Part 4

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, which seems to me to deserve to be considered the most important in this work, namely, that I should disclose and publish to the world the occasion of discovering and observing four PLANETS, never seen from the very beginning of the world up to our own times, their positions, and the observations made during 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. I give them warning however again, so that they may not approach such an inquiry to no purpose, that they will want a very accurate telescope, and such as I have described in the beginning of this account.

On the 7th day of January in the present year, 1610, in the first[14] hour of the following night, when I was viewing the constellations of the heavens through a telescope, the planet Jupiter presented itself to my view, and 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 telescope, namely, that three little stars, small but very bright, were near the planet; and although I believed them to belong to the number of the fixed stars, yet they made me somewhat wonder, because they seemed to be arranged exactly in a straight line, parallel to the ecliptic,[15] and to be brighter than the rest of the stars, equal to them in magnitude. The position of them with reference to one another and to Jupiter was as follows (Fig. 1).

[14] The times of Galileo’s observations are to be understood as reckoned from sunset.

[15] The satellites of Jupiter revolve in planes very nearly, although not exactly, coincident with that of the equator of the planet, which is inclined 3° 5´ 30´´ to the orbit of the planet, and the plane of the orbit is inclined 1° 18´ 51´´ to the ecliptic.

On the east side there were two stars, and a single one towards the west. The star which 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; but when on January 8th, led by some fatality, I turned again to look at the same part of the heavens, I found a very different state of things, for 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 accompanying illustration (Fig. 2) shows.

At this point, although I had not turned my thoughts at all upon the approximation of the stars to one another, yet my surprise began to be excited, 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 became afraid lest the planet might have moved differently from the calculation of astronomers, and so had passed those stars by its own proper motion. I therefore waited for the next night with the most intense longing, but I was disappointed of my hope, for the sky was covered with clouds in every direction.

But on January 10th the stars appeared in the following position with regard to Jupiter; there were two only, and both on the east side of Jupiter, the third, as I thought, being hidden by the planet (Fig. 3). They were situated just as before, exactly in the same straight line with Jupiter, and along the Zodiac.

When I had seen these phenomena, as I knew that corresponding changes of position could not by any means belong to Jupiter, and as, moreover, I perceived that the stars which I saw had been always the same, for there were no others either in front or behind, within a great distance, along the Zodiac,—at length, changing from doubt into surprise, I discovered that the interchange of position which I saw belonged not to Jupiter, but to the stars to which my attention had been drawn, and I thought therefore that they ought to be observed henceforward with more attention and precision.

Accordingly, on January 11th I saw an arrangement of the following kind (Fig. 4), namely, only two stars to the east of Jupiter, the nearer of which was distant from Jupiter three times as far as from the star further to the east; and the star furthest to the east was nearly twice as large as the other one; whereas on the previous night they had appeared nearly of equal magnitude. I therefore concluded, and decided unhesitatingly, that there are three stars in the heavens moving about Jupiter, as Venus and Mercury round the Sun; which at length was established as clear as daylight by numerous other subsequent observations. These observations also established that there are not only three, but four, erratic sidereal bodies performing their revolutions round Jupiter, observations of whose changes of position made with more exactness on succeeding nights the following account will supply. I have measured also the intervals between them with the telescope 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.

Jan. 12.—At the first hour of the next night I saw these heavenly bodies arranged in this manner (Fig. 5). The satellite[16] furthest to the east was greater than the satellite furthest to the west; but both were very conspicuous and bright; the distance of each one from Jupiter was two minutes. A third satellite, 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.

[16] Galileo continues to call these bodies _stars_, perhaps meaning “Medicean stars,” throughout the description of their configurations, but as he had now detected their nature, it is more convenient to call them _satellites_, the term introduced by Kepler.

Jan. 13.—For the first time four satellites were in view in the following position with regard to Jupiter (Fig. 6).

There were three to the west, and one to the east; they made a straight line nearly, but the middle satellite of those to the west deviated a little from the straight line towards the north. The satellite furthest to the east was at a distance of 2´ from Jupiter; there were intervals of 1´ only between Jupiter and the nearest satellite, and between the satellites themselves, west of Jupiter. All the satellites appeared of the same size, and though small they were very brilliant, and far outshone the fixed stars of the same magnitude.

