The Essential Galileo

by Galilei, Galileo, Finocchiaro, Maurice A.

  Now, since this kind of secondary brightness is not inherent in and characteristic of the moon, nor borrowed from the sun or any other heavenly body, and since there now remains in the whole universe no other body whatever except the earth, what, pray, must we think? What must we propose? Shall we propose that the body of the moon, or some other dark and opaque body, receives light from the earth? What is so strange about that? Look: the earth, with fair and grateful exchange, pays back to the moon an illumination like that which it receives from the moon nearly the whole time during the darkest gloom of night. Let me explain the matter more clearly.

  At conjunction, when the moon occupies a position between the sun and the earth, the moon is illuminated by the sun’s rays on her half towards the sun and turned away from the earth, and the other half facing the earth is covered with darkness and so does not illuminate the earth’s surface in any way. When the moon has separated slightly from the sun, straightaway she is partly illuminated on the half directed towards us; she turns towards us a slender silvery crescent and illuminates the earth slightly. The sun’s illumination increases upon the moon as she approaches her first quarter, and the reflection of that light increases on the earth. Next, the brightness of the moon extends beyond the semicircle, and our nights grow brighter. Then the entire face of the moon looking towards the earth is irradiated with the most intense brightness by the sun, which happens when the sun and moon are on opposite sides of the earth; then far and wide the surface of the earth shines with the flood of moonlight. After this the moon, now waning, sends out less powerful beams, and the earth is illuminated less powerfully. Finally, the moon draws near her first position of conjunction with the sun, and forthwith black night invades the earth. In such a cycle the moonlight gives us each month alternations of brighter and fainter illumination. But the benefit of her light to the earth is balanced and repaid by the benefit of the light of the earth to her; for while the moon is found near the sun about the time of conjunction, she has in front of her the entire surface of that hemisphere of the earth which is exposed to the sun and is vividly illuminated with his beams, and so she receives light reflected from the earth. Owing to such reflection, the hemisphere of the moon nearer to us, though deprived of sunlight, appears of considerable brightness. Again, when removed from the sun by a quadrant, the moon sees only one-half of the earth’s illuminated hemisphere (namely the western half), for the other (the eastern) is covered with the shades of night; the moon is then less brightly illuminated by the earth, and accordingly that secondary light appears fainter to us. But if you imagine the moon to be set on the opposite side of the earth to the sun, she will see the hemisphere of the earth, now between the moon and the sun, quite dark, and steeped in the gloom of night; if, therefore, an eclipse should accompany such a position of the moon, [75] she will receive no light at all, being deprived of the illumination of the sun and earth together. In any other position with regard to the earth and the sun, the moon receives more or less light by reflection from the earth, according as she sees a greater or smaller portion of the hemisphere of the earth illuminated by the sun; for such a law is observed between these two globes, that at whatever times the earth is most highly illuminated by the moon, at those times, on the contrary, the moon is least illuminated by the earth; and vice versa.

  Let these few words on this subject suffice in this place. I will consider it more fully in my System of the World. There, by very many arguments and experiments, it is shown that there is a very strong reflection of the sun’s light from the earth, for the benefit of those who urge that the earth must be excluded from the dance of the stars, chiefly for the reason that it has neither motion nor light. For I will prove that the earth has motion, and surpasses the moon in brightness, and is not the place where the dull refuse of the universe has settled down; and I will support these conclusions by countless arguments taken from natural phenomena.

  [§1.5] Hitherto I have spoken of the observations which I have made concerning the moon’s body. Now I will briefly announce the phenomena that have been, as yet, seen by me with reference to the fixed stars. And first of all the following fact is worthy of consideration.

  The stars, fixed as well as wandering, when seen with a spyglass, by no means appear to be increased in magnitude in the same proportion as other objects, and the moon herself, gain increase of size. In the case of the stars such increase appears much less, so that you may consider that a spyglass which (for the sake of illustration) is powerful enough to magnify other objects one hundred times will scarcely render the stars magnified four or five times. The reason for this is as follows. When stars are viewed with our natural eyesight, they do not present themselves to us in their bare, real size, but beaming with a certain vividness, and fringed with sparkling rays, especially when the night is far advanced; and from this circumstance they appear much larger than they would if they were stripped of those adventitious fringes, for the angle which they subtend at the eye is determined not by the primary disc of the star, but by the brightness that so widely surrounds it. Perhaps you will understand this most clearly from the well known circumstance that when stars rise at sunset, in the beginning of twilight they appear very small, although they may be stars of the first magnitude; and even the planet Venus itself, on any occasion when it may present itself to view in broad daylight, is so small to see that it scarcely seems equal to a star of the last magnitude. It is different in the case of other objects, and even of the moon, which, whether viewed in the light of midday or in the depth of night, always appears of the same size. We conclude, therefore, that [76] the stars are seen at midnight in uncurtailed glory, but their fringes are of such a nature that the daylight can cut them off, and not only daylight, but any slight cloud that may be interposed between a star and the eye of the observer. A dark veil or colored glass has the same effect, for upon placing it between the eye and the stars, all the blaze that surrounds them leaves them at once. A spyglass also accomplishes the same result, for it removes from the stars their adventitious and accidental splendors before it enlarges their true globes (if indeed they are of that shape), and so they seem less magnified than other objects; for example, a star of the fifth or sixth magnitude seen through a spyglass is shown as of the first magnitude.

