milanković put the sun at the center of his theory, as the only source of heat and light in the solar system. he considered three cyclical movements of the earth: eccentricity (100,000-year cycle – johannes kepler, 1609), axial tilt (41,000-year cycle – from 22,1° to 24,5°; presently, the earth's tilt is 23.5° – ludwig pilgrim, 1904), and precession (23,000-year cycle – hipparchus, 130 bc). each cycle works on a different time-scale and each affects the amount of solar energy received by the planets. such changes in the geometry of an orbit lead to the changes in the insolation – the quantity of heat received by any spot at the surface of a planet. these orbital variations, which are influenced by gravity of the moon, sun, jupiter, and saturn, form the basis of the milankovitch cycle. his original contribution to celestial mechanics is called milanković's system of vector elements of planetary orbits. he reduced six lagrangean-laplacian elliptical elements to two vectors determining the mechanics of planetary movements. the first specifies the planet's orbital plane, the sense of revolution of the planet, and the orbital ellipse parameter; the second specifies the axis of the orbit in its plane and the orbital eccentricity. by applying those vectors he significantly simplified the calculation and directly obtained all the formulas of the classical theory of secular perturbations. milanković, in a simple but original manner, first deduced newton's law of gravitation from kepler's laws. then milankovich treated the two-body and the many-body problems of celestial mechanics. he accepted but corrected the le verrier and stockwell computation by using newer and more accurate values for the masses of the planets in the solar system.