[size=150]The motion of planets is describedby the three Kepler’s laws, which belong to the usual secondary school curriculum. [br][br][b]Johannes Kepler (1571–1630)[/b], German mathematician and astronomer, whose life was closely connected to Graz and Linz in Austria, and Praha in Bohemia, published the three laws over a period of time in two books. [br][br]He based his findings on observations of the Danish astronomer [b]Tycho Brahe (1546–1601)[/b].[br][br]First, in 1609 in [i]Astronomia nova[/i], Kepler published two statements that are known as his 1[sup]st [/sup]and 2[sup]nd[/sup] laws of planetary motion.[br][br][/size][list][*][size=150] [i]Orbits of all the planets are ellipses with the Sun at one focus ([size=150]Kepler’s 1[sup]st[/sup] law, the law of ellipses).[/size][/i][/size][/*][/list][list][*][size=150] [i]A line segment from the Sun to a planet sweeps out equal areas in equal time ([size=150]Kepler’s 2[sup]nd[/sup] law, the law of equal areas)[/size].[/i][/size][/*][/list][size=150][br]Then, in 1619 in [i]Harmonices mundi[/i], he added the third law.  [br][br][/size][list][*][size=150][b] [/b][i]The orbital period of a planet is proportional to the three-halves power of the size of the semi–major axis of its orbit ([size=150]Kepler’s 3[sup]rd[/sup] law, the law of harmonies)[/size].[/i][/size][/*][/list]