On this day in 1675, construction began on the Royal Observatory in Greenwich, England. John Flamsteed, the first Astronomer Royal, laid the foundation stone on the building that would come to be known as Flamsteed House. The building was completed on 10 July 1676 at a cost of £520.45. King Charles II had commissioned the observatory for the purpose of "rectifying the Tables of the Motions of the Heavens, and the places of the fixed Stars, so as to find out the so much desired Longitude of Places for perfecting the art of Navigation." Though it's hard to believe now, sailors at the time had no reliable way of determining their longitude, and the hope was that observations of the Moon would lead to a better approach. The resulting lunar tables were based on degrees of longitude away from Greenwich, which led to the decision by the 1884 International Meridian Conference to have the Prime Meridian (0° longitude) pass through the observatory.
