The Essential Galileo

by Galilei, Galileo, Finocchiaro, Maurice A.

  [83] On 16 January at the first hour of the night, I saw three stars arranged in this order:

  Jupiter was between two of them that were at a distance of forty seconds from the planet on either side, and the third was west of Jupiter at a distance of eight minutes. The stars near to Jupiter appeared brighter than the star further off, but not larger.

  On 17 January at thirty minutes after sunset, the configuration was of this kind:

  There was only one star to the east, at a distance of three minutes from Jupiter; to the west likewise there was only one star, distant eleven minutes from Jupiter. The star on the east appeared twice as large as the star to the west; and there were no more than these two. But four hours later, that is, at almost the fifth hour, a third star began to emerge on the east side, which I think before its appearance had been joined with the former of the two other stars. The position was this:

  The middle star was very near indeed to the star on the east, namely only twenty seconds from it; and it was a little towards the south of the straight line drawn through the two outermost stars and Jupiter.

  On 18 January at twenty minutes after sunset, the appearance was such as this:

  The star to the east was larger than the western one, and it was at a distance from Jupiter of eight minutes, the western one being at a distance of ten minutes.

  On 19 January at the second hour of the night, the relative position of the stars was such as this:

  That is, there were three stars exactly in a straight line with Jupiter, one to the east at a distance of six minutes from Jupiter. Between Jupiter and the first star to the west, there was an interval of five minutes. This star was four minutes off the other one more to the west. At that time I was doubtful whether or not there was a star between the star 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 star distinctly, [84] by that time occupying exactly the middle position between Jupiter and the eastern star, so that the configuration was thus:

  Moreover, the star that had just come into view was very small. But at the sixth hour it was nearly as large as the rest.

  On 20 January at one hour and fifteen minutes, the arrangement was like this:

  There were three stars, so small as scarcely to be visible. Their distances from Jupiter, and from one another, were not more than one minute. I was doubtful whether on the western side there were two stars or three. At about the sixth hour, they were grouped in this way:

  The eastern star was twice as far away from Jupiter as before, that is two minutes. On the western side, the star in the middle was distant from Jupiter forty seconds, and from the star still further to the west twenty seconds. Finally, at the seventh hour, three stars were seen on the western side:

  The star nearest to Jupiter was distant from the planet twenty seconds. Between this one and the star furthest to the west there was an interval of forty seconds. But between these another star was in view slightly southward of them, and not more than ten seconds off the most westerly star.

  On 21 January at thirty minutes, there were three stars on the east side; they and Jupiter were at equal distances apart:

  The intervals were estimated to be fifty seconds each. There was also one star on the west, distant four minutes from Jupiter. The star on the east side nearest to Jupiter was the smallest of all.

  On January 22 at the second hour, the arrangement of the stars was this:

  There was an interval of five minutes from the star on the east to Jupiter, [85] and from Jupiter to the star furthest to the west seven minutes. The two interior stars on the western side were forty seconds apart, and the star nearer to Jupiter was one minute from the planet. The inner stars were smaller than the outer ones, but they were situated all in the same straight line, along the zodiac, except that the middle of the three western stars was slightly to the south of it. However, at the sixth hour of the night, they appeared in this position:

  The star on the east was very small, at a distance from Jupiter of five minutes, as before. But the three stars on the west were separated by equal distances from Jupiter and from each other; and the intervals were nearly one minute and twenty seconds each. The star nearest Jupiter appeared smaller than the other two on the same side, but they all appeared arranged exactly in the same straight line.

  On 23 January at forty minutes after sunset, the configuration of the stars was nearly after this fashion:

  There were three stars with Jupiter in a straight line along the zodiac, as they have always been. Two were on the east of the planet, one on the west. The star furthest to the east was seven minutes from the next one; this star was two minutes forty seconds from Jupiter; and the star on the west was three minutes twenty seconds from Jupiter. They were all of nearly the same size. But at the fifth hour the two stars that had been previously near Jupiter were no longer visible, being, I suppose, hidden behind Jupiter; and the appearance was this:

  On 24 January three stars, all on the east side, were visible and almost, but not quite, in the same straight line with Jupiter, for the star in the middle was slightly to the south of it:

  The star nearest to Jupiter was two minutes from it; the next was thirty seconds from that star; and the third was nine minutes further still. They were all very bright. But at the sixth hour only two stars presented themselves, in this position:

  namely, in the same straight line with Jupiter exactly. [86] The distance of the nearest to it had lengthened to three minutes, and the next was eight minutes further off. Unless I am mistaken, the two stars previously observed in the middle had come together and appeared as one.

