous orb.
That the bright surface of the Moon is dotted all over with bumps and cavities has been made sufficiently clear, I think, by the account of our observations. It now remains to say something about their size, and to demonstrate that terrestrial ‘prominences’ are much smaller, I mean smaller in absolute terms, not only relative to the size of their respective globes. This is plainly shown as follows.
I have often observed, in various positions of the Moon in reference to the Sun, that some summits in the darker part of the Moon, although fairly removed from the boundary of light, are illuminated. Comparing their distance with the whole diameter of the Moon, I found that it was sometimes greater than one-twentieth of the diameter. Assuming this to be correct, let us represent the lunar globe by the great circle, CAF, with E as its centre:
The diameter, CF, is to the diameter of the Earth as two is to seven, and since the diameter of the Earth, according to the most careful observations, contains 7,000 Italian miles,* CF will be 2,000, and CE 1,000, and the one-twentieth part of the whole CF, 100 miles. Now let CF be the diameter of the great circle that divides the brighter part from the darker one (for owing to the great distance of the Sun from the Moon this circle does not differ appreciably from a great one). Let A be separated from point C by one-twentieth of this diameter. Extend the radius EA to meet at point D the tangent GCD (representing the illuminating ray). Then arc CA, or rather the straight line CD, will consist of 100 such units as CE contains 1,000, and the sum of the squares of DC and CE will be 1,010,000.* The square of DE is equal to this. Therefore the whole of ED will be more than 1,004, and AD will be more than 4 such units of which CE contains 1,000. Therefore, the height of AD, which on the Moon indicates a summit reaching up to the ray of the Sun GCD, is removed from C by the distance CD, and is more than 4 Italian miles. Now on the Earth there are no mountains reaching a perpendicular height of even 1 mile high.* It is clear therefore that mountains on the Moon are higher than on Earth.
I am pleased to assign here the cause of another lunar appearance that is worthy of notice, although it is not a recent observation but one that I made many years ago, and that I had pointed out to a few close friends and students to whom I explained it and provided its cause. But because it can be more easily and more clearly observed with the aid of the spyglass, I do not believe it unsuitable to mention it here, mainly in order to render more apparent the kinship and the similarity between the Moon and the Earth.
When the Moon, just before or after new moon, is found not far from the Sun, we see not only the side where it is adorned with shining horns, but also a slight and faint circumference that marks out the circle of the part in shadow, namely the one that faces away from the Sun, and separates it from the darker background of the aether. If we examine the matter more closely, we shall see that not only does the extreme edge of the part in shadow shine with a dim light but that the entire face of the Moon, I mean that part that does not yet feel the Sun’s rays, is whitened with a not-inconsiderable glow. At first glance, only a thin luminous circumference is noticed on account of the adjacent dark background of the sky, whereas the rest of the surface appears darker because of the contiguity of the shining horns that obscure our vision. But if someone places himself where a roof, a chimney, or some other object (it must not be too close to the eye) conceals the shining horns while leaving the rest of the lunar globe exposed to view, then he shall see that this tract of the Moon, although deprived of sunlight, glows with a not-inconsiderable light. The more so when the gloom of the night has deepened after the departure of the Sun, since the same light appears brighter against a darker background. It is also the case that this secondary light (so to speak) of the Moon is greater the closer the Moon is to the Sun. It decreases gradually as the Moon moves away from the Sun, so that after the first quarter, and before the second, it is weak and very dim even if observed in a darker sky, whereas at an angular distance of 60 degrees or less it is remarkably bright, even in twilight, so bright indeed, that with the help of the good spyglass the great spots can be distinguished in it.
