Properly called the Opik-Oort cloud, it is named for the Estonian astronomer Ernst Opik, who hypothesized its existence in 1932, and for the Dutch astronomer Jan Oort, who refined the calculations eighteen years later.

* Triangulation, their chosen method, was a popular technique based on the geometric fact that if you know the length of one side of a triangle and the angles of two corners, you can work out all its other dimensions without leaving your chair. Suppose, by way of example, that you and I decided we wished to know how far it is to the Moon. Using triangulation, the first thing we must do is put some distance between us, so let’s say for argument that you stay in Paris and I go to Moscow and we both look at the Moon at the same time. Now if you imagine a line connecting the three principals of this exercise-that is, you and I and the Moon-it forms a triangle. Measure the length of the baseline between you and me and the angles of our two corners and the rest can be simply calculated. (Because the interior angles of a triangle always add up to 180 degrees, if you know the sum of two of the angles you can instantly calculate the third; and knowing the precise shape of a triangle and the length of one side tells you the lengths of the other sides.) This was in fact the method use by a Greek astronomer, Hipparchus of Nicaea, in 150 B.C. to work out the Moon’s distance from Earth. At ground level, the principles of triangulation are the same, except that the triangles don’t reach into space but rather are laid side to side on a map. In measuring a degree of meridian, the surveyors would create a sort of chain of triangles marching across the landscape.

4How fast you are spinning depends on where you are. The speed of the Earth’s spin varies from a little over 1,000 miles an hour at the equator to 0 at the poles.

5The next transit will be on June 8, 2004, with a second in 2012. There were none in the twentieth century.

6In 1781 Herschel became the first person in the modern era to discover a planet. He wanted to call it George, after the British monarch, but was overruled. Instead it became Uranus.

7To a physicist, mass and weight are two quite different things. Your mass stays the same wherever you go, but your weight varies depending on how far you are from the center of some other massive object like a planet. Travel to the Moon and you will be much lighter but no less massive. On Earth, for all practical purposes, mass and weight are the same and so the terms can be treated as synonymous. at least outside the classroom.

8There will be no testing here, but if you are ever required to memorize them you might wish to remember John Wilford’s helpful advice to think of the eras (Precambrian, Paleozoic, Mesozoic, an( Cenozoic) as seasons in a year and the periods (Permian, Triassic Jurassic, etc.) as the months.

9Although virtually all books find a space for him, there is a striking variability in the details associated with Ussher. Some books say he made his pronouncement in 1650, others in 1654, still others in 1664. Many cite the date of Earth’s reputed beginning as October 26. At least one book of note spells his name “Usher.” The matter is interestingly surveyed in Stephen Jay Gould’s Eight Little Piggies.

10Darwin loved an exact number. In a later work, he announced that the number of worms to be found in an average acre of English country soil was 53,767.

11In particular he elaborated the Second Law of Thermodynamics. A discussion of these laws would be a book in itself, but I offer here this crisp summation by the chemist P. W Atkins, just to provide a sense of them: “There are four Laws. The third of them, the Second Law, was recognized first; the first, the Zeroth Law, was formulated last; the First Law was second; the Third Law might not even be a law in the same sense as the others.” In briefest terms, the second la states that a little energy is always wasted. You can’t have a perpetual motion device because no matter how efficient, it will always lose energy and eventually run down. The first law says that you can’t create energy and the third that you can’t reduce temperatures to absolute zero; there will always be some residual warmth. As Dennis Overbye notes, the three principal laws are sometimes expressed jocularly as (1) you can’t win, (2) you can’t break even, and (3) you can’t get out of the game.

12The notable exception being the Tyrannosaurus rex, which was found by Barnum Brown in 1902.

13The confusion over the aluminum/aluminium spelling arose b cause of some uncharacteristic indecisiveness on Davy’s part. When he first isolated the element in 1808, he called italumium . For son reason he thought better of that and changed it toaluminum four years later. Americans dutifully adopted the new term, but mai British users dislikedaluminum , pointing out that it disrupted the -ium pattern established by sodium, calcium, and strontium, so they added a vowel and syllable.

