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
Singular term Singular term A singular term is a paradigmatic referring device in a language. Singular terms are of philosophical importance for philosophers of language, because they "refer" to things in the world, and the ability of words to refer calls for scrutiny. Singular terms are defined as expressions that purport to denote or designate particular individual people, places, or other objects. They contrast with "general terms" (such as "car" or "chair") which can apply to more than one thing. There are various kinds of singular terms: proper names (e.g. "Matthew"), definite descriptions (e.g. "the second fisherman in the boat"), singular personal