  On 25 January at one hour and forty minutes, the stars were grouped thus:

  There were only two stars on the east side, and they were rather large. The star furthest to the east was 5 minutes from the star in the middle, and it was six minutes from Jupiter.

  On 26 January at forty minutes, the relative positions of the stars were thus:

  Three stars were in view, of which two were east and the third west of Jupiter. This third star was three minutes from the planet; on the east side, the star in the middle was at a distance of five minutes and twenty seconds, and the further star was six minutes 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 star was emerging on the east side near Jupiter. It was smaller than the rest and was then at a distance of thirty seconds from Jupiter; but it was raised a little above the straight line towards the north, as the following figure shows:

  On 27 January at one hour after sunset, a single star was in view, on the east side of Jupiter, in this position:

  It was very small and at a distance of seven minutes from Jupiter.

  On 28 and 29 January, owing to the interference of clouds, I could make no observation.

  On 30 January at the first hour of the night, the stars were in view, arranged in the following way:

  There was one star on the east side, at a distance of two minutes and thirty seconds from Jupiter; and there were two stars on the west, of which the one nearer to Jupiter was three minutes off the planet, and the other star one minute further. The outer stars and Jupiter were in the same straight line; but the star in the middle was a little above it to the north. The star furthest to the west was smaller than the rest.

  [87] On the last day of the month, at the second hour, two stars on the east side were visible, and one on the west:

  Of the stars east of the planet, the one in the middle was two minutes and twenty seconds from Jupiter, and the star further to the east was thirty seconds from the middle star; the star on the west was ten minutes from Jupiter. They were in the same straight line nearly, except that the star on the east nearest to Jupiter was raised a little towards the north. At the fourth hour, the two stars on the east were even closer together, for they were only twenty seconds apart, thus:

  In these two observa
tions, the western star appeared rather small.

  On 1 February at the second hour of the night, the arrangement was the following;

  The star furthest to the east was six minutes from Jupiter, and the western star eight minutes. On the east side, there was a very small star at a distance of twenty seconds from Jupiter. They made a perfectly straight line.

  On 2 February the stars were seen arranged thus:

  There was only one on the east, at a distance of six minutes from Jupiter, which was four minutes from the nearest star on the west; and between this star and the one further to the west there was an interval of eight minutes. They were in the same straight line exactly and were nearly of the same magnitude. But at the seventh hour there were four stars, two on each side of Jupiter:

  Of these stars, the most easterly was four minutes from the next; this star was one minute forty seconds from Jupiter, which was six minutes from the nearest star on the west; and this one was eight minutes from the star further to the west. And they were all again in the same straight line, drawn along the zodiac.

  On 3 February at the seventh hour, the stars were arranged in the following way:

  The star on the east was one minute and thirty seconds from Jupiter; the nearest star on the west, two minutes; [88] and there was a long distance, ten minutes, from this star to the star further to the west. They were exactly in the same straight line and of equal magnitude.

  On 4 February at the second hour, four stars attended Jupiter, two on the east and two on the west, arranged in one perfectly straight line, as in the following figure:

  The star furthest to the east was three minutes from the next; this one was forty seconds from Jupiter, which was four minutes from the nearest star on the west; and this one was six minutes from the star further on the west. In magnitude they were almost equal, but the star nearest to Jupiter was rather smaller in appearance than the rest. Then at the seventh hour, the eastern stars were only thirty seconds apart:

  Jupiter was two minutes from the nearest star on the east and four minutes from the next star on the west; and this one was three minutes from the star further to the west. They were all equal in magnitude and arranged in a straight line drawn along the ecliptic.

  On 5 February the sky was cloudy.

  On 6 February only two stars appeared, with Jupiter between them, as is seen in the following figure:

  The star on the east was two minutes from Jupiter, and the one on the west three minutes. They were in the same straight line with Jupiter and equal in magnitude.

  On 7 February there were two stars by the side of Jupiter, both of them on the east of the planet, arranged in this manner:

  The intervals between the stars and from Jupiter were equal, namely, one minute; and a straight line would go through them and the center of Jupiter.

  On 8 February at the first hour, there were three stars, all on the east side of Jupiter, as in this diagram:

  The nearest to Jupiter, a rather small one, was one minute and twenty seconds from the planet; the middle one was four minutes from this star and was rather large; the star furthest to the east, a very small one, was twenty seconds from the second star. It was doubtful whether the one nearest to Jupiter [89] was a single star or two starlets, for sometimes it seemed that there was another star next to it further east, extraordinarily small and only ten seconds from it. They were all situated on a straight line drawn along the zodiac. At the third hour, the star nearest to Jupiter was almost touching the planet, for it was only ten seconds from it; but the others had gone further off, for the middle one was six minutes from Jupiter. Finally, at the fourth hour, the star that was previously the nearest to Jupiter joined with the planet and disappeared.