This wonderful brightness has made philosophers marvel, and they have offered various explanations for it. Some have said that it is the inherent, natural brightness of the Moon, some that it is imparted by Venus, others by all the stars, and others still by the Sun whose rays permeate through the solid body of the Moon. Now these opinions are easily refuted and shown to be false. For if this light were the Moon’s own or if it were conferred by the stars, it would assuredly be retained during eclipses when it would show itself because it would be left alone in a particularly dark sky. But this goes against experience: the brightness that is seen on the Moon during eclipses is much less, being somewhat reddish and almost copper-coloured, whereas this secondary light is brighter and whiter. Furthermore, the brightness seen during an eclipse is changeable and shifting, for it wanders over the face of the Moon so that the part nearest the circumference of the circular shadow cast by the Earth is brighter, whereas the rest of the Moon is always seen to be dark. Without doubt, this brightness is due to the way the rays of the Sun just graze some denser region that surrounds the Moon. Through this contact, a kind of dawn is spread over the neighbouring regions of the Moon just as twilight spreads in the morning and in the evening on Earth. But I shall deal with this more fully in my book on The System of the World.*
The suggestion that this kind of light is imparted to the Moon by Venus is so childish that it doesn’t deserve an answer,* for who is so ignorant as not to understand that from the time of the new moon to a separation of 60 degrees between Moon and Sun, no part of the Moon that is turned away from the Sun can possibly be seen from Venus? And it is equally untenable that it should come from the Sun penetrating and permeating with its light the solid body of the Moon,* for then it would never lessen since one hemisphere of the Moon is always illuminated by the Sun except at the time of lunar eclipses. But the light does decrease as the Moon hastens towards the first quarter and becomes completely dull when it has passed it. Since, therefore, this secondary light is not inherent and the Moon’s own, and is not borrowed from any of the stars or from the Sun, and since there remains in this vast world no other body than the Earth, what, I pray, are we to think? What can we suggest? Can it be said that the Moon, just as would any other opaque and dark body, is flooded with light coming from the Earth? What is marvellous about that? Indeed, the Earth, in fair and grateful exchange, gives back to the Moon an illumination like the one that it receives from her during nearly the whole time in the deepest gloom of the night.
Let us explain the matter more fully. When the Moon is between the Earth and the Sun, as happens at new moon, the rays of the Sun illuminate the side of its hemisphere that is turned away from the Earth. The other side of the hemisphere that faces the Earth is cloaked in darkness, and so the Moon does not illuminate the surface of the Earth at all. But as the Moon moves slowly away from the Sun, the side of its hemisphere that faces us gradually grows brighter, and it turns towards us a slender silvery crescent that slightly illuminates the Earth. The Sun’s illumination increases as the Moon approaches her first quarter, and the reflection of its light increases on the Earth. As the illumination on the Moon spreads beyond the first quarter, our nights become brighter. Finally, the whole face of the Moon, on the side that is turned towards us, is bathed in the bright rays of the Sun, which is on the opposite side of the Earth. The surface of the Earth that is covered with moonlight shines forth in all directions. But as the Moon wanes it sends us weaker rays, and the Earth is more faintly illuminated. By the time of new moon, night on Earth is completely dark.
There is thus a monthly period during which the Moon distributes its light, which is alternately brighter and weaker. But the Earth pays the Moon back with an equal benefit, for when the Moon is between the Earth and the Sun it faces the whole surface of the Earth that is exposed to the Sun and is entirely bathed in its vivid rays, which are reflected onto the Moon. And so it happens that the hemisphere of the Moon that faces the Earth, although deprived of sunlight, shines brightly. The same Moon, when it is 90 degrees from the Sun, sees only half of the terrestrial sphere illuminated, namely the western half, for the other, the eastern, is in darkness. Therefore, the Moon is less brightly illuminated by the Earth and, accordingly, its secondary light appears fainter. But if you imagine the Moon when it is on the opposite side of the Sun, it will be facing the hemisphere of the Earth, which is now between itself and the Sun, and will see it as completely dark and steeped in the gloom of night. If this happens when the Moon is in the plane of the ecliptic,* it will receive no light at all, and will be entirely deprived of both solar and terrestrial rays. In its various positions with respect to the Earth and the Sun, the Moon receives more or less light than from the reflection that comes from the Earth according to the greater or smaller portion of the illuminated terrestrial hemisphere that it faces. There is reciprocity between these two globes such that whenever the Earth is more brightly illuminated by the Moon, the Moon is less brightly illuminated by the Earth, and vice versa.
