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criticized by the great charles lyell。 the following year the geological society ofedinburgh passed a resolution conceding that there might be some general merit in the theorybut that certainly none of it applied to scotland。
lyell did eventually e round。 his moment of epiphany came when he realized that amoraine； or line of rocks； near his family estate in scotland； which he had passed hundreds oftimes； could only be understood if one accepted that a glacier had dropped them there。 buthaving bee converted； lyell then lost his nerve and backed off from public support of theice age idea。 it was a frustrating time for agassiz。 his marriage was breaking up； schimperwas hotly accusing him of the theft of his ideas； charpentier wouldn鈥檛 speak to him； and thegreatest living geologist offered support of only the most tepid and vacillating kind。
in 1846； agassiz traveled to america to give a series of lectures and there at last found theesteem he craved。 harvard gave him a professorship and built him a first…rate museum； themuseum of parative zoology。 doubtless it helped that he had settled in new england；where the long winters encouraged a certain sympathy for the idea of interminable periods ofcold。 it also helped that six years after his arrival the first scientific expedition to greenlandreported that nearly the whole of that semicontinent was covered in an ice sheet just like theancient one imagined in agassiz鈥檚 theory。 at long last； his ideas began to find a realfollowing。 the one central defect of agassiz鈥檚 theory was that his ice ages had no cause。 butassistance was about to e from an unlikely quarter。
in the 1860s； journals and other learned publications in britain began to receive papers onhydrostatics； electricity； and other scientific subjects from a james croll of anderson鈥檚university in glasgow。 one of the papers； on how variations in earth鈥檚 orbit might haveprecipitated ice ages； was published in the philosophical magazine in 1864 and wasrecognized at once as a work of the highest standard。 so there was some surprise； and perhapsjust a touch of embarrassment； when it turned out that croll was not an academic at theuniversity； but a janitor。
born in 1821； croll grew up poor； and his formal education lasted only to the age ofthirteen。 he worked at a variety of jobs鈥攁s a carpenter； insurance salesman； keeper of atemperance hotel鈥攂efore taking a position as a janitor at anderson鈥檚 （now the university ofstrathclyde） in glasgow。 by somehow inducing his brother to do much of his work； he wasable to pass many quiet evenings in the university library teaching himself physics；mechanics； astronomy； hydrostatics； and the other fashionable sciences of the day； andgradually began to produce a string of papers； with a particular emphasis on the motions ofearth and their effect on climate。
croll was the first to suggest that cyclical changes in the shape of earth鈥檚 orbit； fromelliptical （which is to say slightly oval） to nearly circular to elliptical again； might explain the onset and retreat of ice ages。 no one had ever thought before to consider an astronomicalexplanation for variations in earth鈥檚 weather。 thanks almost entirely to croll鈥檚 persuasivetheory； people in britain began to bee more responsive to the notion that at some formertime parts of the earth had been in the grip of ice。 when his ingenuity and aptitude wererecognized； croll was given a job at the geological survey of scotland and widely honored：
he was made a fellow of the royal society in london and of the new york academy ofscience and given an honorary degree from the university of st。 andrews； among much else。
unfortunately； just as agassiz鈥檚 theory was at last beginning to find converts in europe； hewas busy taking it into ever more exotic territory in america。 he began to find evidence forglaciers practically everywhere he looked； including near the equator。 eventually he becameconvinced that ice had once covered the whole earth； extinguishing all life； which god hadthen re…created。 none of the evidence agassiz cited supported such a view。 nonetheless； inhis adopted country his stature grew and grew until he was regarded as only slightly below adeity。 when he died in 1873 harvard felt it necessary to appoint three professors to take hisplace。
yet； as sometimes happens； his theories fell swiftly out of fashion。 less than a decade afterhis death his successor in the chair of geology at harvard wrote that the 鈥渟o…called glacialepoch 。 。 。 so popular a few years ago among glacial geologists may now be rejected withouthesitation。鈥�
part of the problem was that croll鈥檚 putations suggested that the most recent ice ageoccurred eighty thousand years ago； whereas the geological evidence increasingly indicatedthat earth had undergone some sort of dramatic perturbation much more recently than that。
without a plausible explanation for what might have provoked an ice age； the whole theoryfell into abeyance。 there it might have remained for some time except that in the early 1900sa serbian academic named milutin milankovitch； who had no background in celestial motionsat all鈥攈e was a mechanical engineer by training鈥攄eveloped an unexpected interest in thematter。 milankovitch realized that the problem with croll鈥檚 theory was not that it wasincorrect but that it was too simple。
as earth moves through space； it is subject not just to variations in the length and shape ofits orbit； but also to rhythmic shifts in its angle of orientation to the sun鈥攊ts tilt and pitch andwobble鈥攁ll affecting the length and intensity of sunlight falling on any patch of land。 inparticular it is subject to three changes in position； known formally as its obliquity；precession； and eccentricity； over long periods of time。 milankovitch wondered if there mightbe a relationship between these plex cycles and the ings and goings of ice ages。 thedifficulty was that the cycles were of widely different lengths鈥攐f approximately 20；000；40；000； and 100；000 years； but varying in each case by up to a few thousand years鈥攚hichmeant that determining their points of intersection over long spans of time involved a nearlyendless amount of devoted p