Astronomical observatory , oldest scientific institution in Britain, founded for navigational purposes in 1675 by Charles II at Greenwich , England.
Its main contributions have been in navigation, timekeeping, determination of star positions, and almanac publication. In 1767 it began publishing The Nautical Almanac , based on the time at the longitude of Greenwich; its popularity among navigators led in part to the Greenwich meridian's being made Earth's prime meridian and the starting point for international time zones in 1884 (see Greenwich Mean Time ).