Answer Reading Homework Questions

64 SIDEREUS NUN C IUS

We have briefly explained our observations thu s far about the Moon, the fixed stars, and the Milky Way. It remains for us to reveal and make known what appears to be most important in the present matter: four planets never seen from the beginning of the world right up to our day, the occasion of their discovery and observation, their positions, and the observations made over the past 2 months 73 concerning their behavior and changes. And I call on all astronomers to devote themselves to investigating and de- termining their periods . Because of the shortness of time, it has not been possible for us to achieve this so far. 74 We advise them again, however, that they will need a very accurate glass like the one we have described at the beginning of this account, lest they undertake such an investigation in vain. 75

Accordingly, on the seventh day of January of the present year 1610, 76 at the first hour of the ni ght , when I inspected the celestial constellations through a spyglass, Jupiter presented himself. And since I had prepared for myself a superlative instrument, I saw (which earlier had not happened because of th e weakness of the other instruments) n that three little stars were positioned near him- small but yet very bri ght. Although I believed them to be among the numb er of fixed stars, they nevertheless intrigued me because they appeared to be arranged exactly along a straight line and par- allel to th e ecliptic, and to be brighter than others of equal size. And their disposition among th emselves and with respect to Jupiter was as follows: 78

73. 7 J an u ary to 2 M a rch 1610. 74. In 1612 Ga lileo published periods for all four satellites. They were virtua lly

th e same as the modern values. See Disco urse on Bodies in Water, tr. Thomas Salusbury, ed. Stillman Drake (Urbana: University of Illinois Press, 196o), 1.

75. Especially in th e case of the satellites of Jupiter, it was necessa r y to h ave a tel- escope that magnified fifteen times or more and was especia ll y adapted for celestial use .

76. All dates us ed b y Gali leo are Gre go rian . 77 . See pp . 13-14, above . 78. Satellites I and II were very close toget her, just ;o the eas t of Jupiter. Gal il eo

saw them as o ne. See J ean M ee us, "Ga lil eo's First Reco rds of Jupiter 's Satellites,"

Sky and Telescop e 24 ( 1962): 137-39 .

SIDEREUS NUN C I US 65

East * * 0 * West That is, two stars were near him on the east and one on the west; the m ore eastern one and the western one appeared a bit larger than the remaining one. I was not in the least concerned with their distan ces from Jupiter, for, as we said above, at first I believed them to be fi xed stars. But when, on the eighth, I returned to the same observation, guided by I know not what fate, 79 I found a very different arrangement. For all three little stars were to the west of Jupiter and closer to each other than the previous night, and sep- arated by equal intervals , as shown in the adjoining sketch. 80 Even thou gh at this point I had by no means turned my thought to the mutual motions of these stars, yet I was aroused by the question

East 0 * * * West of how Jupiter could be to the east of all the said fixed stars when the day before he had been to the west of two of them . I was afraid, therefore, that perhaps , contrary to the astronomical computations, his m otion was direct and that, by his proper motion , he had bypassed those stars. 8 1 For this reason I waited eagerly for the next night . But I was disappointed in my hope , for the sky was every- where covered with clouds .

Then, on the tenth, the stars appeared in this position with regard to Jupiter. Only two stars were near him, both to the east. The

East * * 0 West 79. See pp . 12-13 , above. 80. On this night, satellite IV was at its farthest distan ce from Jupiter to the east,

and it escaped Galileo because of th e smalln ess of th e field of view of his s pyglass. See Meeus , "Galileo's First Records."

81. See pp . 15-16, above.

66 SIDEREUS NUNCIUS

third, as I thought, was hidden behind Jupiter. 82 As before, they were in the same straight line with Jupiter and exac tl y ali gned along the zodiac. When I saw this, and since I knew th at such changes could in no way be assigned to Jupiter, and since I knew, moreover, that the observed stars were always the same ones ( for no others, either preceding or following Jupiter, were present alon g the zodiac for a great distance), now, moving from doubt to astonishment, I found that the observed change was not in Jupiter but in the said stars. And therefore I decided that henceforth they should be ob- served more accurately and diligently.

