<P> In 1687, Isaac Newton stated the law of universal gravitation, described earlier as a hypothesis by Robert Hooke and others . His main achievement was to mathematically derive Kepler's laws of planetary motion from the law of gravitation, thus helping to prove the latter . This introduced gravitation as the force that both kept the Earth and planets moving through the heavens and also kept the air from flying away . The theory of gravity allowed scientists to construct a plausible heliocentric model for the solar system quickly . In his Principia, Newton explained his system of how gravity, previously thought of as an occult force (that is, an unexplained force), directed the movements of celestial bodies, and kept our solar system in its working order . His descriptions of centripetal force were a breakthrough in scientific thought which used the newly developed differential calculus, and finally replaced the previous schools of scientific thought, i.e. those of Aristotle and Ptolemy . However, the process was gradual . </P> <P> Several empirical tests of Newton's theory, explaining the longer period of oscillation of a pendulum at the equator and the differing size of a degree of latitude, gradually became available over the period 1673--1738 . In addition, stellar aberration was observed by Robert Hooke in 1674 and tested in a series of observations by Jean Picard over ten years finishing in 1680 . However, it was not explained until 1729 when James Bradley provided an approximate explanation in terms of the Earth's revolution about the sun . </P> <P> In 1838, astronomer Friedrich Wilhelm Bessel measured the parallax of the star 61 Cygni successfully, and disproved Ptolemy's claim that parallax motion did not exist . This finally confirmed the assumptions made by Copernicus, provided accurate, dependable scientific observations, and displayed truly how far away stars were from Earth . </P> <P> A geocentric frame is useful for many everyday activities and most laboratory experiments, but is a less appropriate choice for solar - system mechanics and space travel . While a heliocentric frame is most useful in those cases, galactic and extra-galactic astronomy is easier if the sun is treated as neither stationary nor the center of the universe, but rotating around the center of our galaxy, and in turn our galaxy is also not at rest in the cosmic background . </P>

Who is the first person to establish a geocentric universe