Pale Blue Dot - Carl Sagan [47]
The existence even of the four satellites of Jupiter that Galileo discovered—hardly planets—was disbelieved on the grounds that it challenged the precedence of the number seven. As acceptance of the Copernican system grew, the Earth was added to the list of planets, and the Sun and Moon were removed. Thus, there seemed to be only six planets (Mercury, Venus, Earth, Mars, Jupiter, and Saturn). So learned academic arguments were invented showing why there had to be six. For example, six is the first “perfect” number, equal to the sum of its divisors (1 + 2 + 3). Q.E.D. And anyway, there were only six days of creation, not seven. People found ways to accommodate from seven planets to six.
As those adept at numerological mysticism adjusted to the Copernican system, this self-indulgent mode of thinking spilled over from planets to moons. The Earth had one moon; Jupiter had the four Galilean moons. That made five. Clearly one was missing. (Don’t forget: Six is the first perfect number.) When Huygens discovered Titan in 1655, he and many others convinced themselves that it was the last: Six planets, six moons, and God’s in His Heaven.
The historian of science I. Bernard Cohen of Harvard University has pointed out that Huygens actually gave up searching for other moons because it was apparent, from such arguments, that no more were to be found. Sixteen years later, ironically with Huygens in attendance, G. D. Cassini* of the Paris Observatory discovered a seventh moon—Iapetus, a bizarre world with one hemisphere black and the other white, in an orbit exterior to Titan’s. Shortly after, Cassini discovered Rhea, the next Saturnian moon interior to Titan.
Here was another opportunity for numerology, this time harnessed to the practical task of flattering patrons. Cassini added up the number of planets (six) and the number of satellites (eight) and got fourteen. Now it so happened that the man who built Cassini’s observatory for him and paid his salary was Louis XIV of France, the Sun King. The astronomer promptly “presented” these two new moons to his sovereign and proclaimed that Louis’s “conquests” reached to the ends of the Solar System. Discreetly, Cassini then backed off from looking for more moons; Cohen suggests he was afraid one more might now offend Louis—a monarch not to be trifled with, who would shortly be throwing his subjects into dungeons for the crime of being Protestants. Twelve years later, though, Cassini returned to the search and found—doubtless with a measure of trepidation—another two moons. (It is probably a good thing that we have not continued in this vein; otherwise France would have been burdened by seventy-some-odd Bourbon kings named Louis.)
WHEN CLAIMS OF NEW WORLDS WERE MADE in the late eighteenth century, the force of such numerological arguments had much dissipated. Still, it was with a real sense of surprise that people heard in 1781 about a new planet, discovered through the telescope. New moons were comparatively unimpressive, especially after the first six or eight. But that there were new planets to be found and that humans had devised the means to do so were both considered astonishing, and properly so. If there is one previously unknown planet, there may be many more—in this solar system and in others. Who can tell what might be found if a multitude of new worlds are hiding in the dark?
The discovery