And so, on the eleventh, I saw the following arrangement:

East

* * 0 West There were only two stars on the east, 83 of which the middle one was three times as far from Jupiter than from the more eastern one, and the more eastern one was about twice as large as the other, although the previous night they had appeared about equal. I there- fore arrived at the conclusion, entirely beyond doubt, that in the heavens there are three stars wandering around Jupiter like Venus and Mercury around the Sun. This was at length seen clear as day in many subsequent observations, and also that there are not only three, but four wandering stars making their revolutions about Jupiter. The following is an account of the changes in their positions, accurately determined from then on. I also measured the distances between them with the glass, by the procedure explained above. 84

I have added the times of the observations, especially when more than one were made on the same night, for the revolutions of these

82 . On this night, Satellite I was so close to Jupiter on the west that it was lost in the planet's g lare. Satellites II and Ill were very close to each ot her and Galileo saw them as one, just to the eas t of the planet . See Mceus, "Galil eo's First Records."

83. Satellites I and II had just ended their tran si ts in front of the plan et and were sti ll too close to be discerned by Galileo. See ibid.

84. See pp . 38-39.

SIDEREUS NUNCIUS 67

planets are so swift that the hourly differences can often be perceived as well.

Thus, on the twelfth, at the first hour of the following night, I saw the stars arranged in this manner. The more eastern star was

East ~ *o * West larger than the western one, but both were very conspicuous and bright. 85 Both were two minutes"'· distant from Jupiter. In the third hour a third little star, not at all seen earlier, also began to appear. This almost touched Jupiter on the eastern side and was very small. All were in the same straight line and aligned along the ecliptic.

On the thirteenth, for the first time four littl e stars were seen by me in this formation with respect to Jupiter. "7 Three were on

East 'll£: Oil'*'* West th e west and one on the east. They formed a very nearly straight line, but the middle star of the western ones was displaced a little to the north from the straight line . The more eastern one was 2 minutes distant from Jupiter; the intervals between the remaining ones and Jupiter were only I minute. All these stars displayed the

85. Note that Galileo initially saw only two of the sa tellites, Ill on the east and II on th e west. Satellites I and IV were both on the eas t and rather close to Jupiter. Apparently Galileo cou ld not see either one until satellite I moved farther away from the planet. See M eeus, "Galileo's First Record s."

86. Galileo took Jupite r 's angular diameter to be about I arcminute, and he used this measure to estimate the distances of the satellites. In his drawings and in Sidereus Nunciu s, howev er, he showed the planet's disk as being about twice as large while keeping the distances of th e satellites the same. The drawings are thus out of pro- portion . See Stillman Drake, Telescopes, Tides, and Tactics (Chicago: University of Chicago Press, 19 83), 214-19.

87. It was thus on this day that Galileo recognized that there were four moons. During the previous observations he had been prevented by various ci rcum s tances from seeing all four moons at once.

68 SIDEREUS NUNCIUS

same size, and although small they were nevertheless very brilliant and much brighter than fixed stars of the same size.

On the fourteenth, the weather was cloudy. On the fifteenth, in the third hour of the night, the four stars

were positioned with respect to Jupiter as shown in the next figure.

East 0 i< # * * West They were all to the west and arranged very nearly in a straight line, except that the third one from Jupiter was raised a little bit to the north . The closest one to Jupiter was the smallest of all, and the rest consequently appeared larger. The intervals between Jupiter and the next three stars were all equal and of 2 minutes; and the most eastern one was 4 minutes from the closest one to it. They were very brilliant and did not twinkle, as indeed was always the case, both before and afterward. But in the seventh hour only three stars were present in this arrangement with Jupiter. They were

East 0 ... * * West indeed precisely"" in the same straight line . The closest one to Jupiter was very small and removed from him by 3 minutes; the second was r minute distant from this one; and the third from the second 4 minutes and 30 seconds. After another hour, however, the two little stars in the middle were still closer to each other, for they were removed from each other by barely 30 seconds.

On the sixteenth, in the first hour of the night, we saw three stars arranged in this order. Two flanked Jupiter, 40 seconds re-

East *O * * West moved from him on either side, and the third was 8 minutes from Jupiter in the west. The one closer to Jupiter appeared not larger but brighter than the farther one.

88 . I hav e tran slated th e Latin ad ung uem as precisely throu g hout thi s section .

SIDE RE USN U NCI US 69

On the seventeenth , 30 minutes after sunset, the configuration was thus . There was only one star on the east, 3 minutes from

East * 0 jj

West

J upiter. Likewise, one was IT minutes from Jupiter to the west. The eastern one appeared twice as large as the western one. There were no more than these two . But after 4 hours, that is, around the fifth hour of the night, on the eastern side a third began to emerge, which, I suspect, had earlier been united with the first one. The formation was thus. The middle star, extremely close to the

East *i11 0 # West eastern one, was only 20 seconds from it, and it was displaced a little bit to the south of the line drawn through the outermost stars and Jupiter.

