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began to aggregate。 virtually all of it鈥�99。9percent of the mass of the solar system鈥攚ent to make the sun。 out of the floating materialthat was left over； two microscopic grains floated close enough together to be joined byelectrostatic forces。 this was the moment of conception for our planet。 all over the inchoatesolar system； the same was happening。 colliding dust grains formed larger and larger clumps。
eventually the clumps grew large enough to be called planetesimals。 as these endlesslybumped and collided； they fractured or split or rebined in endless random permutations；but in every encounter there was a winner； and some of the winners grew big enough todominate the orbit around which they traveled。
it all happened remarkably quickly。 to grow from a tiny cluster of grains to a baby planetsome hundreds of miles across is thought to have taken only a few tens of thousands of years。
in just 200 million years； possibly less； the earth was essentially formed； though still moltenand subject to constant bombardment from all the debris that remained floating about。
at this point； about 4。5 billion years ago； an object the size of mars crashed into earth；blowing out enough material to form a panion sphere； the moon。 within weeks； it isthought； the flung material had reassembled itself into a single clump； and within a year it had formed into the spherical rock that panions us yet。 most of the lunar material； it isthought； came from the earth鈥檚 crust； not its core； which is why the moon has so little ironwhile we have a lot。 the theory； incidentally； is almost always presented as a recent one； butin fact it was first proposed in the 1940s by reginald daly of harvard。 the only recent thingabout it is people paying any attention to it。
when earth was only about a third of its eventual size； it was probably already beginning toform an atmosphere； mostly of carbon dioxide； nitrogen； methane； and sulfur。 hardly the sortof stuff that we would associate with life； and yet from this noxious stew life formed。 carbondioxide is a powerful greenhouse gas。 this was a good thing because the sun wassignificantly dimmer back then。 had we not had the benefit of a greenhouse effect； the earthmight well have frozen over permanently； and life might never have gotten a toehold。 butsomehow life did。
for the next 500 million years the young earth continued to be pelted relentlessly byets； meteorites； and other galactic debris； which brought water to fill the oceans and theponents necessary for the successful formation of life。 it was a singularly hostileenvironment and yet somehow life got going。 some tiny bag of chemicals twitched andbecame animate。 we were on our way。
four billion years later people began to wonder how it had all happened。 and it is there thatour story next takes us。
part ii the size of the earthnature and nature鈥檚 laws lay hid innight；god said； let newton be！ and allwas light。
…alexander pope
4 THE MEASURE OF THINGS
if you had to select the least convivial scientific field trip of all time； you could certainlydo worse than the french royal academy of sciences鈥櫋eruvian expedition of 1735。 led by ahydrologist named pierre bouguer and a soldier…mathematician named charles marie de lacondamine； it was a party of scientists and adventurers who traveled to peru with the purposeof triangulating distances through the andes。
at the time people had lately bee infected with a powerful desire to understand theearth鈥攖o determine how old it was； and how massive； where it hung in space； and how it hade to be。 the french party鈥檚 goal was to help settle the question of the circumference ofthe planet by measuring the length of one degree of meridian （or 1/360 of the distance aroundthe planet） along a line reaching from yarouqui； near quito； to just beyond cuenca in what isnow ecuador； a distance of about two hundred miles。
1almost at once things began to go wrong； sometimes spectacularly so。 in quito； the visitorssomehow provoked the locals and were chased out of town by a mob armed with stones。 soonafter； the expedition鈥檚 doctor was murdered in a misunderstanding over a woman。 thebotanist became deranged。 others died of fevers and falls。 the third most senior member ofthe party； a man named pierre godin； ran off with a thirteen…year…old girl and could not beinduced to return。
at one point the group had to suspend work for eight months while la condamine rode off tolima to sort out a problem with their permits。 eventually he and bouguer stopped speakingand refused to work together。 everywhere the dwindling party went it was met with thedeepest suspicions from officials who found it difficult to believe that a group of frenchscientists would travel halfway around the world to measure the world。 that made no sense atall。 two and a half centuries later it still seems a reasonable question。 why didn鈥檛 the frenchmake their measurements in france and save themselves all the bother and disfort of theirandean adventure？
the answer lies partly with the fact that eighteenth…century scientists； the french in particular；seldom did things simply if an absurdly demanding alternative was available； and partly witha practical problem that had first arisen with the english astronomer edmond halley manyyears before鈥攍ong before bouguer and la condamine dreamed of going to south america；much less had a reason for doing so。
* triangulation； their chosen method； was a popular technique based on the geometric fact that if you know thelength of one side of a triangle and the angles of two corners； you can work out all its other dimensions withoutleaving your chair。 suppose； by way of example； that you and i decided we wished to know how far it is to themoon。 using triangulation； the first thing we must do is put some distance between us； so lets say for argumentthat you stay in paris and i go to moscow and we both look at the moon at the same time。 now if you imagine aline connecting the three principals of this exercise…that is； you and i and the moon…it forms a triangle。 measurethe length of the baseline between you and me and the angles of our two corners and the rest can be simplycalculated。 （because the interior angles of a triangle always add up to 180 degrees； if you know the sum of twoof the angles you can instantly calculate the third； and knowing the precise shape of a triangle and the length ofone 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 moons distance from earth。 at ground level； the principles oftriangulation are the same； except that the triangles dont reach into space but rather are laid side to side on amap。 in measuring a degree of meridian； the surveyors would create a sort of chain of triangles marching acrossthe landscape。
halley was an exceptional figure。 in the course of a long and productive career； he was asea captain； a cartographer； a professor of geometry at the university of oxford； deputycontroller of the royal mint； astronomer royal； and inventor of the deep…sea diving bell。 hewrote authoritatively on magnetism； tides； and the motions of the planets； and fondly on theeffects of opium。 he invented the weather map and actuarial table； proposed methods forworking out the age of the earth and its distance from the sun； even devised a practicalmethod for keeping fish fresh out of season。 the one thing he didn鈥檛 do； interestingly enough；was discover the et that bears his name。 he merely recognized that the et he saw in1682 was the same one that had been seen by others in 1456； 1531； and 1607。 it didn鈥檛bee halley鈥檚 et until 1758； some sixteen years after his death。
for all his achievements； however； halley鈥檚 greatest contribution to human knowledge maysimply have been to take part in a modest scientific wager with two other worthies of his day：
robert hooke； who is perhaps best remembered now as the first person to describe a cell； andthe great and stately sir christopher wren； who was actually an astronomer first and architectsecond； though that is not often generally remembered now。 in 1683； halley； hooke； andwren were dining in london when the conversation turned to the motions of celestial objects。
it was known that planets were inclined to orbit in a particular kind of oval known as anellipse鈥斺�渁 very specific and precise curve；鈥潯o quote richard feynman鈥攂ut it wasn鈥檛understood why。 wren generously offered a prize worth forty shillings （equivalent to a coupleof weeks鈥櫋ay） to whichever of the men could provide a solution。
hooke； who was well known for taking credit for ideas that weren鈥檛 necessarily his own；claimed that he had solved the problem already but declined now to share it on the interestingand inventive grounds that it would rob others of the satisfaction of discovering the answer forthemselves。 he would instead 鈥渃onceal it for some time； that others might know how to valueit。鈥潯f he thought any more on the matter； he left no evidence of it。 halley； however； becameconsumed with finding the answer； to the point that the following year he traveled tocambridge and boldly called upon the university鈥檚 lucasian professor of