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
Encyclopedia of the Central Intelligence Agency if he would instead be interested in writing an encyclopedia detailing the history of the CIA. Smith agreed as he felt the CIA was highly relevant to military history. "Encyclopedia of the Central Intelligence Agency" was first published in 2003 by Facts on File. It was released in both hardcover and paperback formats. An eBook edition was published the same year. Additionally an eBook was also published in 2003 in Great Britain. In the "Encyclopedia of the Central Intelligence Agency", Smith discusses and defines key concepts relating to intelligence gathering, including "clandestine mentality", a colloquial term used to characterize what