Jan. 14.—The weather was cloudy.

Jan. 15.—At the third hour of the night the four satellites were in the state depicted in the next diagram (Fig. 7) with reference to Jupiter.

All were to the west, and arranged nearly in the same straight line; but the satellite which counted third from Jupiter was raised a little to the north. The nearest to Jupiter was the smallest of all; the rest appeared larger and in order of magnitude; the intervals between Jupiter and the three nearest satellites were all equal, and were of the magnitude of 2´ each; but the satellite furthest to the west was distant 4´ from the satellite nearest to it. They were very brilliant, and not at all twinkling, as they have always appeared both before and since. But at the seventh hour there were only three satellites, presenting with Jupiter an appearance of the following kind (Fig. 8). 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 3´; the distance of the second from this one was 1´; and of the third from the second 4´ 30´´. But after another hour the two middle satellites were still nearer, for they were only 30´´, or less, apart.

Jan. 16.—At the first hour of the night I saw three satellites arranged in this order (Fig. 9). Jupiter was between two of them, which were at a distance of 0´ 40´´ from the planet on either side, and the third was west of Jupiter at a distance of 8´. The satellites near to Jupiter appeared brighter than the satellite further off, but not larger.

Jan. 17, after sunset 0 hours 30 minutes, the configuration was of this kind (Fig. 10). There was one satellite only to the east, at a distance of 3´ from Jupiter; to the west likewise there was only one satellite, distant 11´ from Jupiter. The satellite on the east appeared twice as large as the satellite to the west; and there were no more than these two. But four hours after, that is, nearly at the fifth hour, a third satellite began to emerge on the east side, which, before its appearance, as I think, had been joined with the former of the two other satellites, and the position was of this kind (Fig. 11). The middle satellite was very near indeed to the satellite on the east, and was only 20´´ from it; and was a little towards the south of the straight line drawn through the two extreme satellites and Jupiter.

Jan. 18, at 0 h. 20 m. after sunset, the appearance was such as this (Fig. 12). The satellite to the east was larger than the western one, and was at a distance from Jupiter of 8´, the western one being at a distance of 10´.

Jan. 19.—At the second hour of the night the relative position of the satellites was such as this (Fig. 13); that is, there were three satellites exactly in a straight line with Jupiter, one to the east, at a distance of 6´ from Jupiter; between Jupiter and the first satellite to the west in order, there was an interval of 5´; this satellite was 4´ off the other one more to the west. At that time I was doubtful whether or no there was a satellite between the satellite to the east and Jupiter, but so very close to Jupiter as almost to touch the planet; but at the fifth hour I saw this satellite distinctly, by that time occupying exactly the middle position between Jupiter and the eastern satellite, so that the configuration was thus (Fig. 14). Moreover, the satellite which had just come into view was very small; yet at the sixth hour it was nearly as large as the rest.

Jan. 20: 1 h. 15 m.—A similar arrangement was seen (Fig. 15). There were three satellites, so small as scarcely to be distinguishable; their distances from Jupiter, and from one another, were not more than 1´. I was doubtful whether on the western side there were two satellites or three. About the sixth hour they were grouped in this way (Fig. 16). The eastern satellite was twice as far away from Jupiter as before, that is 2´; on the western side, the satellite in the middle was distant from Jupiter 0´ 40´´, and from the satellite still further to the west 0´ 20´´; at length, at the seventh hour, three satellites were seen on the western side (Fig. 17). The satellite nearest to Jupiter was distant from the planet 0´ 20´´; between this one and the satellite furthest to the west there was an interval of 40´´, but between these another satellite was in view slightly southward of them, and not more than 10´´ off the most westerly satellite.

Jan. 21: 0 h. 30 m.—There were three satellites on the east side; the satellites and Jupiter were at equal distances apart (Fig. 18). The intervals were by estimation 50´´ each. There was also one satellite on the west, distant 4´ from Jupiter. The satellite on the east side nearest to Jupiter was the least of all.