Let these few words suffice here. The matter will be considered more fully* in our System of the World, where it will be shown by means of numerous arguments and experiments that the reflection of sunlight from the Earth is indeed very strong. This for the benefit of those who claim that the Earth must be removed from the round of stars,* chiefly for the reason that it has neither motion nor light. We shall demonstrate that it is in motion, that it surpasses the Moon in brightness,* and that it is not the bilge where the rubbish and the refuse of the world have settled down. Furthermore, we shall confirm this with a thousand physical arguments.*
Hitherto, we have spoken of the observations of the lunar body; now we must briefly mention what we have thus far been able to discern about the fixed stars. First of all, it is worth mentioning that when the stars, whether fixed or erratic, are viewed with a spyglass they are by no means magnified in the same ratio as other objects, including the Moon, are enlarged. The increase in the fixed stars is considerably less. For instance, a spyglass that is powerful enough to magnify other objects a hundred times will hardly, I believe, make stars four or five times greater.* The reason for this is that when the stars are looked at with the naked eye they do not present themselves, so to say, in their bare and real size but sparkling all over and fringed with shining rays, especially when night is far advanced. This is why they appear much larger than they would if they were shorn of these adventitious rays, for the angle that they subtend at the eye is determined not by the primary disc of the star but by the brightness that surrounds it.*
This can easily be understood from the fact that when the stars rise at sunset, as twilight is settling in, they appear very small even if they are of the first magnitude, and Venus, if it happens to be visible at midday, appears so small that it hardly seems the size of a star of the last magnitude. Things are different with other objects, including the Moon, that always appears of the same size, whether it is seen in broad daylight or in the depth of night. So the stars are seen unshorn (of their rays) in the dark, but daylight can cut their fringes off, and not only daylight but any slight cloud that passes between them and the eye of the observer. When a dark veil or a piece of coloured glass is placed between the eye and the star, the blaze that surrounds also disappears. The spyglass does the same, for it first removes from the stars their adventitious and accidental splendour before it enlarges their true disc (if indeed they are of that shape), and so they are seen less magnified: a star of the fifth or even the sixth magnitude that is observed through a spyglass is only shown as if it were of the first magnitude.
The difference between the appearance of the planets and that of the fixed stars deserves to be noted. The planets present their discs as perfectly round, just as if they had been traced with a pair of compasses, and they appear as so many little moons, completely illuminated. But the fixed stars never appear bounded by a circular rim, but by blazing light that sparkles all over. Observed through a spyglass, the stars have the same figure as when viewed with the naked eye, but so much larger that a star of the fifth or sixth magnitude seems to equal the Dog Star,* the largest of all fixed stars.
Beyond the stars of the sixth magnitude you will observe through the telescope a host of other stars that escape our natural faculty of sight. They are so numerous as to be almost beyond belief, for you can distinguish more than six new orders of magnitude.* The largest of these, which we may call of the seventh magnitude, or of the first magnitude of invisible stars, appear with the aid of a spyglass larger and brighter than the stars of the second magnitude that are seen with the naked eye. In order that you may see a couple of instances of their inconceivable number, I have described two star-clusters so that from these examples you may decide about the rest. In my first example, I intended to depict the entire constellation of Orion, but I was overwhelmed by the enormous number of stars and the lack of time, and have postponed this until some other occasion. Within the limits of one or two degrees, more than five hundred* are scattered among the old stars. For this reason, I have selected the three stars in the belt of Orion and the six in the sword, which were already known, and I have added in their vicinity another eighty that I recently observed. I have preserved as precisely as possible the intervals between them. In order to distinguish them, I drew the well-known old stars of larger size, and I outlined them with a double line. The others, the invisible ones, I made smaller with one line only. I also preserved the differences of magnitudes as much as possible. As a second example, I depicted the six stars of the constellation of Taurus, called the Pleiades (I say six since the seventh is hardly ever visible).* These are enclosed within very narrow precincts in the heaven, and near them lie more than forty other invisible ones, not a single of which is much more than half a degree from one of the aforementioned six. Of these I have indicated only thirty-six, and I preserved their intervals, their magnitudes, and the distinction between the old and the new stars, as I did for those in Orion.
Asterism of the belt and sword of Orion
Constellation of the Pleiades
What we observed in the third place is the essence, namely the matter, of the Milky Way,* which can be seen so clearly with the aid of the telescope that what philosophers for centuries found an excruciating problem has been solved with ocular certainty, thus freeing us from wordy disputes. For the Galaxy* is nothing else but a collection of innumerable stars heaped together. In whatever part of the Milky Way you point the spyglass, a vast crowd of stars immediately present themselves. Several appear quite large and very bright, but the number of small ones can in no way be determined. This milky brightness, which resembles a whitish cloud, is not only to be seen in the Milky Way, for several patches of like colour are disseminated throughout the aether. If you turn the spyglass upon any of them you encounter a cluster of closely packed stars.
Furthermore (what is even more admirable) the stars that have been called by all the astronomers up to this day nebulous* are really groups of small stars herded together in a wonderful way. Although, on account of their small size or their great distance, each one of them escapes our sight, the commingling of their rays gives rise to that brightness that has hitherto been believed to be a denser part of the heavens that can reflect the rays of the stars or the Sun. We have observed some of these, and below we give the diagrams of a couple of such star-clusters.
