Geocentric model the coordinate system in order to predict the motions of planetary bodies, though doing so may make calculations easier to perform or interpret. A geocentric coordinate system can be more convenient when dealing only with bodies mostly influenced by the gravity of the Earth (such as artificial satellites and the Moon), or when calculating what the sky will look like when viewed from Earth (as opposed to an imaginary observer looking down on the entire solar system, where a different coordinate system might be more convenient). The Ptolemaic model of the solar system held sway into the early modern age;
Truce term credibility". "Pax" was no longer a group word as reported by the Opies. Other terms reported included "pips", "force field" and "quits". The authors concluded that either the Opies had grossly oversimplified the picture or things had radically changed in 30 years (some seven to eight generations of primary school children). They also noted that although some schools reported a marked preference for a particular term, all schools reported at least some children using different terms. The Concise Scots Language Dictionary published in 1999, records the use of "keys" as a truce term in Fife, south west and west central