Jan. 22: 2 h.—The grouping of the satellites was similar (Fig. 19). There was an interval of 5´ from the satellite on the east to Jupiter; from Jupiter to the satellite furthest to the west 7´. The two interior satellites on the western side were 0´ 40´´ apart, and the satellite nearer to Jupiter was 1´ from the planet. The inner satellites were smaller than the outer ones, but they were situated all in the same straight line, along the ecliptic, except that the middle of the three western satellites was slightly to the south of it, but at the sixth hour of the night they appeared in this position (Fig. 20). The satellite on the east was very small, at a distance from Jupiter of 5´ as before; but the three satellites on the west were separated by equal distances from Jupiter and from each other; and the intervals were nearly 1´ 20´´ each. The satellite nearest Jupiter appeared smaller than the other two on the same side, but they all appeared arranged exactly in the same straight line.

Jan. 23, at 0 h. 40 m. after sunset, the grouping of the satellites was nearly after this fashion (Fig. 21). There were three satellites with Jupiter in a straight line along the ecliptic, as they always have been; two were on the east of the planet, one on the west; the satellite furthest to the east was 7´ from the next one, and this satellite 2´ 40´´ from Jupiter; Jupiter was 3´ 20´´ from the satellite on the west; and they were all of nearly the same size. But at the fifth hour the two satellites which had been previously near Jupiter were no longer visible, being, as I suppose, hidden behind Jupiter, and the appearance presented was such as this (Fig. 22).

Jan. 24.—Three satellites, all on the east side, were visible, and nearly, but not quite, in the same straight line with Jupiter, for the satellite in the middle was slightly to the south of it (Fig. 23). The satellite nearest to Jupiter was 2´ distant from the planet; the next in order 0´ 30´´ from this satellite, and the third was 9´ further off still; they were all very bright. But at the sixth hour two satellites only presented themselves to view in this position, namely in the same straight line with Jupiter exactly, and the distance of the nearest to the planet was lengthened to 3´; the next was 2´ further off, and unless I am mistaken, the two satellites previously observed in the middle had come together, and appeared as one.

Jan. 25, at 1 h. 40 m., the satellites were grouped thus (Fig. 24). There were only two satellites on the east side, and these were rather large. The satellite furthest to the east was 5´ from the satellite in the middle, and it was 6´ from Jupiter.

Jan. 26, at 0 h. 40 m., the relative positions of the satellites were thus (Fig. 25). Three satellites were in view, of which two were east and the third west of Jupiter; this one was distant 3´ from the planet. On the east side the satellite in the middle was at a distance of 5´ 20´´; the further satellite was 6´ beyond; they were arranged in a straight line, and were of the same size. At the fifth hour the arrangement was nearly the same, with this difference only, that the fourth satellite was emerging on the east side near Jupiter. It was smaller than the rest, and was then at a distance of 0´ 30´´ from Jupiter; but was raised a little above the straight line towards the north, as the accompanying figure shows (Fig. 26).

Jan. 27, 1 h. after sunset, a single satellite only was in view, and that on the east side of Jupiter in this position (Fig. 27). It was very small, and at a distance of 7´ from Jupiter.

Jan. 28 and 29.—Owing to the intervention of clouds, I could make no observation.

Jan. 30.—At the first hour of the night the satellites were in view, arranged in the following way (Fig. 28). There was one satellite on the east side, at a distance of 2´ 30´´ from Jupiter; and there were two satellites on the west, of which the one nearer to Jupiter was 3´ off the planet, and the other satellite 1´ further. The places of the outer satellites and Jupiter were in the same straight line; but the satellite in the middle was a little above it to the north. The satellite furthest to the west was smaller than the rest.

On the last day of the month, at the second hour, two satellites on the east side were visible, and one on the west (Fig. 29). Of the satellites east of the planet, the one in the middle was 2´ 20´´ distant from Jupiter; and the satellite further to the east was 0´ 30´´ from the middle satellite; the satellite on the west was at a distance of 10´ from Jupiter. They were in the same straight line nearly, and would have been exactly so, only the satellite on the east nearest to Jupiter was raised a little towards the north. At the fourth hour, the two satellites on the east were still nearer together, for they were only 20´´ apart (Fig. 30). The western satellite appeared rather small in these two observations.