14The principle led to the much later adoption of Avogadro’s number, a basic unit of measure in chemistry, which was named for Avogadro long after his death. It is the number of molecules found in 2.016 grams of hydrogen gas (or an equal volume of any other gas). Its value is placed at 6.0221367 x 1023, which is an enormously large number. Chemistry students have long amused themselves by computing just how large a number it is, so I can report that it is equivalent to the number of popcorn kernels needed to cover the United States to a depth of nine miles, or cupfuls of water in the Pacific Ocean, or soft drink cans that would, evenly stacked, cover the Earth to a depth of 200 miles. An equivalent number of American pennies would be enough to make every person on Earth a dollar trillionaire. It is a big number.

15Specifically it is a measure of randomness or disorder in a system. Darrell Ebbing, in the textbookGeneral Chemistry, very usefully suggests thinking of a deck of cards. A new pack fresh out of the box, arranged by suit and in sequence from ace to king, can be said to be in its ordered state. Shuffle the cards and you put them in a disordered state. Entropy is a way of measuring just how disordered that state is and of determining the likelihood of particular outcomes with further shuffles. Of course, if you wish to have any observations published in a respectable journal you will need also to understand additional concepts such as thermal nonuniformities, lattice distances, and stoichiometric relationships, but that’s the general idea.

16Planck was often unlucky in life. His beloved first wife died early, in 1909, and the younger of his two sons was killed in the First World War. He also had twin daughters whom he adored. One died giving birth. The surviving twin went to look after the baby and fell in love with her sister’s husband. They married and two years later she died in childbirth. In 1944, when Planck was eighty-five, an Allied bomb fell on his house and he lost everything-papers, diaries, a lifetime of accumulations. The following year his surviving son was caught in a conspiracy to assassinate Hitler and executed.

17Einstein was honored, somewhat vaguely, “for services to theoretical physics.” He had to wait sixteen years, till 1921, to receive the award-quite a long time, all things considered, but nothing at all compared with Frederick Reines, who detected the neutrino in 1957 but wasn’t honored with a Nobel until 1995, thirty-eight years later, or the German Ernst Ruska, who invented the electron microscope in 1932 and received his Nobel Prize in 1986, more than half a century after the fact. Since Nobel Prizes are never awarded posthumously, longevity can be as important a factor as ingenuity for prizewinners.

18Howc came to be the symbol for the speed of light is something of a mystery, but David Bodanis suggests it probably came from the Latinceleritas , meaning swiftness. The relevant volume of theOxford English Dictionary , compiled a decade before Einstein’s theory, recognizesc as a symbol for many things, from carbon to cricket, but makes no mention of it as a symbol for light or swiftness.

19Named for Johann Christian Doppler, an Austrian physicist, who first noticed the effect in 1842. Briefly, what happens is that as a moving object approaches a stationary one its sound waves become bunched up as they cram up against whatever device is receiving them (your ears, say), just as you would expect of anything that is being pushed from behind toward an immobile object. This bunching is perceived by the listener as a kind of pinched and elevated sound (the yee). As the sound source passes, the sound waves spread out and lengthen, causing the pitch to drop abruptly (the yummm).

20The name comes from the same Cavendishes who producec Henry. This one was William Cavendish, seventh Duke of Devonshire, who was a gifted mathematician and steel baron in Victoriar England. In 1870, he gave the university £6,300 to build an experimental lab.

21Geiger would also later become a loyal Nazi, unhesitatingly betraying Jewish colleagues, including many who had helped him.

22There is a littleuncertainty about the use of the word uncertainty in regard to Heisenberg’s principle. Michael Frayn, in an afterword to his playCopenhagen , notes that several words in German-Unsicherheit, Unscharfe, Unbestimmtheit-have been used by various translators, but that none quite equates to the English uncertainty. Frayn suggests thatindeterminacy would be a better word for the principle andindeterminability would be better still.

23Or at least that is how it is nearly always rendered. The actual quote was: “It seems hard to sneak a look at God’s cards. But that He plays dice and uses ‘telepathic’ methods. . . is something that I cannot believe for a single moment.”

24If you have ever wondered how the atoms determine which 50 percent will die and which 50 percent will survive for the next session, the answer is that the half-life is really just a statistical convenience-a kind of actuarial table for elemental things. Imagine you had a sample of material with a half-life of 30 seconds. It isn’t that every atom in the sample will exist for exactly 30 seconds or 60 seconds or 90 seconds or some other tidily ordained period. Each atom will in fact survive for an entirely random length of time that has nothing to do with multiples of 30; it might last until two seconds from now or it might oscillate away for years or decades or centuries to come. No one can say. But what we can say is that for the sample as a whole the rate of disappearance will be such that half the atoms will disappear every 30 seconds. It’s an average rate, in other words, and you can apply it to any large sampling. Someone once worked out, for instance, that dimes have a half-life of about 30 years.

