7.3 Saturn  (Page 2/3)

Galileo sent his correspondents the solution of the anagram, Altissimum planetam tergeminum observavi , or "I have observed the highest planet tri-form." And the newly configured Saturn now took its place in Galileo's Hall of Fame. But there was something very strange about this planet. For one thing, after being notified other observers often saw the planet oval shaped, but Galileo argued that this was due to inferior telescopes. For another, if these lateral bodies were satellites, they were very different from the satellites of Jupiter for they were much larger with respect to the planet and never moved with respect to it. Or did they?

In his third sunspot letter, dated December 1612, Galileo revealed another mystery about the planet: the lateral bodies had disappeared. Although Galileo confidently predicted that they would return, which they did, Saturn's appearances remained an enigma. If Saturn was sometimes seen oval (denied by Galileo), sometimes with two lateral bodies, and at other times round and solitary, how could one explain all these appearances? And the mystery grew deeper as time went on. In 1616 Galileo announced to his patrons that he had now observed Saturn in yet another shape, and he published this without commentary in his Asayer of 1623.

Although the planet had again appeared solitary in 1626, few noticed this. But by the next solitary appearance in 1642, there was a growing community of telescopic astronomers who now made observation of the planet a central part of their research programs. Pierre Gassendi and Johannes Hevelius played central roles in this quest, but there were a number of others. Astronomers now routinely published figures of the shapes in which they had observed Saturn, a sampling of which can be seen in fig. 3. Near the solitary appearances, virtually all astronomers still saw the planet triple-bodied as Galileo had first seen it; at other times, however, they saw two arms, or handles (Latin, ansae) attached to the central body and, representations of this handled appearance varied greatly.

If in 1642 there was a lack of information about Saturn's appearances, by 1655 when the handles had again shrunk into little disks and the planet was approaching its solitary appearance, there was a plethora of information. What was needed now was a model or theory that would make sense out of all these divergent observations. In 1656 Hevelius pubished De Nativa Saturni Facie (On the Real Appearance of Saturn"), in which he proposed that Saturn's body was ellipsoidal in shape with two crescents attached to its extremeties. Rotation about the minor axis in the plane of the crescents would, according to Hevelius, explain all the planet's appearances.

His book convinced few. In 1658 Christopher Wren (remembered more for his later architecture) proposed a model in which a "corona" so thin it could be considered a mere surface was attached to the planet; the entire formation rotated or librated about its major axis.

In the meantime, Christiaan Huygens had discovered a satellite of Saturn, now named Titan. In 1656 he published a brief tract on the discovery and included an anagram containing his own theory about Saturn's appearances. He unveiled his theory in 1659, in a substantial book entitled Systema Saturnium ("The Saturnian System"). Huygens's theory was that the planet was surrounded by a thin flat ring that nowhere touched it. Although Huygens did think that the ring had an appreciable thickness, this was basically the modern solution of the problem.

But Huygens's solution was a geometrical one. The question now facing astronomers was how such a ring could be stable. Huygens thought the ring was a solid structure; others opined that it was made up of a huge swarm of minute satellites. The argument went on for several centuries until James Clerk Maxwell published his mathematical analysis of the ring structure in 1858, proving that the ring had to be made up of particles no larger than a few inches. At the end of the nineteenth century, spectrographic studies showed that the angular rotation of the inside of the ring was greater than that of the outside of the ring, and that the ratio obeyed Kepler's third law. The problem was now solved, although Saturn's ring system still held surprises, as can be seen from the results of the recent flybys of the planet.