In the first, you find the nebula of Orion’s Head, as it is called, and in which we counted twenty-one stars. The second contains the nebula known as Praesepe, which is not one star only but a mass of more than forty starlets. Besides the Aselli,* I have indicated thirty-six in the order in which they are arranged:
Nebula of Orion
Nebula of Praesepe
I have briefly recounted what I observed in the Moon, the fixed stars, and the Milky Way. There remains what deserves to be considered the most important of all, namely the disclosure of four planets that were never seen from the creation of the world up to our own times, and to declare how they were found and observed, what are their positions, and what observations I made during almost two months concerning their motion and their changes. I call upon all astronomers to devote themselves to the study and the determination of their periods, which so far I have not been able to ascertain because of lack of time. I warn them again, however, in order that they may not undertake such an inquiry in vain, that they will need a very sharp spyglass of the kind I described at the beginning of this account.
On the 7th of January of the present year 1610, at the first hour of night,* when I was observing the celestial bodies with the spyglass, Jupiter came forward. As I had just made an excellent instrument for myself, I saw (what I had not done before because of the weakness of the former instrument) three little stars,* small but very bright. Although I assumed that they belonged to the number of fixed stars, they caused me no little surprise because they appeared to lie exactly on a straight line, parallel to the ecliptic,* and brighter than other stars of the same magnitude. Their arrangement with respect to Jupiter and among themselves was as follows:
There were two stars on the eastern side, but just one on the western. The furthest to the east and the western one seemed slightly larger than the third. I paid no attention to the distance between them and Jupiter for, as I have said, I thought at first that they were fixed stars. When on the 8th of January, led by I know not what Fate, I turned to look at them again, I found a very different arrangement. There were now three small stars to the west of Jupiter, closer to each other than the night before, and separated by equal intervals as shown in the diagram below. At the time I gave no thought as to how these small stars could have come together, but I began to wonder how Jupiter could be found to the east of all those fixed stars when it had been to the west of two of them the day before.
I therefore asked myself whether, contrary to the computations of astronomers, Jupiter might not be moving eastward at this time and had passed in front of the stars by its own proper motion. I eagerly awaited the next night, but my hopes were dashed: the sky was everywhere covered with clouds.
On the 10th of January the stars appeared in the following position with regard to Jupiter: there were only two and both were to the east:
The third one, I conjectured, was hiding behind Jupiter. They were, as before, in the same straight line with Jupiter, and located exactly along the line of the Zodiac. When I saw this, and knowing that such changes of position could in no way be ascribed to Jupiter, and moreover, having had to recognize that the stars that I saw had always been the same (there were no other stars within a great distance before or after on the line of the Zodiac), I realized that the changes of position were not due to Jupiter but to the stars that had been observed. My perplexity gave way to amazement, and I therefore resolved to observe them with greater care and attention from then on.
On the 11th of January I saw the following arrangement:
There were two stars to the east, and the one in the middle was three times as far from Jupiter as it was from the star further east. This last star was almost twice the size of the other whereas, on the previous night, they had appeared about equal in size. I now decided that without the shadow of a doubt there were in the heavens three stars wandering about Jupiter, just as Venus and Mercury go around the Sun. This became clearer than daylight when I later made numerous other observations. Nor were there only three, but four wandering stars revolving around Jupiter. The account that follows will describe the changes of position that I subsequently observed with greater accuracy. I also measured the intervals between them with the spyglass in the manner already explained. Besides this, I have also given the time of the observations, especially when several were made in the same night, for the revolutions of those planets are so swift that it is generally possible to note their hourly changes.
On the 12th of January, at the first hour of night, I saw the stars disposed in this manner:
The star further to the east was bigger than the westerly one, but both were very conspicuous and bright. The distance of each one from Jupiter was two minutes. A third small star, not visible before, began to appear at the third hour. It almost touched Jupiter on the eastern side and was exceedingly small. They were all on a straight line along the ecliptic.
On the 13th of January, for the first time, I saw four small stars in this arrangement with regard to Jupiter:
Three were to the west, and one to the east. They made a line that was almost straight, but the one in the middle of those that were to the west deviated a little from the straight line towards the north. The farthest to the west was at a distance of two minutes from Jupiter. The interval between Jupiter and the closest star, and between the stars themselves, was only one minute. All the stars appeared of the same magnitude, and although small were nevertheless very brilliant and far outshone the fixed stars of the same magnitude.