  The difference between the appearance of the planets and of the fixed stars seems also deserving of notice. The planets present their bodies perfectly delineated and round, and appear as so many little moons, completely illuminated and of a globular shape. However, the fixed stars do not look to the naked eye bounded by a circular periphery, but rather like blazes of light, shooting out beams on all sides and very sparkling; and with a spyglass they appear of the same shape as when they are viewed by simply looking at them, but so much larger that a star of the fifth or sixth magnitude seems to equal the Dog Star, the largest of all the fixed stars.

  Beyond the stars of the sixth magnitude, you will behold through the spyglass a host of other stars that escape the unassisted eye, so numerous as to be almost beyond belief. You may see more than six other magnitudes, and the largest of these (which one could call stars of the seventh magnitude, or of the first magnitude of invisible stars) appear with the aid of the spyglass larger and brighter than stars of the second magnitude seen with the unassisted sight. In order that you may see one or two proofs of the inconceivable manner in which they are crowded together, I have wanted to give you drawings of two star clusters, so that from them as specimen you may decide about the rest.

  As my first example, I had determined to depict the entire constellation of Orion, but I was overwhelmed by the vast quantity of stars and by want of time, and so I have deferred attempting this to another occasion; for there are adjacent to, or scattered among, the old stars more than five hundred new stars within the limits of one or two degrees. For this reason I have selected the three stars in Orion’s Belt and the six in his Sword, which have long been well known groups, and I have added eighty other stars recently disc
overed in their vicinity, and I have preserved as exactly as possible the intervals between them. The well-known or old stars, for the sake of distinction, I have depicted of larger size, and I have outlined them with a double line; the others, invisible to the naked eye, I have marked smaller and with one line only. I have also preserved the differences in magnitude as much as I could.

  [78] As a second example, I have depicted the six stars of the constellation Taurus, called the Pleiades (I say six intentionally, since the seventh is scarcely ever visible). This is a group of stars that appear in the heavens within very narrow limits. Near these there lie more than forty others invisible to the naked eye, no one of which is much more than half a degree off any of the aforesaid six. In my diagram, I have marked only thirty-six stars; I have preserved their intervals, magnitudes, and the distinction between the old and the new stars, just as in the case of the constellation Orion.

  The third thing which I have observed is the essence or substance of the Milky Way. By the aid of a spyglass anyone may behold this in a manner which so distinctly appeals to the senses that all the disputes which have tormented philosophers through so many ages are exploded at once by the indubitable evidence of our eyes, and we are freed from wordy disputes upon this subject. In fact, the galaxy is nothing but a mass of innumerable stars planted together in clusters. For upon whatever part of it you direct the spyglass, straightaway a vast crowd of stars presents itself to view; many of them are tolerably large and extremely bright, but the number of small ones is quite beyond determination.

  That milky brightness, like the brightness of a white cloud, is seen not only in the Milky Way, but also in several spots of a similar color that shine here and there in the heavens. If you turn the spyglass upon any of them, you will find a cluster of stars packed close together. [79] Furthermore—and you will be more surprised at this—the stars that have been called by every one of the astronomers up to this day nebulous are groups of small stars set thick together in a wonderful way. Although each star escapes our sight on account of its smallness, or of its immense distance from us, from the commingling of their rays there arises that brightness which has hitherto been believed to be the denser part of the heavens, able to reflect the rays of the stars or the sun.

  I have observed some of these, and I wish to reproduce the star clusters of two of these nebulas. First you have a diagram of the nebula called Orion’s Head, in which I have counted twenty-one stars. The second cluster contains the nebula called Praesepe, which is not a single star but a mass of more than forty small stars; besides the two Aselli, I have marked thirty-six stars, arranged as in the following diagram.

  [§1.6] I have now finished my brief account of the observations which I have thus far made with regard to the moon, the fixed stars, and the galaxy. There remains the matter that seems to me to deserve to be considered the most important in this work. That is, I should disclose and publish to the world the occasion of discovering and observing four planets never seen from the beginning of the world up to our own times, their positions, and the observations made during [80] the last two months about their movements and their changes of magnitude. And I summon all astronomers to apply themselves to examine and determine their periodic times, which it has not been permitted me to achieve up to this day owing to the restriction of my time. However, I give them warning again that they will need a very accurate spyglass, such as I have described at the beginning of this account, so that they may not approach such an inquiry to no purpose.