On the eighteenth, 20 minutes after sunset, the appearance was

East * 0 * West

thus. The eastern star was larger than the western one and 8 minutes distant from Jupiter, while the western one was ro minutes from J upiter.

On the nineteenth, at the second hour of the night, the formation was like this. There were three stars exactly on a straight line

East " * 0 * * West

through Jupiter, one to the east, 6 minutes distant; between Jupiter and the first western one was an interval of 5 minutes, while this star was 4 minutes from the more western one. At this time I was uncertain whether between the eastern star and Jupiter there was a little star, very close to Jupiter, so that it almost touched him. And at the fifth hour, I clearly saw this little star now occupying a place

70 SIDEREUS NU NCI US

precisely in the middle between Jupiter and the eastern star, so that the formation was as follows:

East * • 0 * * West Further, the newly perceived star was very sma ll ; yet by the sixth hour it was almost equal in magnitude to the others.

On the twentieth, at I hour, 15 minutes, a similar configuration appeared. There were three little stars so small that they could

East 4 0 .. If. West hardly be perceived . They were not more than I minute from Jupiter and each other. I was uncertain whether on the west there were two or three little stars. Around the sixth hour they were arranged in thi s manner:

East If. 0"'"' West The eastern one was twice as far from Jupiter as before, that is , 2 minutes; the middle one to the west was 40 seconds from Jupiter but 20 seco nds from the western one . At length, in the seventh hour, three little stars were seen to the west; the nearest was 20 seconds from him; between this one and the westernmost one there

East • 0"'•"' West was an interval of 40 seconds; and between these another was seen, displaced a little to the south and not more than IO seconds from the westernmost one.

On the twenty-first, at 30 minutes, there were three little stars to the east, equally spaced from each other and Jupiter.

East ~l\l«o ~ West

SIDEREUS NUNCIUS 71

The intervals were estimated to be 50 seconds. There was also a star to the west, 4 minutes from Jupiter. The closest one to Jupiter to the east was the smallest of all. The rest were somewhat larger

and equal to each other. On the twenty-second, at the second hour, the configuration

was similar. The distance from the eastern one to Jupiter was 5 minutes; the distance from Jupiter to the westernmost one was 7

East * 0 *~ * West

minutes ; the two western stars in the middle were 40 seconds from each other while the nearer one was I minute from Jupiter. The little stars in th e middle were smaller than the outermost ones, but they were on the same straight lin e extended along the length of the zodiac except that of the three western ones the middle one was displaced a bit to the south. But at the sixth hour of the night they appeared in this arrangement. The eastern one was very small and, as before, 5 minutes distant from Jupiter; the three western

East IC 0 * * * West ones were separated equally from Jupiter and each other, and the spaces were nearly I minute, 20 seconds each; and the star closer to Jupiter than the other two appeared smaller; and they all appeared to lie exactly on the same straight lin e.

On the twenty-third, 40 minutes after sunset, the configuration

of stars was about like this:

East * * 0 * West

There were three stars in a straight line with Jupiter along the length of the zodiac, as they have always been ; two were to the east and one to the west. The easternmost one was 7 minutes from the next one, this one 2 minutes, 40 seconds from Jupiter, and Jupiter 3 minutes , 20 seconds from the western one; and they were all about

72 SIDEREUS NUNCIUS

equal in magnitude. But at the fifth hour the two stars which earlier were closest to Jupiter were no longer visible, hiding behind Jupiter in my opinion; and the appearance was as follows:

East * 0 West On the twenty-fourth, three stars appeared, all to the east, and

nearly in the same straight line with Jupiter, for the middle one

East * ** 0 West

deviated slightly to the south. The star closest to Jupiter was 2 minutes from him, the next one 30 seconds from this one, and the easternmost one 9 minutes from that one; and all were very bright. But at the sixth hour only two stars presented themselves in this arrangement,

East [*] * 0 West that is, precisely on a straight line with Jupiter. The nearer one was removed from Jupiter by 3 minutes while the other one was 8 minutes from this one. If I am not mistaken, the two middle little stars observed earlier had united into one.

On the twenty-fifth, at I hour, 40 minutes, the formation was

thus:

East * * 0 West

There were only two stars to the east, and those fairly large. The easternmost was 5 minutes from the middle one, and the middle one 6 minutes from Jupiter.

