Milankovic moved to Belgrade in fall of 1909. He brought with him all the necessary scientific literature available at that time for lecture preparation.
He was invited to become a professor at the Department for Applied Mathematics which consisted of rational and celestial mechanics and theoretical physics.
Lectures lasted for six classes per week during three years and ranged over areas like: rational mechanics, vector analysis, general theory of physical field, science of heat conduction, electrostatics and magnetostatics, Maxwell’s theory of electricity, electrons theory, celestial mechanics and higher dynamics. There were twenty students on average. He acquired the necessary literature mostly by his own means.
Milankovic has accepted the notion formulated by Kant that in every natural science the amount of real science is equal to amount of mathematics in it. Because of that he has decided to study crossings between sciences and possibilities of applying mathematics for solving problems in these areas. He started with meteorology.
He decided to commit himself to problems solving in climatology while reading the paper on problem of allocation of the Sun’s heat on the surface of the Earth, in which the starting point was a wrong equation. He analyzed contemporary literature in a paper published in herald of the Royal Serbian Academy of Science in 1913
''This paper has been a cornerstone for my later papers which were resolving a much broader task.''
''A connection should be found between planets’ insolation and their atmosphere and surface temperatures…such a theory should have the capacity of crossing over the borders of our direct observation, not only in space but also in time. '
''Those who study the climate of the Earth, meteorologists, don’t care about climates of other planets, and as far as the Earth’s climate is concerned they are all pure empirics who don’t care about complex mathematical theories, let alone the fact they could not operate them. The other reason which added to the fact that no one seriously tried to formulate mathematical theory of climate is certainly the fact that its formulation calls for solution of complex problem in various scientific areas: spherical astronomy, celestial mechanics and theoretical physics. These sciences are sharply divided and on the top of that each scientist has in his scientific area his own particular burrow which he very unwillingly leaves. The third reason…was that the strength of the insolation has never been recorded properly. ''
During the first three years of his life in Belgrade he has written and published seven papers on mathematical theory of climate both on the Earth and on the other planets.
He calculated that ''the yearly average temperature of the surface on Mars is at the equator -3 degrees, at the thirtieth parallel -12 and -52 at the poles. The yearly average temperature of entire surface of Mars is -17 degrees.''
Atmosphere of Mars was studied and the temperature of its surface was measured by satellites and probes, half the century after the publication of the first paper written by Milankovic. His calculations were right. At the fifteenth International astronomy conference held in Sydney in 1973. a decision was made to name one of Mars craters after Milankovic.