Galileo Galilei (1610)
Siderius Nuncius, OR
The Sidereal Messenger*
. . .  We have briefly explained our observations thus 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: tour 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 concerning their behavior and changes. And I call on all astronomers to devote themselves to investigating and determining their periods Because of the shortness of time, it has not been possible for us to achieve this so far. 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.
Accordingly, on the seventh day of January of the present year 1610, at the first hour of the night, 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 the weakness of the other instruments) that three little stars were positioned near him-- small but yet very bright. Although I believed them to be among the number of fixed stars, they nevertheless intrigued me because they appeared to be arranged exactly along a straight line and parallel to the ecliptic, and to be brighter than others of equal size. And their disposition among themselves and with respect to Jupiter was as follows: 
That is, two stars were near him on the east and one on the west; the more eastern one and the western one appeared a bit larger than the remaining one. I was not in the least concerned with their distances from Jupiter, for, as we said above, at first I believed them to be fixed stars. But when, on the eighth, I returned to the same observation, guided by I know not what fate, 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 separated by equal intervals, as shown in the adjoining sketch. Even though at this points I had by no means turned my thought to the mutual motions of these stars, yet I was aroused by the question
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 motion was direct and that, by his proper motion, he had bypassed those stars 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
< third, as I thought, was hidden behind Jupiter. As before, they were in the same straight line with Jupiter and exactly aligned along the zodiac. When I saw this, and since I knew that 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 along 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 observed more accurately and diligently
And so, on the eleventh, I saw the following arrangement:
There were only two stars on the east, 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 therefore 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 . I have added the times of the observations, especially when more than one were made on the same night, for the revolutions of these  planets are so swift that the hourly differences can often be perceived.
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
larger than the western one, but both were very conspicuous and bright. 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 little stars were seen by me in this formation with respect to Jupiter. Three were on
the 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 1 minute. All these stars displayed the  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.
 ... These are the observations of the four Medicean planets recently, and for the first time, discovered by me From them, although it  is not yet possible to calculate their periods, something worthy of notice 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 revolutions about him while, in the meantime, all together they complete a 12-year 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 never be seen united while, on the other hand, near Jupiter two, three, and occasionally all four planets are found crowded together at the same time. It is further seen that the revolutions of the planets describing smaller circles around Jupiter are faster. For the stars closer to Jupiter are often seen to the east when the previous day they appeared to the 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. We have moreover an excellent and splendid argument for taking away the scruples of those who, while tolerating with equanimity the revolution of the 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. For here we have only one planet revolving around another while both run through a great circle around 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  12 years. Finally, we must not neglect the reason why it happens that the Medicean stars, while completing their very small revolutions around Jupiter, are themselves now and then seen twice as large. We can in no way seek the cause in terrestrial vapors, for the stars appear larger and smaller when the 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. 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 larger 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. From what we have said above as well as from those things that will be discussed  more amply in our system, it is moreover certain that not only the Earth but also the Moon has its surrounding vaporous orb. 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."" Lack of time prevents me from proceeding further. The fair reader may expect more about these matters soon.
[*] Gaileo Galilei (1610), Siderius Nuncius, or the Sidereal Messenger, tr. by Albert van Helden (Chicago: Univ. of Chicago Press, 1989)