On the twenty-sixth, at o hours, 40 minutes, the formation of stars was like this. For three stars were observed, of which two

East

* * 0 * West

SIDEREUS NUNCIUS 73

were to the east and one to the west. This last one was 5 minutes from him, while the middle one in the east was 5 minutes, 20 seconds from him. The easternmost was 6 minutes from the middle one. They were on the same straight line and of the same magni- tude. Then at the fifth hour the arrangement was nearly the same, differing only in this, that near Jupiter a fourth star had emerged

East * * ~o * West

on the east, smaller than the rest, at that time 30 seconds removed from Jupiter but elevated a little to the north above the straight

line, as shown in the adjoining figure. On the twenty-seventh, at I hour after sunset, only a single star

was perceived, and that one to the east, in this arrangement:

East • 0 West It was very small and 7 minutes removed from Jupiter.

On the twenty-eighth and twenty-ninth, nothing could be ob-

served because of interposed clouds . On the thirtieth, at the first hour of the night, the stars were

observed arranged in this order. One was to the east, 2 minutes ,

East * 0 * ,. West 30 seconds from Jupiter, and two were to the west, of which the one closest to Jupiter was 3 minutes from him and the other 1 minute from this one. The outermost stars and Jupiter were ar- ranged in a straight line, and the middle star was elevated a little to the north. The westernmost star was smaller than the others.

On the last day [of January], at the second hour, two stars ap- peared to the east and one to the west. The middle of the eastern

East ** 0 * West

74 SIDEREUS NUNCIUS

ones was 2 minutes, 20 seconds from Jupiter, the easternmost one 30 seconds from the middle one. The western star was IO minutes from Jupiter. They were nearly in the same straight line, only the eastern one, closest to Jupiter, was a little bit elevated to the north . But at the fourth hour the two to the east were still closer to each

East ** 0 ~ West other, for they were only 20 seconds apart. In these observations the western star appeared very small.

On the first day of February, at the second hour of the night, the formation was similar. The eastern star was 6 minutes from

East * ~JO * West

Jupiter and the western one 8. To the east a very small star was 20 seconds distant from Jupiter. They traced out a precisely straight

line . On the second, the stars appeared in this order. A single star to

the east was 6 minutes from Jupiter; Jupiter was 4 minutes distant

East * 0 * * West from the nearer one to the west; and between this one and the westernmost star there was an interval of 8 minutes. They were precisely in a straight line and of nearly the same magnitude . But at the seventh hour there were four stars, among which Jupiter

East )jC' *o * * West occupied the middle position . Of these stars the easternmost one was 4 minutes from the next, this one r minute , 40 seconds from Jupiter, Jupiter 6 minutes from the western one closest to him, and this one 8 minutes from the westernmost one. They were all to- gether on the same straight line extended along the line of the zodiac.

SIDEREUS NUNCIUS 75

On the third, at the seventh hour, the stars were arranged in this sequence. The eastern one was I minute, 30 seconds from Jupiter; the closest western one 2 minutes; and the other western one was

East * 0 * * West 10 minutes removed from this one. They were abso lutel y on the same straight line and of equal magnitude .

On the fourth, at the second hour, there were four stars around Jupiter, two to the east and two to the west, and arranged precisely

East * ~o * * West on a straight line, as in the adjoining figure . The easternmost was distant 3 minutes from the next one, while this one was 40 seconds from Jupiter; Jupiter was 4 minutes from the nearest western one, and this one 6 minutes from the westernmost one. Their magnitudes were nearly equal; the one closest to Jupiter appeared a little smaller than the rest . But at the seventh hour the eastern stars were only 30 seconds apart. Jupiter was 2 minutes from the nearer eastern

East ** 0 * * West one, while he was 4 minutes from the next western one, and this one was 3 minutes from the westernmost one. They were all equal and extended on the same straight line along the ecliptic.

On the fifth, the sky was cloudy. On the sixth, on ly two stars appeared flanking Jupiter, as is seen

East * 0 * West

in the adjoining figure. The eastern one was 2 minutes and the western one 3 minutes from Jupiter. They were on the same straight line with Jupiter and equal in magnitude .

On the seventh, two stars stood near Jupiter, both to the east,

arranged in this manner.

