Jean-Dominique Cassini

“ Cassini, name of a family of astronomers of the 17th and 18th centuries, of Italian origin; distinguished for their observations and discoveries affecting the comets, the planets, and the moon; they settled, father and son and grandson, in Paris, and became in succession directors of the observatory of Paris, the last of whom died in 1864, after completing in 1793 a great topographical map of France begun by his father.
Cassiodo`rus, a Latin statesman and historian, born in Calabria; prime minister of Theodoric the Great and his successor ”

“ Even so, there must be collisions absorbing the energy of rotation, and tending to make the rings eventually fall into the planet. The slower motion of the external parts has been proved by the spectroscope in Keeler’s hands, 1895.
Saturn has perhaps received more than its share of attention owing to these rings. This led to other discoveries. Huyghens in 1655, and J. D. Cassini in 1671, discovered the sixth and eighth satellites (Titan and Japetus) . Cassini lost his satellite, and in searching for it found Rhea (the fifth) in 1672, besides his old friend, whom he lost again. ”

“ The Sun was behind Cassini, so nearly the full disc was illuminated. South polar clouds are seen at the bottom.
Surface features are best seen near the center of the moon. The surface features become fuzzier toward the outside of the image, where the spacecraft is peering through more haze. The brighter region on the right side near the equator is named Xanadu Regio. Scientists are debating what processes may have created the bizarre surface brightness patterns seen there. Titan's lack of obvious craters is a hint of a young surface. ”

“ Cassini, nurtured in France, soon afterward added four more satellites to the system of Saturn, those now called the first, second, third, and fifth, and he detected the black list, or dark, elliptical line bisecting the surface of the ring, and dividing it into two. Astronomy is under immense obligations to a measure adopted by the courts of France and England at nearly the same period, for the patronage of scientific associations, and the founding of national observatories. ”

“ It is no wonder, then, that men also, whether educated or ignorant, do not escape the impression. A party of the courtiers of Louis XV. is said to have gathered round Cassini to witness an eclipse from the terrace of the Paris observatory, and to have been laughing at the populace, whose cries were heard as the light began to fade; when, as the unnatural gloom came quickly on, a sudden silence fell on them too, the panic terror striking through their laughter. Something common to man and the brute speaks at such times, if never before or again ”

“ That vision by aid of a Telescope or Microscope, is not deceitful: And an Observation of the likeness between the Earth and the Moon.
This same Author in his Letter to Mr F. C. suggests, that Cassini hath observed the Motion of Jupiter about his Axis in 10 hours; of Mars in 23 hours; that Venus hath the like Rotations: but the precise period not yet known; that Cassini hath publish'd Tables of the Motion of the Safellits of Jupiter, with an Ephemeris of the same for this present year: All which are there much applauded. ”

“ Pioneer Saturn image of Saturn and its rings from a distance of 2,800,000 km (August 29, 1979, 72 hours before closest approach) . The Cassini Division at the inner edge of the A-ring is clearly resolved and is bright. The A-ring is dark outside the Cassini Division because it has more particulate matter there. Polar belts are becoming more visible on the face of the planet. Irregularities that appear in the ring silhouette and shadow are due to stepping anomalies in the imaging photopolarimeter and will be removed in further processing. ”

“ It must revive in every one who contemplates anew the unique arrangements of that world apart known to us as the Saturnian system. The resolution of the so-called ansæ, or "handles," into one encircling ring by Huygens in 1655, the discovery by Cassini in 1675 of the division of that ring into two concentric ones, together with Laplace's investigation of the conditions of stability of such a formation, constituted, with some minor observations, the sum of the knowledge obtained, up to the middle of the last century, on the subject of this remarkable formation. ”

“ The difficulties of observation with these unwieldy monsters must have been of the most exasperating type, while their magnifying power did not exceed that of an ordinary modern achromatic of, perhaps, 36 inches focal length. Cassini, for instance, seems never to have gone beyond a power of 150 diameters, which might be quite usefully employed on a good modern 3-inch refractor in good air. Yet with such tools he was able to discover four of the satellites of Saturn and that division in Saturn's ring which still bears his name. Such facts speak volumes for the quality of the observer. ”

“ The conclusion to be drawn from the increase of forces of attraction (in the direction from the equator to the poles) with respect to the figure of a planet is dependent on the distribution of density in its interior. Newton, from theoretical principles, and perhaps likewise prompted by Cassini's discovery, previously to 1666, of the compression of Jupiter,* determined, in his immortal work, 'Philosophiae Naturalis Principia', that the compression of the Earth, as a homogeneous mass, was 1/230th. ”

“ Cassini [1045] and Schröter both noticed that markings on Jupiter travelled quicker the nearer they were to his equator; and Cassini even hinted at their possible assimilation to sun-spots. [1046] It is now well ascertained that, as a rule (not without exceptions) , equatorial spots give a period some 5-1/2 minutes shorter than those in latitudes of about 30°. But, as Mr. Denning has pointed out, [1047] no single period will satisfy the observations either of different markings at the same epoch, or of the same markings at different epochs. ”

“ The B-Ring appears dark from below the ring plane because it is dense enough to reflect most of the sunlight, causing it to appear very bright when seen from the sunward side. The opaline brightness of the Cassini Division here indicates a great deal of sunlight being scattered through this region. ”

“ The spacecraft was below the ring plane during most of the 3-hour observation period and was much closer to the planet at the end of the period (right of image) than at the start (left of image) . Thus, the images are quite distorted. The small-scale pattern is instrument noise. In the left image, the warm infrared radiation from the planet is seen through Cassini’s Division between the A- and B-rings. The brightness level in the right image implies that Saturn emits heat at a rate that is 2.5 times the rate that it absorbs energy from the sun. ”

