<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> Historically, the apparent motions of the planets were described by European and Arabic philosophers using the idea of celestial spheres . This model posited the existence of perfect moving spheres or rings to which the stars and planets were attached . It assumed the heavens were fixed apart from the motion of the spheres, and was developed without any understanding of gravity . After the planets' motions were more accurately measured, theoretical mechanisms such as deferent and epicycles were added . Although the model was capable of reasonably accurately predicting the planets' positions in the sky, more and more epicycles were required as the measurements became more accurate, hence the model became increasingly unwieldy . Originally geocentric it was modified by Copernicus to place the sun at the centre to help simplify the model . The model was further challenged during the 16th century, as comets were observed traversing the spheres . </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> <P> Isaac Newton demonstrated that Kepler's laws were derivable from his theory of gravitation and that, in general, the orbits of bodies subject to gravity were conic sections (this assumes that the force of gravity propagates instantaneously). Newton showed that, for a pair of bodies, the orbits' sizes are in inverse proportion to their masses, and that those bodies orbit their common center of mass . Where one body is much more massive than the other (as is the case of an artificial satellite orbiting a planet), it is a convenient approximation to take the center of mass as coinciding with the center of the more massive body . </P>

Who developed most of our current understanding of planetary motion