Eccentricity, THEORY

OF. Result of one of several attempts to find a cause for the cold of the Glacial Era or Ice Age. Its author, Dr. James Croll of Edinburgh, following a suggestion of Herschel, investigated the probable effect on climate of the actual secular variation in the eccentricity of the earth's orbit, whereby her distance from the sun varies from a minimum of 84,293,650 m. to a maximum of 98,506,350 m., involving a difference of 14,212,700 m. Dr. Croll believed that the result of this variation must be a great increase in the length of the winters and in the consequent accumulation of ice and snow about the Pole, whose winter occurred when the earth was furthest from the sun: to this accumulation he at tributed the continental ice-sheet and the accompanying phenomena of glacial action. According to this theory, an Ice Age must occur during every period of 21,000 years in times of high eccentricity. This period is determined by the prece ion of the equinoxes, which throws each hemisphere alternately into an aphelion position every 25,000 years, and by the retrograde motion of the perihelion point, which shortens this period to 21,000 years. Here has lain one of the greatest difficulties of the theory, in the opinion of many geologists, the evidence failing to establish during the Ice Age so rapid an alternation of glacial and temperate conditions.