76 SIDEREUS NUN CIUS

East **O West The intervals between them and with Jupiter were equal , that is, 1 minute , and a straight line ran through them and the center of

Jupiter. On the eighth, at the first hour, three stars were present, all to

the east, as in the figure . The small star closest to Jupiter was 1

East "* *o West

minute, 20 seconds distant from him; the middle star was 4 minutes from this one and rather large; and the very small easternmost star was 20 seconds from that one. I was of two minds whether the one closest to Jupiter was only one, or two little stars, for it seemed now and then that there was another star near it, toward the east, extremely small, and separated from it by only IO seconds. They were all extended on the same straight line along the zodiac. But at the third hour the star closest to Jupiter nearly touched him . It was only 10 seconds from him , while the others had moved farther from Jupiter, for the middle one was 6 minutes away from Jupiter. Finally, at the fourth hour, the one that before was closest to Jupiter,

united with him, was seen no longer. On the ninth, at 30 minutes, two stars were near Jupiter to the

east and one to the west, in this formation. The easternmost star,

East • * 0 • West

which was rather small, was 4 minutes from the next one; the larger middle star was 7 minutes distant from Jupiter; Jupiter was 4 min- utes removed from the western star, which was small.

On the tenth, at 1 hour, 30 minutes, two very small stars, both to the east, appeared in this arrangement. The farther one was IO

East ¥ ·O West

SIDEREUS NUNCIUS 77

minutes from Jupiter and the nearer one 20 seconds. They were on the same straight line. But in the fourth hour the star close to Jupiter did not appear any longer and the other appeared so diminished that it could hardly be perceived, although the air was very clear, and it was farther from Jupiter than it had been before, since it was

now 12 minutes distant. On the eleventh, at the first hour, two stars were present to the

east and one to the west. The western one was 4 minutes from

East * * 0 * West

Jupiter; the nearer one to the east was likewise 4 minutes away from Jupiter, while the easternmost star was 8 minutes from this one. They were moderately conspicuous and on the same straight line . But at the third hour a fourth star appeared close to Jupiter to the east, smaller than the other ones, separated from Jupiter by 30

East * * •O * West

seconds and slightly displaced to the north from the straight line drawn through the rest of the stars. They were all most brilliant and very conspicuous . But at the fifth hour plus a half the star closest to Jupiter to the east, already more remote from him, had attained a position in the middle between him and the more eastern star close to itself. And they were all precisely on the same straight line and of the same magnitude, as can be seen in the adjoining

figure.

East * * * 0 * West

On the twelfth, at 40 minutes, two stars were present to the east and likewise two to the west. The farther one to the east was IO

East * "0*': * West

78 SIDEREUS NUNCIUS

minutes from Jupiter while the more remote star to the west was 8 minutes away. They were both rather conspicuous. The other two stars were very close to Jupiter and very small, especially the eastern one, which was 40 seconds distant from Jupiter, while the western one was I minute away. But at the fourth hour the little star that was close to Jupiter to the east no longer appeared.

On the thirteenth, at 30 minutes, two stars appeared to the east and two also to the west. The eastern star closer to Jupiter, fairly

East * * 0 [*]* West conspicuous, was 2 minutes from him, and the more eastern one, appearing smaller, was 4 minutes removed from this one . The western star farther from Jupiter, exceedingly conspicuous, was separated from him by 4 minutes. Between it and Jupiter fell a small starlet closer to the westernmost star, since it was not more than 30 seconds from it. They were all precisely on the same straight line along the length of the ecliptic.

On the fifteenth ( for on the fourteenth the sky was covered by clouds), at the first hour, the position of the stars was as follows.

East *H o West That is, there were three stars to the east, but none were seen to the west. The star to the east closest to Jupiter was 50 seconds from him , the next one was 20 seconds from this one, and the easternmost star 2 minutes from this one. And it was larger than the others, for th e two nearer ones were exceedingly small. But at about the fifth hour, of the stars close to Jupiter only one was seen, 30 seconds

East * ~o West

distant from Jupiter. The elongation of the more eastern one from Jupiter was increased for it was then 4 minutes. But at the sixth hour, in addition to the two positioned to the east, as was stated a

SIDEREUS NUNCIUS 79

East * ·O * West moment ago, one little star, exceedingly small, was seen toward the west, 2 minutes removed from Jupiter.

On the sixteenth, at the sixth hour, they were in the following arrangement. That is, one star was 7 minutes away from Jupiter to the east, Jupiter 5 minutes from the next star to the west, and this

East * 0 * * West

one 3 minutes from the remaining western one. They were all of about the same magnitude, fairly conspicuous, and exactly on the same straight line drawn along the zodiac.