25There are practical side effects to all this costly effort. The World Wide Web is a CERN offshoot. It was invented by a CERN scientist, Tim Berners-Lee, in 1989.

26You are of course entitled to wonder what is meant exactly by “a constant of 50″ or “a constant of 100.” The answer lies in astronomical units of measure. Except conversationally, astronomers don’t use light-years. They use a distance called theparsec (a contraction ofparallax andsecond ), based on a universal measure called the stellar parallax and equivalent to 3.26 light-years. Really big measures, like the size of a universe, are measured in megaparsecs: a million parsecs. The constant is expressed in terms of kilometers per second per megaparsec. Thus when astronomers refer to a Hubble constant of 50, what they really mean is “50 kilometers per second per megaparsec.” For most of us that is of course an utterly meaningless measure, but then with astronomical measures most distances are so huge as to be utterly meaningless.

27It is KT rather than CT because C had already been appropriated forCambrian. Depending on which source you credit, the K comes either from the GreekKreta or GermanKreide . Both conveniently mean “chalk,” which is also whatCretaceous means.

28For those who crave a more detailed picture of the Earth’s interior, here are the dimensions of the various layers, using average figures: From 0 to 40 km (25 mi) is the crust. From 40 to 400 km (25 to 250 mi) is the upper mantle. From 400 to 650 km (250 to 400 mi) is a transition zone between the upper and lower mantle. From 650 to 2,700 km (400 to 1,700 mi) is the lower mantle. From 2,700 to 2,890 km (1,700 to 1,900 mi) is the “D” layer. From 2,890 to 5,150 km (1,900 to 3,200 mi) is the outer core, and from 5,150 to 6,378 km (3,200 to 3,967 mi) is the inner core.

29The discovery of extremophiles in the boiling mudpots of Yellowstone and similar organisms found elsewhere made scientists realize that actually life of a type could range much farther than that-even, perhaps, beneath the icy skin of Pluto. What we are talking about here are the conditions that would produce reasonably complex surface creatures.

30Of the remaining four, three are nitrogen and the remaining atom is divided among all the other elements.

31Oxygen itself is not combustible; it merely facilitates the combus tion of other things. This is just as well, for if oxygen were corn bustible, each time you lit a match all the air around you would bur into flame. Hydrogen gas, on the other hand, is extremely corn bustible, as the dirigibleHindenburg demonstrated on May 6, 193 in Lakehurst, New Jersey, when its hydrogen fuel burst explosive) into flame, killing thirty-six people.

32If you have ever been struck by how beautifully crisp and well defined the edges of cumulus clouds tend to be, while other clouds are more blurry, the explanation is that in a cumulus cloud there is a pronounced boundary between the moist interior of the cloud and the dry air beyond it. Any water molecule that strays beyond the edge of the cloud is immediately zapped by the dry air beyond, allowing the cloud to keep its fine edge. Much higher cirrus clouds are composed of ice, and the zone between the edge of the cloud and the air beyond is not so clearly delineated, which is why they tend to be blurry at the edges.

33The term means a number of things to different people, it appears. In November 2002, Carl Wunsch of MIT published a report inScience , “What Is the Thermohaline Circulation?,” in which he noted that the expression has been used in leading journals to signify at least seven different phenomena (circulation at the abyssal level, circulation driven by differences in density or buoyancy, “meridional overturning circulation of mass,” and so on)-though all have to do with ocean circulations and the transfer of heat, the cautiously vague and embracing sense in which I have employed it here.

34The indigestible parts of giant squid, in particular their beaks, accumulate in sperm whales’ stomachs into the substance known as ambergris, which is used as a fixative in perfumes. The next time you spray on Chanel No. 5 (assuming you do), you may wish to reflect that you are dousing yourself in distillate of unseen sea monster.

35There are actually twenty-two naturally occurring amino acids known on Earth, and more may await discovery, but only twenty of them are necessary to produce us and most other living things. The twenty-second, called pyrrolysine, was discovered in 2002 by researchers at Ohio State University and is found only in a single type of archaean (a basic form of life that we will discuss a little further on in the story) calledMethanosarcina barkeri .

