Milutin Milanković (1879–1958) · Kanon der Erdbestrahlung und seine Anwendung auf das Eiszeitenproblem · Royal Serbian Academy, Belgrade, 1941 (in German)
The cosmic problem
Milanković opens by setting himself a single, audacious problem: to compute, from celestial mechanics alone, exactly how much solar radiation each latitude of a planet receives in each season — and how that has changed over geological time. He solves it for the Earth and sketches it for Mars and Venus. The whole edifice rests on Newtonian gravitation and spherical astronomy; nothing is fitted to the geological record in advance.
The three orbital elements
Three slow changes in the Earth's motion govern the answer. The eccentricity of the orbit — how far from a circle the ellipse is — breathes in and out over roughly 100,000 and 413,000 years. The obliquity, the tilt of the spin axis, nods between about 22.1° and 24.5° over roughly 41,000 years. And the precession of the equinoxes — the slow swivel of the tilted axis, combined with the turning of the orbit's own long axis — shifts the season at which the Earth is nearest the Sun, with periods near 23,000 and 19,000 years.
The insolation integral
From these elements Milanković integrates the radiation arriving over a day and over a 'caloric' half-year. With Köppen's advice he fixes attention on the summer half-year at high northern latitudes, expressing each result as the latitude that would receive the same summer radiation today — the 65°N equivalent latitude. The governing quantity is the high-latitude summer insolation, not the annual total.
The curves of the past 600,000 years
He computes the summer-insolation curve back through six hundred millennia, entirely by hand. The criterion, again Köppen's: ice ages are born of cool summers that fail to melt the winter snow, not of cold winters. The dips in the curve are read as glacial epochs — and they line up with the four Alpine glaciations (Günz, Mindel, Riss, Würm) that Penck and Brückner had mapped from the field.
What lies beyond the astronomy
Milanković is candid that the orbital forcing is only the trigger. The amplification — growing ice that reflects more sunlight, shifting oceans and carbon — belongs to a physics he does not claim to have solved. The Canon supplies the metronome; the orchestra of feedbacks is left to others.
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Belgrade, 1941