On the seventeenth, at the first hour, two stars were present, one to the east 3 minutes from Jupiter and another to the west,

East * 0 * West

distant IO minutes. This star was somewhat smaller than the eastern one. But at the sixth hour the eastern one was closer to Jupiter and was only 50 seconds distant from him. The western star was farther, that is, 12 minutes . In both observations they were on the same straight line, and both were rather small, especially the one to the

east in the second observation. On the eighteenth, at the first hour, three stars were present, of

which two were to the west and one to the east. The eastern star

East * 0 * * West

was 3 minutes from Jupiter, the closest one to the west 2 minutes, and the remaining more westerly star was 8 minutes from the middle one. All were precisely on the same straight line and of nearly the same magnitude. But at the second hour the stars closer to Jupiter were removed from Jupiter by equal spaces, for the west- ern one [of these] was now also 3 minutes away from him . But at

Bo SIDEREUS NUNCIUS

the sixth hour a fourth star appeared between the eastern one and Jupiter in the following configuration. The easternmost star was 3 minutes from the next one, this star I minute, 50 seconds from Jupiter, Jupiter 3 minutes from the next western star, and this one

East * *o * * West 7 minutes from th e westernmost star. They were nearly equal, only the eas tern one close to Jupiter was a bit smaller, and they were all on the same strai g ht line parallel to th e ecliptic.

On the nineteenth, at 40 minutes, only two stars, rather large,

East 0 * * West were seen to the west of Jupiter and precisely arrayed with Jupiter on th e sa m e straight lin e drawn along the ecliptic. The nearer star was 7 minutes from Jupiter and 6 minutes from the westernmost star.

On the twentieth, the sky was cloudy. On the twenty-first, at r hour, 30 minutes, three little stars,

rather sma ll, were observed in thi s arrangement. The eastern star

East '!I< 0 "'* "' West was 2 minutes from Jupiter, Jupiter 3 minutes from the next western one, and this star 7 minutes from the westernmost one. They were precisely in the same straight line, parallel to the ecliptic.

On the twenty-fifth, at I hour, 30 minutes ( for during the three precedin g nights the sky was covered by clouds), three stars ap-

East ... ... 0 * West peared , two to the east, whose distances between themselves and from Jupiter were eq ual at 4 minutes . To the west one star was 2 minutes from Jupiter. They were precisely on the same straight lin e extending along the ecliptic.

SIDEREUS NUNCIUS 81

On the twenty-sixth , at 30 minutes, only two stars were present. One was to the east ro minutes from Jupiter, and the other was to

East -lit 0 * West the west 6 minutes distant . The eastern star was somewhat smaller than the western one. But at the fifth hour three stars appeared. Besides th e two already noted, a third was perceived close to Jupiter,

East :JI o~ * West to the west and very small, which earlier had been hidden behind Jupiter, and it was r minute from him . The eastern star appeared farther than before, being Ir minutes from Jupiter. On this night I decided for the first time to observe the progress of Jupiter and his adjacent planets along the length of the zodiac by reference to some fixed star, for a fixed star was observed to the east, r I minutes from the easternmost planet and displaced somewhat to the south

in the following manner: s9

East 11: o~ * West *fin

On the twenty-seventh , at r hour, 4 minutes, 90 the stars appeared in this configuration. The easternmost star was IO minutes from Jupiter, the next star, close to Jupiter, 30 seconds; the next one, to

East * ~JO ** West

* fixa 89. This is a seventh-m agnitude s tar, just below th e ecliptic, at R.A. 5 hours, 4

minutes and decl. + 22° .4, in th e con stellati on Taurus. 90. Galileo clearly meant "40 minutes," but this is not a printer's error: the

manu sc ript also has "4 minutes " (Opere, 3:44) .

82 SIDEREUS NUNCIUS

the west , was 2 minutes, 30 seconds from Jupiter, and the west- ernmost star was r minute distant from this one. The stars nearer Jupiter appeared small, especially the eastern one, but the outermost stars were very conspicuous, especially the western one. And they formed a straight line exactly drawn along the ecliptic. The progress of these planets tow ard the east was clearly discerned through a comparison with the aforesaid fixed star, for Jupiter with his at- tendant planets was closer to it, as can be seen in the adjoining figure. But at the fifth hour the eastern star near Jupiter was r minute away from him.

On the twenty-eighth , at th e first hour, only two stars were seen, an eas tern one 9 minutes , and a western one 2 minutes from Jupiter. They were fairly conspicuous and on the same straig ht line. This line was perpendicularly intersec ted by a line from the fixed star to the eastern planet, as shown in the figure .

East * 0 * West

*fixa

But at the fifth hour a third littl e star was perceived to the east 2 minutes distant from Jupiter in thi s arrangement.

