VOL. XI.] PHILOSOPHICAL TRANSACTIONS. 325 



and another superior to him ; and this being compared to the centre of the ap- 

 parent disk of Jupiter, is sometimes turned to the south, sometimes to the 

 north, by a perpetual change of inclination to our visual ray. Galilaeo be- 

 lieved formerly, that he had found rules for this phenomenon, or perpetual 

 change of inclination, by supposing the planes of those circles to be always 

 parallel to the ecliptic ; for by Galilaeo's supposition, the satellites in the 

 superior part of their circles should have their latitude, in respect of the 

 centre of Jupiter, always contrary to the latitude of Jupiter in respect of 

 the ecliptic; which the observations of this year contradict, forasmuch as 

 the satellites, being in the superior part of their circles, near to their con- 

 junction with Jupiter, have also the meridional latitude in respect of his centre, 

 as Jupiter has since the month of March, in respect of the ecliptic— The con- 

 trariety of latitude between one satellite being in the superior part of his circle, 

 and another being in the inferior part of his, is more sensible in the encounter 

 of a direct, which is always superior, with a retrograde, which is always inferior, 

 and particularly near to Jupiter. 



Signor Cassini foresees, 1. That at the end of March next, the satellites 

 will no more have any latitude in respect of Jupiter's centre, and that they will 

 appear in a straight line in all their configurations between themselves and with 

 Jupiter, and will eclipse each other: which, according to Galilaeo, should have 

 come to pass ever since the first months of this present year, when Jupiter 

 passed from the north- side to that of the south, and not the next year, when 

 Jupiter will have a great southern latitude. 2. That the straight line of the 

 satellites will be inclined to the ecliptic, contrary to the Galilasan hypothesis. 



3. That this disposition of the satellites, in a straight line in their encounter, 

 will last but a few days, though Galilaeo assures us that it lasts many months. 



4. That the next summer the situation of the circles of the satellites will be 

 found inverted, in respect of that which they have now ; for, the superior 

 semicircles which at present are turned to the south, will then be turned to 

 the north : which will overthrow the hypotheses of Marius and Hodierna, 

 who suppose them always turned the same way. 



These observations will serve to verify the nodes of the orbs of the satellites 

 with the orbit of Jupiter, and the obliquity of the one to the others ; which are 

 the two keys to the theory of the satellites. Signor Cassini settles these nodes 

 towards the 13th degree of Leo and Aquarius; but Galilaeo supposed them 

 always to be with the nodes of Jupiter, which are towards the beginning of 

 Cancer and Capricorn. He finds the obliquity of their circles to the orbit of 

 Jupiter almost double to the obliquity of this orbit to the ecliptic ; whereas 

 Galilaeo supposes it equal. — Lastly, Cassini retracts the motion, which he in- 



