Monsieur Cassini, after he had discovered (by the means of those Excellent Glasses of 50. palmes, or 35. feet, made by M. Campani) the Shadows, cast by the 4 Moons or Satellits of Jupiter upon his Diske, when they happen to be between the Sun and Him; after he had also distinguished their Bodies upon the Diske of Jupiter; made the last year some Prædictions for the Months of August and September, noting the dayes and hours, when the Bodies of the said satellits and their Shadows should appear upon Jupiter, to the end that the Curious might be convinced of this matter by their own Observations.
Some of these Prædictions have been verified not only at Rome, and in other places of Italy, but also at Paris by M. Auzout, the most Celebrated and the most Exact of our Astronomers; and in Holland, by M. Hugens. And we can now doubt no longer, of the rotation of the Satellits about Jupiter, as the Moon turns about the Earth; nor believe, that Jupiter or his Attendants have any other Light, than that, which they receive from the Sun; as some did
assure before these Observations. There remained to find by Experience, whether Jupiter did turn about his Axis, as many believe, that the Earth turns about her's. And although most Astronomers had conjectur'd, it did so, either by this Analogy, or by other Congruities, yet it was much wish'd, that we might be assured thereof by Observations. And this it is, for which we are obliged to M. Cassini, who, having by the advantage of the same Glasses discover'd several changes, as well in the three obscure Belts, commonly seen in Jupiter, as in the rest of his Diske, and having also observed Spots in the midst of that Planet, and sometimes Brightnesses, such as have bin formerly seen in the Sun, hath at length discover'd a Permanent Spot in the Northern part of the most Southern Belt; by the means whereof, he hath concluded, that Jupiter turns about his Axis in 9. hours, 56. minutes, and makes 29. whole circumvolutions in 12 dayes 4. minutes of ours, and 360 in 149. dayes. For he has found, that this Spot was not caused by the Shadow of any Satellit, as well by reason of its Situation, as because it appeared, when there could be no Shadow. Besides, that its motion differed from that of the Shadows, which is almost equal, as well towards the Edges as towards the Middle of Jupiter: Whereas, on the contrary, this Spot hath all the accidents, that must happen to a thing, which is upon the surface of a round Body moving; for example, to move much more slowly towards the Edges, than towards the Middle, and to pass over that part, which is in the middle of the Diske, equal to the half of the Diameter, in the sixth part of the time, it takes to make the whole revolution: he having seen this half pass'd over, in 99 or 100 minutes just, as it must happen, supposing the whole circumrotation is made in 9. hours 56. minutes.
He hath not yet been able to determine the Situation of the Axis, upon which this motion is made, because the Belts, according to which it is made, have for some years appeared streight, though in the precedent years, other Astronomers have seen them a little crooked: Which sheweth, that the Axis of the diurnal motion of Jupiter is a little inclined to the plain of the Ecliptick. But in time we may discover, what certainty there is in this matter.
These Tables are not yet sent over, but, 'tis hoped, will be, ere long.
After this excellent Discovery, he hath calculated many Tables, whereof he gives the Explication and Use in the Letters by him addressed to the Abbot Falconieri. By the means of them, one may know, when this Spot may be seen by us. For, having first
considered it in relation to the Sun, in respect whereof, its motion is regular, he considers the same in relation to the Earth, where We observe it; and shews by the means of his Tables, what is to be added or subtracted, to know, at what time the said Spot is to come into the middle of Jupiter's Diske, according as he is Oriental or Occidental. He hath also considered it in relation to an unmovable point, which he has supposed to be the first point of Aries, because we thither refer here upon Earth the beginning of all the Celestial motions, and there is the Primum mobile, that one would imagine, if we were in Jupiter, as we do here imagine Ours of 24. hours.
The Discovery is one of the best, that have been yet made in the Heavens; and those, that hold the Motion of the earth, find in it a full Analogy. For, Jupiter turning about the Sun, does nevertheless turn about his Axis; and although he be much bigger than the Earth, he does nevertheless turn much more swiftly than it, since he makes more than two Turns, and a third part, for its one; and carries with him 4. Moons, as the Earth does one.
This Observation ought to excite all Curious persons to endeavour the perfecting of Optick Glasses, to the end that it may be discovered, whether the other Planets, as Mars, Venus and Mercury, about whom no Moon hath as yet been discovered, do yet turn about their Axes, and in how much time they do so; especially Mars, in whom some Spot is discover'd, and Venus, wherein M.Burattini hath signified from Poland, he has observ'd Inequalities, as in the Moon.
It will be worth while, to watch for the seeing of Jupiter again this Spring, that this happy Observation may be confirmed in divers places, and endeavours used to make new ones.