The sidereal messenger of Galileo Galilei

by Galileo Galilei; Johannes Kepler

Feb. 11: 1 h.—There were two satellites on the east, and one on the west ([Fig. 42]). The western satellite was at a distance of 4´ from Jupiter. The satellite on the east, nearest to the planet, was likewise 4´ from Jupiter; but the satellite further to the east was at a distance from this one of 8´; they were fairly clear to view, and in the same straight line; but at the third hour the fourth satellite was visible near to Jupiter on the east, less in magnitude than the rest, separated from Jupiter by a distance of 0´ 30´´, and slightly to the north out of the straight line drawn through the rest ([Fig. 43]). They were all very bright and extremely distinct, but at 5 h. 30 m. the satellite on the east nearest to Jupiter had moved further away from the planet, and was occupying a position midway between the planet and the neighbouring satellite further to the east. They were all in the same straight line exactly, and of the same magnitude, as may be seen in the accompanying diagram ([Fig. 44]).

Feb. 12: 0 h. 40 m.—A pair of satellites on the east, a pair likewise on the west, were near the planet ([Fig. 45]). The satellite on the east furthest removed from Jupiter was at a distance of 10´, and the further of the satellites on the west was 8´ off. They were both fairly distinct. The other two were very near to Jupiter, and very small, especially the satellite to the east, which was at a distance of 0´ 40´´ from Jupiter. The distance of the western satellite was 1´. But at the fourth hour the satellite which was nearest to Jupiter on the east was visible no longer.

Feb. 13: 0 h. 30 m.—Two satellites were visible in the east, two also in the west ([Fig. 46]). The satellite on the east near Jupiter was fairly distinct; its distance from the planet was 2´. The satellite further to the east was less noticeable; it was distant 4´ from the other. Of the satellites on the west, the one furthest from Jupiter, which was very distinct, was parted from the planet 4´. Between this satellite and Jupiter intervened a small satellite close to the most westerly satellite, being not more than 0´ 3´´ off. They were all in the same straight line, corresponding exactly to the direction of the ecliptic.

Feb. 15 (for on the 14th the sky was covered with clouds), at the first hour, the position of the satellites was thus ([Fig. 47]); that is, there were three satellites on the east, but none were visible on the west. The satellite on the east nearest to Jupiter was at a distance of 0´ 50´´ from the planet; the next in order was 0´ 20´´ from this satellite, and the furthest to the east was 2´ from the second satellite, and it was larger than the others, for those nearer to Jupiter were very small. But about the fifth hour only one of the satellites which had been near to Jupiter was to be seen; its distance from Jupiter was 0´ 30´´. The distance of the satellite furthest to the east from Jupiter had increased, for it was then 4´ ([Fig. 48]). But at the sixth hour, besides the two situated as just described on the east, one satellite was visible towards the west, very small, at a distance of 2´ from Jupiter ([Fig. 49]).

Feb. 16: 6 h.—Their places were arranged as follows ([Fig. 50]); that is, the satellite on the east was 7´ from Jupiter, Jupiter 5´ from the next satellite on the west, and this 3´ from the remaining satellite still further to the west. They were all of the same magnitude nearly, rather bright, and in the same straight line, corresponding accurately to the direction of the Zodiac.

Feb. 17: 1 h.—Two satellites were in view, one on the east, distant 3´ from Jupiter; the other on the west, distant 10´ ([Fig. 51]). The latter was somewhat less than the satellite on the east; but at the sixth hour the eastern satellite was nearer to Jupiter, being at a distance of 0´ 50´´, and the western satellite was further off, namely 12´. At both observations they were in the same straight line with Jupiter, and were both rather small, especially the eastern satellite in the second observation.

Feb. 18: 1 h.—Three satellites were in view, of which two were on the west and one on the east; the distance of the eastern satellite from Jupiter was 3´, and of the nearest satellite on the west 2´; the remaining satellite, still further to the west, was 8´ from the middle satellite ([Fig. 52]). They were all in the same straight line exactly, and of about the same magnitude. But at the second hour the satellites nearest to the planet were at equal distances from Jupiter, for the western satellite was now also 3´ from the planet. But at the sixth hour the fourth satellite was visible between the satellite on the east and Jupiter, in the following configuration ([Fig. 53]). The satellite furthest to the east was at a distance of 3´ from the next in order; this one was at a distance of 1´ 50´´ from Jupiter; Jupiter was at a distance of 3´ from the next satellite on the west, and this 7´ from the satellite still further to the west. These were nearly equal in magnitude, only the satellite 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.

Feb. 19: 0 h. 40 m.—Two satellites only were in view, west of Jupiter, rather large, and arranged exactly in the same straight line with Jupiter, in the direction of the ecliptic ([Fig. 54]). The nearer satellite was at a distance of 7´ from Jupiter and of 6´ from the satellite further to the west.

Feb. 20.—The sky was cloudy.