milanković's works on astronomical explanations of ice ages, especially his curve of insolation for the past 130,000 years, received support from the climatologist wladimir köppen and from the geophysicist alfred wegener. köppen noted the usefulness of milanković's theory for paleoclimatological researchers. milanković received a letter on 22 september 1922 from köppen, who asked him to expand his studies from 130,000 years to 600,000 years. they agreed that summer insolation is a crucial factor for climate. after developing the mathematical machinery enabling him to calculate the insolation in any given geographical latitude and for any annual season, milanković was ready to start the realization of the mathematical description of climate of the earth in the past. milanković spent 100 days doing the calculations and prepared a graph of solar radiation changes at geographical latitudes of 55°, 60° and 65° north for the past 650,000 years. milanković believed that these were the latitudes on earth most sensitive to the change of thermal balance. from that time there is an anecdote, when milanković's good friend and professor of geography, jovan cvijić, asked him: "why do you calculate temperature conditions at the top of the atmosphere, what is the purpose?!"