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Joseph Adhemar

French mathematician Joseph Alphonse Adhemar (1797 − 1862) was the first to suggest that ice ages were controlled by astonomical forcing in his 1842 book "Revolutions of the Sea".

The earths orbit is elliptical, with the sun at one focus; lines drawn through the summer and winter solstice, and the spring and autumn equinoxes, intersect at right angles at the sun. Currently, the earth is closest to the sun (perihelion) near the (northern hemisphere) winter solstice, and the earth moves faster through its orbit when closer to the sun. Hence, the period from (northern hemisphere) autumn equinox, to winter, to spring, is shorter (by approx 7 days) than the period from sping to summer to autumn; and the reverse in the southern hemisphere. Hence, northern hemisphere winter is shorter.

Because of this, Adhemar reasoned that because the southern hemisphere had more hours of darkness in winter, it must be cooling, and attributed the Antarctic ice sheet to this. Adhemar knew of the 22,000 year cycle of precession of the equinoxes, and theorised that the ice ages occurred in this cycle.

One immeadiate objection to the theory was that the total insolation during a year does not vary at all during the precessional cycle, only its seasonal distribution. Another, that the timing was wrong, could not be tested by the observations then available.

The theory was developed, much modified, by James Croll then Milutin Milankovitch.