It was during the reign of Charles II., in the year 1675, that the famous Greenwich Observatory was founded, chiefly with a view to determining the longitude of ships at sea. The Observatory, which is built of materials taken from the Tower of London and Tilbury Fort, is set on a hill 180 feet high in Greenwich Park. Here the standard time is decided daily, and all manner of astronomical calculations are made with the fine instruments with which the Observatory is fitted. On the outer walls are standard measures of length, and a clock, showing the correct time. Mr. \V. H. M. Christie is the Astronomer Royal, being the eighth holder of the office; and the annual cost of the institution is about £11,000.
source: The Queen's London : a Pictorial and Descriptive Record of the Streets, Buildings, Parks and Scenery of the Great Metropolis, 1896