Geocentric model 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 heliocentric frame is most useful in those cases, galactic and extragalactic astronomy is easier if
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