East * * 0 * West On the first of March , at 40 minutes , four stars were perceived ,

all to th e eas t. The neares t star to Jupiter was 2 minutes from him, the next star r minute from this one, and the third 20 seconds, and

East • **~ 0 West

* fiXl

SIDEREUS NUNCIUS 83

it was brighter than the rest . The fourth star was 4 minutes from this one, and it was smaller than the rest . They formed nearly a straight line excep t that the third star from Jupiter was elevated a little . The fixed star formed an equilateral triangle with Jupiter and the easternmost star, as shown in the figure .

On the second, at 40 minutes, three planets were present, two to the east and one to the west, in this configuration.

East *· 0 * West

*fixa

The easternmost planet was 7 minutes from Jupiter, while this one was 30 seconds from the next planet . The western planet was 2 minutes removed from Jupiter. And the outermost planets were brighter and larger than the other one, which appeared very small. The easternmost planet appeared somewhat elevated toward the north above the straight line drawn through Jupiter and the other ones . The fixed star already noted was 8 minutes distant from the western planet along the line drawn to that planet perpendicular to the straight line extended through all the planets, as the figure

shows. I decided to add these comparisons of Jupiter and his adjacent

planets with the fixed star so that from them anyone could see that the progress of these planets, in lon gitude as well as latitude, agrees exactly with the motions that are derived from the tables. 91

These are the observations of the four Medicean planets recently, and for the first time, discovered by me. From them, although it

91. Jupiter h ad passed its station at th e end of J anuary and was slowly moving from west to east. Its daily motion in lo n g itude was abo ut 4 arcminutes at th e e nd of February. See Bryant Tuckerman , Planetary, Lunar and Solar Positio'ls A.D. 2 to A .D . 1649 at Five-Day and Ten-Day Int ervals, Am e rican Philosophical Societ y, Mem-

oirs 59 ( 1964): 823.

84 SIDEREUS NUNCIUS

is not yet possible to calculate their periods , something worthy of noti ce may at least be said. And first, since they sometimes follow and at other times precede Jupiter by similar intervals, and are removed from him toward the east as well as the west by only very narrow limits , and accompany him equally in retrograde and direct motion, no one can doubt that they complete their revolu- tion s about him while, in the meantime, all together they complete a 12-yea r period about the center of the world . Moreover, they whirl around in unequal circles, which is clearly deduced from the fact that at the greatest separations from Jupiter two planets could neve r be seen united while, on the other hand , near Jupiter two , three, an d occasionally all four pl anets are found crowded together at th e same time. It is further seen that the revolutions of the planets describing smaller circles around Jupiter are faster. 92 For the stars closer to Jupiter are often seen to the eas t when the previous day they appeared to th e west, and vice versa, while from a careful examination of its previously accurately noted returns , the planet traversing the largest orb appears to have a semimonthly period. 93

We have moreover an excellent and splendid argument for taking away th e scruples of those who, while tolerating with equanimity the revo lution of th e planets around the Sun in the Copernican system, are so disturbed by the attendance of one Moon around the Earth while the two together complete the annual orb around the Sun that they conclude that this constitution of the universe must be overthrown as impossible. 94 For here we have only one planet revolving around another while both run through a great circle aro und the Sun : but our vision offers us four stars wandering around Jupiter like the Moon around the Earth while all together with Jupiter traverse a great circle around the Sun in the space of

92. Kepler 's third law, relating th e mean radi i of th e orbi ts of the planets to th eir periods , was not published until 1619.

93. The actu al period is abo ut 16 days, 18 hours . 94. This was one of th e a rg uments aga inst th e Co pernican h ypo th es is. If the

Earth is a planet , w h y sho uld it be the on ly plane t to h ave a moon? Alternatively, how co uld there be tw o centers of rotation in the universe?

SIDEREUS NUN CIUS 85

12 years. 95 Finally, we must not neglect th e reason why it happens that the Medicean stars, while completing their very small revo- lutions around Jupiter, are themselves now and th en seen twice as large . We can in no way seek the cause in terrestrial vapors, for the stars appear larger and smaller when th e sizes of Jupiter and nearby fixed stars are seen completely unchanged . It seems inconceivable, moreover, that they approach and recede from the Earth by such a degree around the perigees and apogees% of their orbits as to cause such large changes. For smaller circular motions can in no way be responsible , while an oval motion (which in this case would have to be almost straight) appears to be both inconceivable and by no account harmonious with the appearances. 97 I gladly offer what occurs to me in this matter and submit it to the judgment and censure of right-thinking men. It is well known that because of the interposition of terrestrial vapors the Sun and Moon appear larger but the fixed stars and planets smaller. For this reason, near the horizon the luminaries appear larger98 but the stars [ and planets] smaller and generally inconspicuous , and they are diminished even more if the same vapors are perfused by light. For that reason the stars [ and planets] appear very small by day and during twilight, but not the Moon, as we have already stated above. 99 From what we have said above as well as from thos e things that will be di scuss ed

