The Discovery of a Moon of Saturn

[The Hague, 1656]
 

When, on 25 March 1655, I observed the planet Saturn through a telescope, my attention was caught by a little star that was very close to it on the West, outside the ears or arms that protrude from the planet on either side. The little star was at a distance of about three arc-minutes, and its position was marked by a straight line that could be drawn through both arms. And because I wondered if this was perhaps a planet of the same sort of which four go around Jupiter, I made a note of the position of Saturn and this little star, as well as their positions with respect to another star that was about equally distant from Saturn, but on the other side. This star, as opposed to the first one, I held to be a fixed star because it was not on that straight line.

I was not mistaken. When I repeated the observation the following day, I determined that the star that was to the West still had the same position with respect to Saturn, at the same distance as before. The other star had, however, receded to almost double the distance from the first one. From this I believed I had to conclude that this latter one was one of the fixed stars, which had been left further behind by Saturn (which was then in its retrograde arc), while the former, which moved together with the planet, accompanied it as a companion [or satellite].

Through further observations during the following days, all doubt was removed. For from that time, for three months, whenever the heaven was sufficiently clear, I have observed the new planet and showed it to my friends, while it was now to the right and now to the left of Saturn. And from the notes that I made of my observations, I learned that it completed its orbit in sixteen days. The greatest distances at which it was seen from Saturn was a little less than three arc-minutes. At that point of its path it was most easily visible, but when it approached closely to Saturn in order to pass in front or behind it, it hid itself for two days because of the brightness of the planet. The period of sixteen days so accurately measures the orbit of the planet, that now when a year and more has passed since the discovery, no deficit or remainder can be found: in the place we predicted, there it is in the heavens.

I know that some years ago Anton Maria de Rheita attributed not one, but as many as six satellites to Saturn. But he was mistaken about this, as he was mistaken in the five planets that, according to him, revolve around Jupiter in addition to the Medicean Planets. This is clear from the fact that the renowned Hevelius, although he demonstrates that he used a better telescope, could not detect a single companion of Saturn, even though he observed it many times and with great diligence. Hevelius himself has said this voluntarily.

Besides Rheita, no one, as far as I know, has made such a claim about Saturn. For of the two companions that Galileo discovered we know in the meantime that they are something very different from what they appeared to be at first sight. What they in fact are is a riddle, and astronomers do not yet dare to state an opinion.
 

But this new phenomenon of Saturn has shown me the way on this point, too. We have finally discovered the reason why Saturn sometimes is flanked by two things that look like ears, sometimes by something that protrudes as two straight arms, and sometimes also is lacking all this and appears round, as it was seen in the year 1642 and now again for the last three months. And it will not be difficult to determine when these changes will happen again, if we will be allowed two more months of observation, during which we will be able to see if they [the observations] agree with our hypothesis.

For we expect that toward the end of April, or even a bit sooner, Saturn's arms will reappear, not curved, as you can see them in the illustrations of Francesco Fontana and Hevelius, but in a straight line that protrudes on both sides, provided you study them with a superior telescope because if you use ordinary telescopes they will represent them again as two little circles as they showed themselves the first time to Galileo.
 

Our instrument, with which we discovered the companion of Saturn, is twelve feet long and magnifies the diameter of the observed object fifty times. We have since built one of double that length, which magnifies a hundred times. Because, as is reported, even longer telescopes – of thirty and even forty feet – have been built by others, it must be assumed that the lenses had defects or that they did not have the correct mutual proportions, otherwise the new satellite of Saturn would not have escaped the acuity of their sight up to the present.

We will publish the observations that we gathered last year and this year, which show the period of the moon, all together when we have completed the entire system of Saturn. In the meantime, I think it appropriate to hide the main point in the following anagram, so that anyone who perhaps thinks he has found the same will have the opportunity to come forward with it and it cannot be said that he got it from us or we from him.

aaaaaaa ccccc d eeeee g h iiiiiii llll mm nnnnnnnnn oooo pp q rr s ttttt uuuuu

The Hague, 5 March 1656.