36To illustrate, humans are in the domain eucarya, in the kingdom animalia, in the phylum chordata, in the subphylum vertebrata, in the class mammalia, in the order primates, in the family hominidae, in the genus homo, in the speciessapiens . (The convention, I’m informed, is to italicize genus and species names, but not those of higher divisions.) Some taxonomists employ further subdivisions: tribe, suborder, infraorder, parvorder, and more.

37The formal word for a zoological category, such asphylum orgenus . The plural istaxa .

38We are actually getting worse at some matters of hygiene. Dr. Maunder believes that the move toward low-temperature washing machine detergents has encouraged bugs to proliferate. As he puts it: “If you wash lousy clothing at low temperatures, all you get is cleaner lice.”

39Actually, quite a lot of cells are lost in the process of development, so the number you emerge with is really just a guess. Depending on which source you consult the number can vary by several orders of magnitude. The figure of ten thousand trillion (or quadrillion) is from Margulis and Sagan, 1986.

40Leeuwenhoek was close friends with another Delft notable, the artist Jan Vermeer. In the mid-1660s, Vermeer, who previously had been a competent but not outstanding artist, suddenly developed the mastery of light and perspective for which he has been celebrated ever since. Though it has never been proved, it has long been suspected that he used a camera obscura, a device for projecting images onto a flat surface through a lens. No such device was listed among Vermeer’s personal effects after his death, but it happens that the executor of Vermeer’s estate was none other than Antoni van Leeuwenhoek, the most secretive lens-maker of his day.

41An auspicious date in history: on the same day in Kentucky, Abraham Lincoln was born.

42Darwin was one of the few to guess correctly. He happened to be visiting Chambers one day when an advance copy of the sixth edition of Vestiges was delivered. The keenness with which Chambers checked the revisions was something of a giveaway, though it appears the two men did not discuss it.

43By coincidence, in 1861, at the height of the controversy, just such evidence turned up when workers in Bavaria found the bones of an ancient archaeopteryx, a creature halfway between a bird and a dinosaur. (It had feathers, but it also had teeth.) It was an impressive and helpful find, and its significance much debated, but a single discovery could hardly be considered conclusive.

44In 1968, Harvard University Press canceled publication ofThe Double Helix after Crick and Wilkins complained about its characterizations, which the science historian Lisa Jardine has described as “gratuitously hurtful.” The descriptions quoted above are after Watson softened his comments.

45Junk DNA does have a use. It is the portion employed in DNA fingerprinting. Its practicality for this purpose was discovered accidentally by Alec Jeffreys, a scientist at the University of Leicester in England. In 1986 Jeffreys was studying DNA sequences for genetic markers associated with heritable diseases when he was approached by the police and asked if he could help connect a suspect to two murders. He realized his technique ought to work perfectly for solving criminal cases-and so it proved. A young baker with the improbable name of Colin Pitchfork was sentenced to two life terms in prison for the murders.

46Though Dutch, Dubois was from Eijsden, a town bordering the French-speaking part of Belgium.

47Humans are put in the lamely Homimdae. Its members, traditionally called hominids, include any creatures (including extinct ones) that are more closely related to us than to any surviving chimpanzees. The apes, meanwhile, are lumped together in a family called Pongidae. Many authorities believe that chimps, gorillas, and orangutans should also be included in this family, with humans and chimps in a subfamily called Homininae. The upshot is that the creatures traditionally called hominids become, under this arrangement, hominins. (Leakey and others insist on that designation.) Hominoidea is the name of the aue sunerfamily which includes us.

48Absolute brain size does not tell you everything-or possibly sometimes even much. Elephants and whales both have brains larger than ours, but you wouldn’t have much trouble outwitting them in contract negotiations. It is relative size that matters, a point that is often overlooked. As Gould notes, A.africanus had a brain of only 450 cubic centimeters, smaller than that of a gorilla. But a typicalafricanus male weighed less than a hundred pounds, and a female much less still, whereas gorillas can easily top out at 600 pounds (Gould pp. 181-83).

49One possibility is that Neandertals and Cro-Magnons had different numbers of chromosomes, a complication that commonly arises when species that are close but not quite identical conjoin. In the equine world, for example, horses have 64 chromosomes and donkeys 62. Mate the two and you get an offspring with a reproductively useless number of chromosomes, 63. You have, in short, a sterile mule.