Editing Geocentric model (section)

==Gravitation==

[[Johannes Kepler]] analysed [[Tycho Brahe]]'s famously accurate observations, and afterwards constructed his [[Kepler's laws of planetary motion|three laws]] in 1609 and 1619, based upon a heliocentric model wherein the planets move in elliptical paths. Using these laws, he was the first astronomer to successfully predict a [[transit of Venus]] for the year 1631. The change from circular orbits to elliptical planetary paths dramatically improved the accuracy of celestial observations and predictions. Because the heliocentric model devised by Copernicus was no more accurate than Ptolemy's system, new observations were needed to persuade those who still adhered to the geocentric model. However, Kepler's laws based upon Brahe's data became a problem that geocentrists could not easily overcome.

In 1687, [[Isaac Newton]] stated the [[law of universal gravitation]], which was 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 which kept Earth and the planets moving through the universe, and also kept the atmosphere from flying away. The theory of gravity allowed scientists to rapidly construct a plausible heliocentric model for the Solar System. In his ''[[Philosophiæ Naturalis Principia Mathematica|Principia]]'', Newton explained his theory of how gravity, previously thought to be a mysterious, unexplained occult force, directed the movements of celestial bodies, and kept our Solar System in working order. His descriptions of [[centripetal force]]<ref name= "Densmore2004"/> were a breakthrough in scientific thought, using the newly developed mathematical discipline of [[differential calculus]], finally replacing the previous schools of scientific thought, which had been dominated by Aristotle and Ptolemy. However, the process was gradual.

Several [[Earth's rotation#Empirical tests|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, would gradually become available between 1673 and 1738. In addition, [[stellar aberration]] was observed by [[Robert Hooke]] in 1674, and tested in a series of observations by [[Jean Picard]] over a period of 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.

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, providing accurate, dependable scientific observations, and conclusively displaying how distant stars are from Earth.

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 [[Heliocentrism|heliocentric frame]] is most useful in those cases, galactic and extragalactic astronomy is easier if the Sun is treated as neither stationary nor the center of the universe, but rather rotating around the center of our galaxy, while in turn our galaxy is also not at rest in the [[Cosmic Microwave Background#Velocity relative to CMB anisotropy|cosmic background]].