The sidereal messenger of Galileo Galilei

by Galileo Galilei; Johannes Kepler

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.