On the 14th of January the sky was overcast.
On the 15th of January,* close to the third hour, the four stars occupied, with respect to Jupiter, the positions depicted below:
All were to the west and situated on a nearly straight line, but the third from Jupiter was raised a little to the north. The nearest to Jupiter was the smallest of all, and the following ones appeared successively larger. The interval between Jupiter and the first star and the intervals between the next two were all equal and each of two minutes, but the one furthest to the west was four minutes away from the one nearest to it. They were very brilliant and did not twinkle at all, and thus they always appeared either before or since. But by the seventh hour there were only three stars in the following configuration with respect to Jupiter:
They lay on a perfectly straight line, but the nearest to Jupiter was very small and at a distance of three minutes. The distance of the second star from this one was one minute, and the third was 4 minutes and 30 seconds from the second. After another hour, the two middle stars were even closer, hardly 30 seconds apart.
[The observations of the satellites of Jupiter that Galileo went on making until 2 March 1610 are omitted.]
These relations of Jupiter and its adjacent planets to a fixed star were recorded in order that anyone may grasp that the progress of these planets, both in longitude and latitude, agree exactly with the movements of Jupiter that are calculated from tables.
These are the observations of the Medicean planets recently, and for the first time, discovered by me. Although it has not yet been possible, from these observations, to determine their periods numerically, some things are worthy of mention. And, in the first place, since they sometimes follow and sometimes precede Jupiter by the same interval, and since they remain within very narrow limits to the east or to the west of Jupiter, and since they accompany it whether its motion is retrograde or direct, it is beyond doubt that they make their revolutions around him while at the same time completing, together with him, twelve-year periods around the centre of the world. Moreover, they revolve in circles of unequal size, as is easy to deduce from the fact that two planets are never seen in conjunction at their greatest elongation from Jupiter, although two, three, or indeed all of them have been seen bunched together close to Jupiter.
Furthermore, it has been found that the planets that describe the smallest circles around Jupiter are faster, for those closest to Jupiter are often to be seen to the east when on the day before they had appeared in the west, and vice versa, whereas the planet that traces out the largest orbit appears, on careful examination of its returns, to have a semi-monthly period. Furthermore, we have a particularly strong argument to remove the scruples of those who are willing to examine dispassionately the revolution of the planets about the Sun in the Copernican system, yet are so troubled by the fact that our one and only Moon should go around the Earth while at the same time both carry out an annual revolution around the Sun, that they consider that this theory about the constitution of the universe should be rejected as impossible. But now we have not only one planet revolving about another one, while both trace out an annual circle around the Sun, but our own eyes show us four stars travelling around Jupiter as the Moon travels around the Earth, while, at the same time, they make a grand revolution around the Sun.
Finally, we should not omit the reason why the Medicean stars, in making their very small revolutions around Jupiter, seem sometimes to be more than doubled in size. We can by no means look for the explanation in terrestrial vapours, since Jupiter and the neighbouring fixed stars do change in size when this increase and decrease is taking place. It is utterly untenable that the cause of such a variation should be due to the way they approach and recede from the Earth at the perigee and the apogee of their revolutions, for the narrow circle along which they travel can in no way cause such an effect.* Furthermore, an oval motion (which in this case would be almost rectilinear) seems equally untenable, and would by no means agree with the appearances.* I gladly offer the explanation that occurs to me, and submit it to the appraisal and criticism of all true philosophers. It is well known that the interposition of terrestrial vapours makes the Sun and the Moon appear larger, but the fixed stars and the planets smaller. Thus, the great lights,* when near the horizon, are larger than at other times, whereas the stars appear smaller and often barely visible. They diminish even more if these vapours are flooded with light, and so the stars appear very small by day and in the twilight, whereas the Moon does not,* as I remarked above. Moreover, not only the Earth but the Moon also is surrounded by a vaporous sphere, as is obvious from what we said above, and especially from what will be declared more fully in my System.* We can reasonably say the same of the other planets, so that it is by no means unthinkable to put around Jupiter a sphere that is denser than the rest of the aether and around which the Medicean planets revolve, as the Moon goes around the sphere of elements.* The planets look smaller at apogee when this sphere is interposed, and when it is removed, at perigee, they look bigger. Want of time prevents me from going further into these matters, but the kind reader may expect more, soon.
