The Mathematical Principles of Natural Philosophy, Book III: Of the System of the World
Phænomenon II
That the circumsaturnal planets, by radii drawn to Saturn’s centre, describe areas proportional to the times of description; and that their periodic times, the fixed stars being at rest, are in the sesquiplicate proportion of their distances from its centre.
For, as Cassini from his own observations has determined, their distances from Saturn’s centre and their periodic times are as follow.
The periodic times of the satellites of Saturn.
1d. 21h. 18’ 27’’. 2d. 17h. 41’ 22’’. 4d. 12h. 25’ 12’’. 15d. 22h. 41’ 14’’.
79d. 7h. 48’ 00’’.
The distances of the satellites from Saturn’s centre, in semi-diameters of its ring.
From observations 8 24
From the periodic times 1,93 2,47 3,45 8 23,35
The greatest elongation of the fourth satellite from Saturn’s centre is commonly determined from the observations to be eight of these semi-diameters very nearly. But the greatest elongation of this satellite from Saturn’s centre, when taken with an excellent micrometer in Mr. Huygens’ telescope of 123 feet, appeared to be eight semi-diameters and of a semi-diameter. And from this observation and the periodic times the distances of the satellites from Saturn’s centre in semi-diameters of the ring are 2,1. 2,69. 3,75. 8,7. and 25,35. The diameter of Saturn observed in the same telescope was found to be to the diameter of the ring as 3 to 7; and the diameter of the ring, May 28–29, 1719, was found to be 43’’; and thence the diameter of the ring when Saturn is at its mean distance from the earth is 42’’, and the diameter of Saturn 18’’. These things appear so in very long and excellent telescopes, because in such telescopes the apparent magnitudes of the heavenly bodies bear a greater proportion to the dilatation of light in the extremities of those bodies than in shorter telescopes. If we, then, reject all the spurious light, the diameter of Saturn will not amount to more than 16’’.