  On the seventh day of January of the present year, 1610, at the first hour of the following night, when I was viewing the constellations of the heavens through a spyglass, the planet Jupiter presented itself to my view. As I had prepared for myself a very excellent instrument, I noticed a circumstance which I had never been able to notice before, owing to want of power in my other spyglass. That is, three little stars, small but very bright, were near the planet. Although I believed them to belong to the number of fixed stars, yet they made me wonder somewhat, because they seemed to be arranged exactly in a straight line parallel to the ecliptic, and to be brighter than the rest of the stars equal to them in magnitude. Their position with reference to one another and to Jupiter was as follows:

  On the east side there were two stars, and a single one towards the west. The star that was furthest towards the east, and the western star, appeared rather larger than the third. I scarcely troubled at all about the distance between them and Jupiter, for, as I have already said, at first I believed them to be fixed stars.

  However, when on 8 January, led by some fatality, I turned again to look at the same part of the heavens, I found a very different state of things. There were three little stars all west of Jupiter, and nearer together than on the previous night; and they were separated from one another by equal intervals as the following illustration shows:

  At this point, although I gave no thought at all to the fact that the stars appeared closer to one another, yet I began to wonder how Jupiter could one day be found to the east of all the aforesaid fixed stars when the day before it had been west of two of them. And forthwith I wondered whether the planet might have been moving with direct motion, contrary to the calculation of astronomers, and so might have passed those stars by its own proper motion. I therefore waited for the next night with the most intense longing, but I was disappointed in my hope, for the sky was covered with clouds in every direction.

  [81] But on 10 January the stars appeared in the following position with regard to Jupiter:

  There were only two, and both on the east side of Jupiter, the third, as I thought, being hidden by the planet. They were situated, just as before, exactly in the same straight line with Jupiter, and along the zodiac. After seeing this, I understood that the corresponding changes of position could not by any means belong to Jupiter. Moreover, I knew that the stars I saw had always been the same, for there were no others either in front or behind, within a great distance along the zodiac. Finally, changing from perplexity to amazement, I became certain that the observed interchange of position was due not to Jupiter but to the said stars. Thus, I thought that henceforth they ought to be observed with more attention and precision.

  Accordingly, on 11 January I saw an arrangement of the following kind:

  That is, there were only two stars to the east of Jupiter, the nearer of which was three times as far from it as from the star further to the east; and the star furthest to the east was nearly twice as large as the other one. But on the previous night they had appeared nearly of equal magnitude. I therefore concluded, and decided unhesitatingly, that there were three stars in the heavens moving around Jupiter, like Venus and Mercury around the sun. This was finally established as clear as daylight by numerous other subsequent observations. These observations also established that there are not only three, but four, wandering sidereal bodies performing their revolutions around Jupiter. The following account will report on the observations of these changes of position made with more exactness on succeeding nights. I have also measured the intervals between them with the spyglass, in the manner already explained. Besides this, I have given the times of observation, especially when several were made in the same night, for the revolutions of these planets are so swift that an observer may generally get differences of position every hour.

  On 12 January at the first hour of the next night, I saw these heavenly bodies arranged in this manner:

  The star furthest to the east was greater than the one furthest to the west, but both were very conspicuous and bright. The distance of each one from Jupiter was two minutes. [82] A third star, certainly not in view before, began to appear at the third hour; it nearly touched Jupiter on the east side and was exceedingly small. They were all arranged in the same straight line, along the ecliptic.

  On 13 January, for the first time, four stars were in view in the following position with regard to Jupiter:

  There were three to the west, and one to the east. They made almost a
straight line, but the middle star of those to the west deviated a little from the straight line towards the north. The star furthest to the east was at a distance of two minutes from Jupiter. There were intervals of only one minute between Jupiter and the nearest star, and between the stars themselves, west of Jupiter. All the stars appeared of the same size, and though small they were very brilliant and far outshone the fixed stars of the same magnitude.

  On 14 January the weather was cloudy.

  On 15 January at the third hour of the night, the four stars were in a state with reference to Jupiter depicted in the next diagram:

  All were to the west and arranged almost in the same straight line; but the star that counted third from Jupiter was raised a little to the north.

  The nearest to Jupiter was the smallest of all; the rest appeared successively larger. The intervals between Jupiter and the three nearest stars were all successively equal and of the magnitude of two minutes each; but the star furthest to the west was four minutes distant from the star nearest to it. They were very brilliant, and not at all twinkling, and such they have always appeared both before and since. But at the seventh hour there were only three stars, presenting with Jupiter an appearance of the following kind:

  They were, that is to say, in the same straight line to a hair. The nearest to Jupiter was very small and distant from the planet three minutes; the distance of the second from this one was one minute; and of the third from the second four minutes and thirty seconds. But after another hour the two middle stars were still nearer, for they were only thirty seconds, or less, apart.