“ Two days after the close encounter with icy Titan, Cassini captured the images used in the mosaic of the battered and cratered moon Tethys. The result is the best-ever natural color view of Titan. As seen here, the surface of Tethys has a neutral hue. Three images form this natural color composite. The mosaic reveals a world nearly saturated with craters -- many small craters lie on top of older, larger ones, suggesting an ancient surface. Grooves can be seen at the top and along the boundary between day and night. ”

“ 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. ”

“ The rings thus bear to Saturn a relation having some analogy to that which the minor planets bear to the sun; and Kirkwood pointed out in 1867 that Cassini's division between the two main rings can be explained by the perturbations due to certain of the satellites, just as the corresponding gaps in the minor planets can be explained by the action of Jupiter (§ 294) .
The great distance of Uranus and Neptune naturally makes the study of them difficult, and next to nothing is known of the appearance or constitution of either; their rotation periods are wholly uncertain. ”

“ Most of the satellites of Saturn are difficult to be perceived with ordinary telescopes, excepting the 4th, which may be seen with powers of from 60 to 100 times. It was discovered by Huygens in 1655, by means of a common refracting telescope 12 feet long, which might magnify about 70 times. The next in brightness to this is the 5th satellite, which Cassini discovered in 1671, by means of a 17 feet refractor, which might carry a power of above 80 times. The 3rd was discovered by the same astronomer in 1672, by a longer telescope ”

“ A 20-foot objective, power 90, gave Cassini the division in Saturn’s ring.
A struggle was still being kept up for the non-spherical curves urged by Descartes. It is quite evident that Huygens had a go [Pg 19] at them, and Hevelius thought at one time that he had mastered the hyperbolic figure, but his published drawings give no indication that he had reduced spherical aberration to any perceptible degree. At this time the main thing was to get good glass and give it true figure and polish, in which Huygens and Campani excelled, as the work on Saturn witnesses. ”

“ Regarding the outermost member of the system, Japetus, discovered by Cassini in 1671, Herschel noticed, in 1792, a singular circumstance. It was already known to vary in brightness; we receive from it, in fact, four and a-half times more light at certain epochs than at others. The novelty consisted in showing that this variation depended upon the satellite’s situation in its orbit in such a manner as to leave no doubt that, like our moon, it keeps the same face always directed inwards towards its primary. ”

“ In the year 1672, Cassini saw a Star in the Neck of the Bull, which he thought was not visible in Tycho’s time; nor when Bayer made his Figures.
Cannot be Comets. 404. Many Stars, besides those above-mentioned, have been observed to change their magnitudes: and as none of them could ever be perceived to have tails, ’tis plain they could not be Comets; especially as they had no parallax, even when largest and brightest. It would seem that the periodical Stars have vast clusters of dark spots, 237and very slow rotations on their Axis ”

“ Cassini on one occasion failed to find the shadow of satellite I. when it should have been on the planet’s disc, [137] an observation which again points to the glowing light of Jupiter’s surface. Sadler and Trouvelot saw the shadow of satellite I. double! an observation difficult to explain—but the same phenomenon was again seen on the evening of September 19, 1891, by Mr. H. S. Halbert of Detroit, Michigan (U.S.A.) . He says that the satellite “was in transit nearing egress, and it appeared as a white disc against the dark southern equatorial belt ”

“ Whence it is, the some Astronomers have made use of Obelisks of a vast bigness, to take the Altitudes; and Signior Cassini, after the example of Egnatio Dante, caused a hole to be made on the highest part of a Wall of 95. feet in a Church at Bononia, through which the beams of the Sun falling on the Floor, mark as exactly as is possible, the height of that Luminary.
Fifthly, That the Author reasons for the Immobility of the Earth after this manner. He supposes for certain, that the swiftness of the Motion of heavy bodies doth still increase in their descent ”

“ The same observer showed that the disappearance which had so puzzled Galilei arose from the varying inclination in the ring: at times it would become invisible, when presenting its narrow edge to the Earth, and this actually occurred again in 1671, as Huygens had predicted. In 1676 Cassini detected a belt upon the planet, and also a dark division in the ring. Dr. Smith’s ‘Optics’ (1738) thus alludes to these discoveries:—
Fig. ”

“ Revolution. Distance.
Saturn's
radius = 1. Discoverer. Year of
Discovery. d. h. m. s. 1 Mimas 0 22 36 18 3.1408 Herschel 1789 2 Enceladus 0 8 53 3 4.0319 Herschel 1789 3 Tethys 1 21 18 33 4.9926 Cassini 1684 4 Dione 2 17 44 51 6.3990 Cassini 1684 5 Rhea 4 12 25 11 8.9320 Cassini 1672 6 Titan 15 22 41 25 20.7060 Huyghens 1655 7 Hyperion 21 4 20 0 25.0290 Bond & Lassell 1848 8 Japetus 79 7 54 41 64.3590 Cassini 1671 Hyperion discovered by Bond and Lassell on same night, year 1848 Greatest inclination to the plane of the ring (Japetus) ,
12° 14′ Apparent diameter of the largest. ”

“ After he had taken deacon’s orders, however, he devoted himself exclusively to science, and, through the patronage of J. Cassini, obtained employment, first in surveying the coast from Nantes to Bayonne, then, in 1739, in remeasuring the French arc of the meridian. ”

“ The truth came to me in a flash when I first perceived the canoe approaching yonder. I felt that hate rather than love urged Cassion to make search for us. He knew of your attempt at rescue, and if he found us here together alone, he would care for nothing save revenge. He has the power, the authority to condemn you, and have you shot. I saw no way to preserve your life, but to keep you out of his grip, until you were with your friends at Fort St. Louis. ”