The sidereal messenger of Galileo Galilei

by Galileo Galilei; Johannes Kepler

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.