191
Much fuller tables, extending to 3,000,000 years of past timeand 1,000,000 years of future time, with diagrams of illustrativecurves, are to be found in Dr. Croll’s treatise. It is thereshown that there have been various intermediate and minoroscillations of value, and that more than a million years agothe eccentricity was occasionally reduced to so small a value,that it was only equal to |, or or even I of the smallestvalue, '0109, mentioned above. But the most interestingpoint to be noticed is, that during a period of some 200,000years, from about 100,000 to about 300,000 years ago, itmaintained an exceptionally large value; and that about themiddle of that period it reached a maximum value of '0575.
It is also of great importance to remember that, if the eccen-tricity remained unaltered (whatever its value might be), thenorthern and southern hemispheres would interchange anyeffect produced by it every 10,500 years. We cannot go intothe proofs of this last statement, which indicate that the pre-cession of the equinoxes, in consequence of its period being26,000 years, would produce such an alternation of effect oncein each 13,000 years, were it not for a modification caused by amovement of the perihelion of the Earth ’s orbit, which reducesthe 13,000 years to 10,500. The result, however, is, that about10,000 years ago, the summer of the southern hemisphere (instead of, as at the present time, that of the northern hemi-sphere) must have occurred when the Earth was in that partof its orbit in which it is at its nearest to the Sun, and that thesame will be the case about 10,000 years hence. As far as thisprecessional effect is concerned, there might consequently resultat such intervals, even with the present value of the eccentricity,an increased accumulation of snow and ice in the northern hemi-sphere, similar to that which at present exists in the southernhemisphere .
But during the period of great eccentricity, which, as we havestated, appears to have lasted from about 100,000 to 300,000
Mathematics,’’ p. 51, where the whole question is very ably discussed.There is also an interesting table of the past and future eccentricitiesof the orbits of Mars , Mercury, and Venus, as well as of that of theEarth, in Lardner’s “Astronomy, ” edited by Dunkin, article 785a.