A letter from Christian Huygens which was sent to us by the worthy Mr. Hartlib; in it he writes to a friend in England about his new observation about the Moon of Saturn.
On the 25th. March in the year 1655, when observing the planet Saturn through the dioptric tube, I noticed, beyond the appendages or arms which cling to it on either side, a certain small star standing close by from the west about 3 scrupuli distant; and it was situated near the straight line that had been drawn through both the arms. And when I was undecided as to whether perhaps it was a planet of the same kind as those four
which move around Jupiter, I took note of the position of Saturn and of the small star, and also of the position of each of them in relation to a certain other, which was located almost as far distant in the other direction from Saturn; I thought that the one nearer [to Saturn]
, rather than the one farther from it, would be a fixed star, because it deflected in a straight line from the one to which I referred. Nor was I mistaken in my view on it. For the next day by making repeated observations and when it faced the west , I discovered that the star was at the same time close to Saturn, separated from it by the same distance as before and that the other had receded to almost twice the previous distance. From this I seemed to myself to understand that the farther one was indeed one of the fixed stars, but that the nearer was more distant from Saturn (which at that particular time was receding) but had, however, at the same time progressed with it and was present with it as a satellite. But by the observations of the following 4 days all doubt was removed. For from that time through 3 continuous months, as often as the stillness of the air allowed me, I observed the new planet and showed it to my friends, now to the right of Saturn, now to the left, and having noted down my observations in commentaries I came to the conclusion that it completed its orbit on the 16th. day. The total digression seemed a little less than 3o
and when it reached that point it was the most visible. But when it approaches Saturn, passing through before it, or nearly so, it lies hid for two days because of [Saturn's]
brightness. But the orbit of the planet is so exactly measured at 16 days that, now that a year and more has passed since the first observations, and it has not so far been discovered either to wax or wane, it stations itself in the sky exactly in the position that we predict. I know that many years before Antonius de Reita attributed not just one but six wandering stars to Saturn. But it is observed that he had been just as mistaken as regards these as he was about those others which he had placed close to the Medicæan planets of Jupiter, because, although the renowned Johannes Hevelius shows that he was using a better telescope, his perception, after very frequent and diligent observation, was that there was no satellite present with Saturn. For this he openly admits. Apart from Reita, no one, as far as I know has produced anything similar about Saturn. For the twin stars to the side [of Jupiter]
which Galileo uncovered have been found to be something far different from what it seemed they were on first appearance. What they may be, then, is uncertain, and Astronomers do not as yet venture an opinion. But for me a new phenomenon of Saturn's day opened the way to these matters also, and at length we found out the reason why sometimes Saturn may be kept in the middle as though between two appendages, while at other times it may stretch out seemingly straight arms and then sometimes may lose them all and be found to be spherical, as it was observed in the year 1642, and now again continues to be in its three-monthly appearance. And it will not indeed be difficult in future to designate the times of these changes
if we may be allowed to wait for two months' observations to see if they accord with our hypothesis. For we expect that the arms will be recreated in Saturn towards the end of April, if not before, not those curved arms of the kind that are depicted by Francis Fontana and Hevelius, but extending a straight line on both sides, if anyone of better repute is seen to have made the observation. For if you employ the ordinary [telescopes]
, they represent the two spheres just as they were first put forward by Galilæo. But if you use that one with which we found our satellite of Saturn, it magnifies the diameter fifty times to the eye and is twelve feet in length: and afterwards we constructed one of double its length and a hundred times it magnification. However since longer telescopes than these, namely of 30 and 40 feet are said to be built by others, some problem may be assumed, either that there are faults in the lenses, or that they do not relate to one another in the proper ratio. For the new satellite of Saturn would not otherwise have escaped their focus.
We shall then publish together the observations collected during this and the previous year once we have completed the whole system of Saturn. Meanwhile it seemed right to record its summary in the following puzzle, and if anyone should happen to think he has discovered that system let him have the courage to declare so, lest he by us or we by him be said to have appropriated it. aaaaaaa ccccc d eeeee g h iiiiiiilllm m nnnnnnnnn oooo pp q rr sttttt uuuu