Since pre–historic times man has viewed the relative movement of the Sun, Moon, and the five visible planets against the backdrop of the stars. Saturn is the furthest planet visible with the naked eye from Earth, and appears as a bright yellow disk. The planet’s movement in the sky was first recorded in Mesopotamia in the mid–seventh century BC.
Further observations occurred in the classical world, which the Greeks used to try to work out the shape of the orbits of the planets. Later the Hellenistic astronomer Claudius Ptolemaeus (AD c.100– c.170) made various observations of Saturn. Saturn’s position continued to be recorded into the late eighth century and early ninth century AD by Islamic astronomers in observatories at Damascus and Baghdad. Techniques in observations changed and improved over time, but celestial bodies were still only seen with the naked eye in the sixteenth century by astronomers such as Tycho Brahe (1546–1601). New information on the physical nature of Saturn could not be gathered until astronomers developed a new piece of equipment.
All astronomical observations until the early seventeenth century had to be performed with the unaided eye. Though Saturn’s motion could be followed through the sky and its future positions predicted, Saturn’s disk could not be resolved. But in July 1610 AD the mathematician Galileo Galilei (1564–1642) looked at Saturn through two lenses encased in a tube. Galileo’s telescope could magnify Saturn by only 32 times, and imperfections in the telescope’s lenses meant that Saturn’s Ring appeared as two satellites (see I). During subsequent observations Galileo saw Saturn as a single disk and then with two great handles attached to the planet. In bringing to light the strange appearance of Saturn, Galileo presented astronomers with a problem that took them almost half a century to solve.
GIAN DOMENICO CASSINI (1625–1712)
Gian Domenico Cassini, also known as Jean–Dominique Cassini, was born in Pernaldo, Italy on 8th June 1625. In 1648 he began working at the Panzano observatory near Bologna, and in 1650 he was appointed to the principle chair of astronomy at the University of Bologna. In 1653 Cassini designed and commissioned a new and larger meridian for the church of San Petronio of Bologna. During the period 1656 to 1662 he produced a number of publications based on observations using his innovative and precise meridian. In 1664 Cassini obtained lenses from the famous optical lens makers Giuseppe Campani and Eustachio Divini. He used these in celestial telescopes to make a number of important discoveries relating to the periods of rotation of Jupiter (1664), Mars (1666), and Venus (1667). In 1667 Cassini was invited to join the Academie Royale des Sciences and also to assist in the creation of the Observatoire Royale in Paris. Before leaving for Paris in 1669 Cassini published Emphermerides Bononienses mediceorem siderum (1668): a table of the movements of the satellites of Jupiter.
Once in Paris, Cassini set about establishing the observatory and began numerous observations and discoveries, including Japetus (VII) (a second satellite of Saturn) in 1671, Rhea (V) (a third satellite of Saturn) in 1672, and finally two more satellites of Saturn; Tethys (III) and Dione (V) (March 1684). It was during this time that, using his highly developed observational talents, Cassini discerned a band on the surface of Saturn and proposed that it was ring around the planet. Furthermore, he discovered that this ring was subdivided in two thinner rings separated by a narrow band, now known as Cassini’s division. At the beginning of the eighteenth century Cassini’s astronomical activities declined and his son, Jacques, gradually replaced him. Cassini died in Paris on 14 September 1712.
The Cassini orbiter is named after this famous Italian-French astronomer for his work involving Saturn.
CHRISTIAAN HUYGENS (1629–1695)
Christiaan Huygens was born in The Hague, the Netherlands, on 14th April 1629 into a prominent and intellectual Dutch family. Huygens received the best possible education, having been taught by, amongst others, the French philosopher Rene Descartes. In 1645 Huygens began studying mathematics and law at the University of Leiden. By 1651 he had published his first work on geometry, entitled Theoremata de quadratura hyperboles, ellypsis et circuli. When Huygens visited Paris in 1655, he made contact with various lens makers with the ultimately successful aim of improving the design and techniques of his own lenses. Returning to The Hague he made several observational discoveries that would make him famous, including the discovery of Titan (a satellite of Saturn) in 1655 which he published in De Saturni luna observation nova (1656). Furthermore, he observed the ’arms’ of Saturn and hypothesised that the planet was surrounded by a ring in Systema Saturnium (1659).
Between 1660 and 1663, Huygens travelled back and forth between Paris and The Hague – working on lenses, pendulum clocks, mathematics and other activities. In 1663 he was appointed a founding member of the Academie Royale Des Sciences by Louis XIV. From 1666 to 1681 Huygens worked in Paris, mainly on pendulum clocks and in 1673 he published his seminal work Horologium oscillatorium. During this period he also developed a wave theory of light, which he discussed in Traite de la lumiere (1678). In 1681 Huygens returned to The Hague, where he worked on optics and clocks until his death in 1695.
The Huygens probe was named after this Dutch astronomer for his work involving Saturn and Titan.