95. This passage in its e ntiret y remov es an impo rt ant objec ti o n aga inst the

Copernican th eo r y, fo r Jupit er 's moons demonstrate that ou r M oon can revo lv e around a moving Earth. It h as been su ggested, howeve r, that it is an arg um en t against the geo-he li oce ntri c syste m ofTycho Brahe . See Wade L. Robison , "Galileo

on the M oon s of Jupiter," Annals of Science 3 1 (1974): 165-69. 96. Apogee an d pe ri gee are the points at which a heave nl y body is fa rth es t from,

and closest to, th e Earth, an·d Ga lileo is usin g th e term s here in their litera l meanings. 97. Alth oug h th e or bits of Jupite r 's satellites are v irtuall y circles, technically they

are ellipses. E lli p tica l as tron omy was introduced by J o h annes Kepler in his Asrronomia

Nova of 1609. 98. In fact, atmospheric refra ction makes th e ve rti ca l diam e ter of th ese bodies

smaller than th e horizontal diameter. The lar ge sizes of the Moon and Su n when

close to th e h o ri zo n are op ti ca l illu sio ns.

99. See pp . 57-58.

86 SIDEREUS NUNCJUS

more amply in our system, it is moreover certain that not only the Earth but also the Moon has its surrounding vaporous orb.

100 And

we can accordingly make the same judgment about the remaining planets, so that it does not appear inconceivable to put around Jupiter an orb denser than the rest of the ether around which the Medicean planets are led like the Moon around the sphere of the elements. And at apogee, by the interposition of this orb, they are smaller, but when at perigee, because of the absence or attenuation of this orb, they appear larger. 101 Lack of time prevents me from proceeding further. The fair reader may expect more about these

matters soon .

100. See note 43 , above . 1 o I. The variations in brightness reported by Galileo cannot be accounted for by the

varying brightness of individual satellites. Since in Galileo's reports satellites were seen dim only when they were close to Jupiter, this effect must be asc ribed to a combination of the

glare of the planet and the poor resolution of Galileo's telescope .

CONCLUSION: THE RECEPTION OF SIDEREUS NUNCIUS

Sidereus Nuncius made Galileo into an international celebrity almost overnight. Although news did not travel instantaneously in the seventeenth century as it does today, it traveled surprisingly fast throu gh diplomatic and commercial channels. From Venice it took less than 2 weeks for letters to reach southern Germany, and about a month to reach distant England . And some of the letters trans- mitted through these channels in the spring of 1610 contained mentions of the astounding discoveries made by the mathematics professor at the University of Padua . Quickly, therefore, Galileo's name was on the lips of learned men all over Europe. The exact nature of the discoveries, however, was usually not known until copies of the book itself arrived in the wake of the rumors . At that point, scholars could read for themselves the claims made by Galileo and the process of evaluation could beg in . But few sci- entists had access to spyglasses other than the ordinary low- pow ered ones, and even the best instruments could not initially compete for quality with those made by Galileo . High-quality glass was difficult to obtain, 1 while the spectacle-making craft was too tradition-bound to respond quickly to the demand for lenses outside the normal range of strengths. 2 Before scientists could verify or disprove the discoveries they h ad to procure appropriate instruments, and this took time. Not before the autumn of 1610 was independent verification, in Italy and abroad, forthcoming.

1. Olaf Pedersen , "Sagredo's Optical Researches," Centaurus 13 (1968): 139-50; Silvio Bedini, "The M akers of Ga lileo's Scientific Inst rument s," in A tti de/ sirnposio interna ziona le di storia, metodologia, logica e jilosojia de/la scienza "Ga lileo nella storia e nella ji losojia de/la scien za," 4 vols. ( Florence: G. Barbera, 1967), 2 (part 5): 89- 11 5.

2 . A . 0 . Pri ckard , "The 'Mundus J ov ialis' of Simon M arius," Obse rva tory 39 (1916): 370-71; Giro la m o Sirturi, Telescopiurn: Sive ars perficiendi (Frankfu rt, 1618),

22-30 .

87