How come longitude lines start in Greenwich...

The Royal Observatory Greenwich is located at the National Maritime Museum in London. The original site of the observatory was arbitrarily chosen as longitude 0 degrees in 1884. A plaque in the original structure marks the zero point from which longitude is calculated. The observatory was founded in 1675 by King Charles II to keep accurate tables of the position of the Moon for the calculation of longitude by English ships. In 1750 those tables were published as the Astronomical Observations, and after 1838 they were published annually. Meridian observations of the Sun, stars, and planets also were made at the observatory. Photographs of the Sun were taken daily, conditions permitting, and a continuous photographic record of sunspots was kept starting in 1873. Today the observatory is primarily a museum with a small planetarium.