Tuesday, 26 March 2013

Early Views of Saturn: Cassini Populates the Saturnian System

My last blog entry talked about the first two Saturnian years between Galileo's first observations of Saturn's unusual 'appendages' and acceptance of Huygen's explanation of these features as a broad, flat ring around the planet's equator.  In 1656 Huygens had also discovered Saturn's largest moon, Titan.  But the latter part of the 17th century was dominated by the observations and discoveries of one Jean Dominique Cassini (1625-1712), who later gave his name to the most sophisticated spacecraft ever sent into the outer solar system.
Jean Dominique Cassini.

Cassini was an Italian, and served as professor of astronomy at Bologna for 19 years, distinguishing himself as a great observer of the planets.  He was invited to Paris in 1669 to become the first director of the new observatory, and became a naturalised French citizen, changing his name from Giovanni Domenico to Jean Dominique.  Cassini's first Saturnian satellite was discovered in 1671, just as the planet was passing the northern autumnal equinox (i.e., ring plane crossing) for the third time since Galileo's first observations.  The discovery using a 17-foot telescope tube, was record in Phil. Trans. volume 8, p5178 - Cassini compared the motion of a new point of light, compared to Saturn and Huygen's discovery of Titan:  its distance to Saturn increased westward from October 25th to November 1st, and then decreased in distance until November 6th.  He noted that the satellite had a long orbital duration (80 days), a maximum distance from Saturn of 10.5x the diameter of the ring, and an inclination to the plane of Saturn's equator.  Cassini had made his first discovery - the icy satellite Iapetus.

The discovery of Rhea came less than a year later in 1672, reported in Phil. Trans. volume 12, p. 831.  Cassini measured an orbital period of 4.5 days, and an orbital distance of 5/3 the ring diameter.  One remark shows how difficult these observations were:  "The apparent magnitude of these planets is so little, that posterity will have cause to wonder, that their discovery was begun by a glass of 17 foot."  In the same report, Cassini made an astonishing discovery - Iapetus was not always so readily visible, and suggested that one side was less effective at reflecting sunlight than the other, and that Iapetus orbited with one face always towards Saturn.  The bright trailing hemisphere was visible when Iapetus was on the western side of Saturn, the dark leading hemisphere when on the eastern side.  So Iapetus' two-tone appearance was first observed 332 years before the spacecraft arrived!  The dark terrain is named Cassini Regio in his honour.

Cassini's diagram showing the gap in the rings and the SEB,
Phil. Trans. volume 11, 1676.
Saturn's rings continued to open up towards southern summer solstice, and Cassini's report in Phil. Trans. volume 11, in August 1676 reported two new discoveries:  the first suggestions of a South Equatorial Belt ("we have discerned on the globe of Saturn a dusky zone [zona obscura], a little farther south than the centre, similar to the zones of Jupiter"), and of a division in Saturn's ring ("also the breadth of the ring was divided into two parts [dividebatur bifarium] by a dark line, apparently eliptical but in reality circular, as if into two concentric rings").  He may have discovered further atmospheric banding in later years, but the dark line separating the A (outer) and B (inner) rings is called the Cassini Division in his honour.

Dione and Tethys were discovered in 1684, as Saturn's northern winter hemisphere was slowly approaching the spring equinox (the end of Saturn's 'third year').  These two orbited closer to Saturn than Rhea, and completed a complement of four icy satellites (in addition to Titan) that Cassini wished to name for his sponsor, Louis XIV.  Their names were actually suggested from classical mythology by John Herschel in the 19th century.

Huygens and Cassini continued to observe the Saturnian system through its fourth year (1685-1715) since Galileo's observations, and in 1714 the ring ansae were again observed to disappear at the northern spring equinox.  As the end of the 17th century was dominated by Cassini's discoveries, the end of the 18th would be dominated by another great astronomer:  William Herschel, subject of the next blog post.

Summary of Saturn Years, Measured from Spring Equinox (Heliocentric Longitude of Zero)
Saturn Year One:  1597-1627:  Galileo discovers Saturn's 'strange appendages'.
Saturn Year Two:  1627-1656:  Several theories proposed to explain Saturn's servants.
Saturn Year Three:  1656-1685:  Huygen's solves the Saturn problem and discovers Titan, Hooke and Flamsteed observe Saturn's ring progression; Cassini discovers Iapetus, Rhea, Dione and Tethys.
Saturn Year Four:  1685-1715:  Repeated observations of the new satellites, early evidence of C ring.
Saturn Year Five:  1715-1744:  Rev. James Bradley accurately measures ring diameters and satellite orbits.

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