<P> The basis for the modern understanding of orbits was first formulated by Johannes Kepler whose results are summarised in his three laws of planetary motion . First , he found that the orbits of the planets in our solar system are elliptical , not circular ( or epicyclic ) , as had previously been believed , and that the Sun is not located at the center of the orbits , but rather at one focus . Second , he found that the orbital speed of each planet is not constant , as had previously been thought , but rather that the speed depends on the planet 's distance from the Sun . Third , Kepler found a universal relationship between the orbital properties of all the planets orbiting the Sun . For the planets , the cubes of their distances from the Sun are proportional to the squares of their orbital periods . Jupiter and Venus , for example , are respectively about 5.2 and 0.723 AU distant from the Sun , their orbital periods respectively about 11.86 and 0.615 years . The proportionality is seen by the fact that the ratio for Jupiter , 5.2 / 11.86 , is practically equal to that for Venus , 0.723 / 0.615 , in accord with the relationship . Idealised orbits meeting these rules are known as Kepler orbits . </P>
<P> The basis for the modern understanding of orbits was first formulated by Johannes Kepler whose results are summarised in his three laws of planetary motion . First , he found that the orbits of the planets in our solar system are elliptical , not circular ( or epicyclic ) , as had previously been believed , and that the Sun is not located at the center of the orbits , but rather at one focus . Second , he found that the orbital speed of each planet is not constant , as had previously been thought , but rather that the speed depends on the planet 's distance from the Sun . Third , Kepler found a universal relationship between the orbital properties of all the planets orbiting the Sun . For the planets , the cubes of their distances from the Sun are proportional to the squares of their orbital periods . Jupiter and Venus , for example , are respectively about 5.2 and 0.723 AU distant from the Sun , their orbital periods respectively about 11.86 and 0.615 years . The proportionality is seen by the fact that the ratio for Jupiter , 5.2 / 11.86 , is practically equal to that for Venus , 0.723 / 0.615 , in accord with the relationship . Idealised orbits meeting these rules are known as Kepler orbits . </P>
who developed most of our current understanding of planetary motion
who developed most of our current understanding of planetary motion
Johannes Kepler
Johannes Kepler