pale blue dot -carl sagan-第18章
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is endowed with supernatural powers。 Seven is a 〃lucky〃 number。 In the New Testament's Book of Revelations； seven seals on a scroll are opened； seven trumpets are sounded； seven bowls are filled。 St。 Augustine obscurely argued for the mystic importance of seven on the grounds that three 〃is the first whole number that is odd〃 （what about one？）； 〃four the first that is even〃 （what about two？）； and 〃of these 。 。 。 seven is posed。〃 And so on。 Even in our time these associations linger。
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 acmodate 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； Cassim discovered Rhea； the next Saturnian moon interior to Titan。
* After whom the European…American mission to the Saturn system is named。
Here was another opportunity for numerology； this time harnessed to the practical task of flattering patrons。 Cassim 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 Cassim'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； Cassim 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； Cassim 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 paratively 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 was made not even by a professional astronomer but by William Herschel； a musician whose relatives had e to Britain with the family of another anglified German； the reigning monarch and future oppressor of the American colonists； George III。 It became Herschel's wish to call the planet George （〃George's Star；〃 actually）； after his patron。 but； providentially； the name didn't stick。 （Astronomers seem to have been very busy buttering up kings。） Instead； the planet that Herschel found is called Uranus （an inexhaustible source of hilarity renewed in each generation of English…speaking nine…year…olds）。 It is named after the ancient sky god who； according to Greek myth； was Saturn's father and thus the grandfather of the Olympian gods。
We no longer consider the Sun and Moon to be planets； and ignoring the paratively insignificant asteroids and ets； count Uranus as the seventh planet in order from the Sun （Mercury； Venus； Earth； Mars； Jupiter； Saturn； Uranus； Neptune； Pluto）。 It is the first planet unknown to the ancients。 The four outer； Jovian； planets turn out to be very different from the four inner； terrestrial； planets。 Pluto is a separate case。
As the years passed and the quality of astronomical instruments unproved； we began to learn more about distant Uranus。 What reflects the dim sunlight back to us is no solid surface； but atmosphere and clouds just as for Titan； Venus； Jupiter Saturn； and Neptune。 The air on Uranus is made of hydrogen and helium； the two simplest gases。 Methane and other hydrocarbons are also present。 Just below the clouds visible to Earthbound observers is a massive atmosphere with enormous quantities of ammonia； hydrogen sulfide； and； especially； water。
At depth on Jupiter and Saturn； the pressures are so great that atoms sweat electrons； and the air bees a metal。 That does not seem to happen on less massive Uranus； because the pressures at depth are less。 Still deeper； discovered only by its subtle tugs on Uranus' moons； wholly inaccessible to view； under the crushing weight of the overlying atmosphere； is a rocky surface。 A big Earthlike planet is hiding down there； swathed in an immense blanket of air。
The Earth's surface temperature is due to the sunlight it intercepts。 Turn off the Sun and the planet soon chills—not to trifling Antarctic cold； not just so cold that the oceans freeze； but to a cold so intense that the very air precipitates out； forming a ten…meter…thick layer of oxygen and nitrogen snows covering the whole planet。 The little bit of energy that trickles up from the Earth's hot interior would be insufficient to melt these snows。 For Jupiter； Saturn； and Neptune it's different。 There's about as much heat pouring out from their interiors as they acquire from the warmth of the distant Sun。 Turn off the Sun； and they would be only a little affected。
But Uranus is another story。 Uranus is an anomaly among the Jovian planets。 Uranus is like the Earth： There's very little intrinsic heat pouring out。 We have no good understanding of why this should be； why Uranus—which in many respects is so similar to Neptune—should lack a potent source of internal heat。 For this reason； among others； we cannot say we understand what is going on in the deep interiors of these mighty worlds。
Uranus is lying on its side as it goes around the Sun。 In the 1990s。 the south pole is heated by the Sun； and it is this pole that Earthbound observers at the end of the twentieth century see when they look at Uranus。 It takes Uranus 84 Earth years to make one circuit of the Sun。 So in the 2030s； the north pole will be sunward （and Earthward）。 In the 2070s the south pole will be pointing to the Sun once again。 In between； Earthbound astronomers will be looking mainly at equatorial latitudes。
All the other planets spin much more upright in their orbits。 No one is sure of the reason for Uranus' anomalous spin； the most promising suggestion is that sometime in its early history； billions of years ago； it was struck by a rogue planet； about the size of the Earth； in a highly eccentric orbit。 Such a collision； if it ever happened； must have worked much tumult in the Uranus system； for all we know； there may be other vestiges of ancient havoc still left for us to find。 But Uranus' remoteness tends to guard its mysteries。
In 1977 a team of scientists led by James Elliot； then of Cornell University； accidentally discovered that； like Saturn； Uranus has rings。 The scientists were flying over the Indian Ocean in a special NASA airplane—the Kuiper Airborne Observatory—to witness the passage of Uranus in front of a star。 （Such passages； or occultations as they're called； happen from time to time； precisely because Uranus slowly moves with respect to the distant stars。） The observers were surprised to find that the star winked on and off several times just before it passed behind Uranus and its atmosphere； then several times more just after it emerged。 Since the patterns of winking on and off were the same before and after occultation； this finding （and much subsequent work） has led to the discovery of nine very thin； very dark circumplanetary rings； giving Uranus the appearance of a bull's…eye in the sky。
Surrounding the rings； Earthbound observers understood were the concentric orbits of the five moons then known： Miranda； Ariel； Umbriel； Titania； and Oberon。 They're named after characters in Shakespeare's A Midsummer Night's Dream and The Tempest； and in Alexander Pope's The Rape of the Lock。 Two of them were found by Herschel himself。 The innermost of the five； Miranda； was discovered as recently as 1948； by my teacher G。 P。 Kuiper。* I remember how great an achievement the discovery of a new moon of Uranus was considered back then。 The near…infrared light reflected by all five moons subsequently revealed the spectral signature of ordinary water ice on their Surfaces。 And no wonder—Uranus is so far from the Sun that it is no brighter there at noontime than it is after sunset on Earth。 The temperatures are frigid。 Any water must be frozen。
* He so named it because of the words spoken by Miranda； the heroine of The Tempest： 〃O brave new world； That has such people in't。〃 （To Which Prospero replies； 〃'Tis new to thee。〃 Just so。 Like all the other worlds in the Solar System； Miranda is about 4。5 billion years old。）
A REVOLUTION IN OUR UNDERSTANDING of the Uranus system—the planet； its rings； and its moons—began on January 24； 1986。 On that day； after a journey of 8？ years； the Voyager 2 spacecraft sailed very near to Miranda； and hit the bull's…eye in the sky。 Uranus' gravity then flung it on to Neptune。 The spacecraft returned 4；300 close…up pictures of the Uranus system and a wealth of other data。
Uranus was found to be surrounded by an intense radiation belt； electrons and protons trapped by the planet's magnetic field。 Voyager flew through this radiation belt； measuring the magnetic field and the trapped charged particles as it went。 It also detected—in changing timbres； harmonies； and nuance； but mainly in fortissimo—a cacophony of radio waves generated by the speeding； trapped particles。 Something similar was discovered on Jupiter and Saturn and would be later found at Neptune—but always with a theme and counterpoint characteristic of each world。
On Earth the magnetic and geographical poles are qui