Feb. 1.—At the second hour of the night the arrangement was similar (Fig. 31). The satellite furthest to the east was at a distance of 6´ from Jupiter, and the western satellite 8´. On the east side there was a very small satellite, at a distance of 20´´ from Jupiter. They made a perfectly straight line.

Feb. 2.—The satellites were seen arranged thus (Fig. 32). There was one only on the east, at a distance of 6´ from Jupiter. Jupiter was 4´ from the nearest satellite on the west; between this satellite and the satellite further to the west there was an interval of 8´; they were in the same straight line exactly, and were nearly of the same magnitude. But at the seventh hour four satellites were there—two on each side of Jupiter (Fig. 33). Of these satellites, the most easterly was at a distance of 4´ from the next; this satellite was 1´ 40´´ from Jupiter; Jupiter was 6´ from the nearest satellite on the west, and this one from the satellite further to the west, 8´; and they were all alike in the same straight line, drawn in the direction of the Zodiac.

Feb. 3: 7 h.—The satellites were arranged in the following way (Fig. 34):—The satellite on the east was at a distance of 1´ 30´´ from Jupiter; the nearest satellite on the west, 2´, and there was a long distance, 10´, from this satellite to the satellite further to the west. They were exactly in the same straight line, and of equal magnitude.

Feb. 4: 2 h.—Four satellites attended Jupiter, two on the east and two on the west, arranged in one perfectly straight line, as in the adjoining figure (Fig. 35). The satellite furthest to the east was at a distance of 3´ from the next satellite. This one was 0´ 40´´ from Jupiter; Jupiter 4´ from the nearest satellite on the west; and this one from the satellite further to the west 6´. In magnitude they were nearly equal; the satellite nearest to Jupiter was rather smaller in appearance than the rest. But at the seventh hour (Fig. 36) the eastern satellites were at a distance of only 0´ 30´´ apart. Jupiter was 2´ from the nearest satellite on the east; and from the satellite on the west, next in order, 4´; this one was distant 3´ from the satellite further to the west. They were all equal in magnitude, and in a straight line, drawn in the direction of the ecliptic.

Feb. 5.—The sky was cloudy.

Feb. 6.—Two satellites only appeared, with Jupiter between them, as is seen in the accompanying figure (Fig. 37). The satellite on the east was 2´ from Jupiter, and that on the west 3´. They were in the same straight line with Jupiter, and were equal in magnitude.

Feb. 7.—There were two satellites by the side of Jupiter, and both of them on the east of the planet, arranged in this manner (Fig. 38). The intervals between the satellites and Jupiter were equal, and of 1´ each; and a straight line would go through them and the centre of Jupiter.

Feb. 8: 1 h.—Three satellites were there, all on the east side of Jupiter, as in the diagram (Fig. 39). The nearest to Jupiter, a rather small one, was distant from the planet 1´ 20´´; the middle one was 4´ from this satellite, and was rather large; the satellite furthest to the east, a very small one, was at a distance of 0´ 20´´ from this satellite. It was doubtful whether there was one satellite near to Jupiter or two, for sometimes it seemed that there was another satellite by its side towards the east, wonderfully small, and only 10´´ from it. They were all situated at points in a straight line drawn in the direction of the Zodiac. At the third hour the satellite nearest to Jupiter was almost touching the planet, for it was only distant 10´´ from it; but the others had become further off, for the middle one was 6´ from Jupiter. At length, at the fourth hour, the satellite which was previously the nearest to Jupiter joined with the planet and disappeared.

Feb. 9: 0 h. 30 m.—There were two satellites on the east side of Jupiter, and one on the west, in an arrangement such as this (Fig. 40). The satellite furthest to the east, which was a rather small one, was distant 4´ from the next satellite; the satellite in the middle was larger, and at a distance of 7´ from Jupiter. Jupiter was distant 4´ from the western satellite, which was a small one.

Feb. 10: 1 h. 30 m.—A pair of satellites, very small, and both on the east of the planet, were visible, in the following position (Fig. 41). The further satellite was distant from Jupiter 10´, the nearer 0´ 20´´, and they were in the same straight line; but at the fourth hour the satellite nearest to Jupiter no longer appeared, and the other seemed so diminished that it could scarcely be kept in sight, although the atmosphere was quite clear, and the satellite was further from Jupiter than before, since its distance was now 12´.