  On 9 February at thirty minutes, there were two stars on the east side of Jupiter and one on the west, in an arrangement like this:

  The star furthest to the east, which was a rather small one, was four minutes from the next star. The star in the middle was larger and seven minutes from Jupiter. And Jupiter was four minutes from the western star, which was a small one.

  On 10 February at one hour and thirty minutes, a pair of stars, very small, and both on the east side of the planet, were visible, in the following configuration:

  The further star was ten minutes from Jupiter; the nearer, twenty seconds; and they were in the same straight line. But at the fourth hour the star 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; it was further from Jupiter than before, since its distance was now twelve minutes.

  On 11 February at the first hour, there were two stars on the east and one on the west:

  The western star was four minutes from Jupiter; the star on the east nearest to the planet was likewise four minutes from Jupiter; and the star further to the east was eight minutes from this one. They were fairly clear to view and in the same straight line. But at the third hour, the fourth star was visible near to Jupiter on the east, less in magnitude than the rest, separated from Jupiter by a distance of thirty seconds, and slightly to the north out of the straight line drawn through the rest:

  They were all very bright and extremely distinct. At the fifth hour and a half, the star on the east nearest to Jupiter had moved further away from the planet, [90] and was occupying a position midway between the planet and the neighboring star further to the east. They were all in the same straight line exactly, and of the same magnitude, as may be seen in the following diagram:

  On 12 February at forty minutes, a pair of stars on the east and likewise a pair on the west were near the planet:

  The star on the east further removed from Jupiter was at a distance of ten minutes, and the further of the stars on the west was eight minutes off. They were both fairly distinct. The other two were very near to Jupiter and very small, especially the star to the east, which was forty seconds from Jupiter; the distance of the western star was one minute. But at the fourth hour, the star that was nearest to Jupiter on the east was no longer visible.

  On 13 February at thirty minutes, two stars were visible in the east and two also in the west:

  The star on the east near Jupiter was fairly distinct; its distance from the planet was two minutes. The star further to the east was less noticeable; it was four minutes from the other. Of the stars on the west, the one furthest from Jupiter, which was very distinct, was parted from the planet by four minutes. Between this star and Jupiter intervened a small star close to the most westerly star, being not more than thirty seconds off. They were all in the same straight line, corresponding exactly to the direction of the ecliptic.

  On 15 February (for on the 14th the sky was covered with clouds) at the first hour, the position of the stars was thus:

  That is, there were three stars on the east, but none were visible on the west. The star on the east nearest to Jupiter was fifty seconds from the planet; the next was twenty seconds from this star; and the furthest to the east was two minutes from the second star. The third star was larger than the others, and those nearer to Jupiter were very small. Then about the fifth hour, only one of the stars that had been near to Jupiter was to be seen; its distance from Jupiter was thirty seconds. The distance of the star furthest to the east from Jupiter had increased, for it was then four minutes:

  But at the sixth hour, besides the two [91] situated as just described on the east, one star was visible towards the west, very small, at a distance of two minutes from Jupiter:

  On February 16, at the sixth hour, their places were arranged as follows:

  That is, the star on the east was seven minutes from Jupiter, which was five minutes from the next star on the west, and this was three minutes from the remaining star still further to the west. They were all approximately of the same magnitude, rather bright, and in the same straight line, corresponding accurately to the direction of the zodiac.

  On 17 February at the first hour, two stars were in view, one on the east, th
ree minutes from Jupiter; the other on the west, ten minutes distant:

  The latter was somewhat smaller than the star on the east. But at the sixth hour, the eastern star was nearer to Jupiter, being at a distance of fifty seconds, and the western star was further off, namely twelve minutes. At both observations, they were in the same straight line with Jupiter, and were both rather small, especially the eastern star in the second observation.

  On 18 February at the first hour, three stars were in view, of which two were on the west and one on the east:

  The distance of the eastern star from Jupiter was three minutes, and of the nearest star on the west two minutes; the remaining star, still further to the west, was eight minutes from the middle star. They were all in the same straight line exactly, and of about the same magnitude. Then at the second hour, the stars nearest to the planet were at equal distances from Jupiter, for the western star was now also three minutes from the planet. At the sixth hour, the fourth star was visible between the star on the east and Jupiter, in the following configuration:

  The star furthest to the east was three minutes from the next; this one was one minute and fifty seconds from Jupiter, which was three minutes from the next star on the west; and the latter was seven minutes from the star still further to the west. They were about equal in magnitude, except that the star on the east nearest to Jupiter was a little smaller than the rest. And they were all in the same straight line parallel to the ecliptic.