During the 19th century sailors and travelers didn’t have the amenities that we do in the 21st century, like GPS. Many calculations were made based on celestial navigation, the stars. And in order to accurately decipher the stars, navigators had to know their longitude and time of day, no matter where in the world they were.
On this day October 13th, in 1884, Greenwich, England is made the universal time meridian of longitude. This was decided at the International Meridian Conference, which took place in Washington DC, of the United States.
It was proposed that the day would have 24 hours beginning with midnight in Greenwich. This also made Greenwich have the longitude of 0 degrees. A few nations did not sanction this, including San Domingo (the Dominican Republic) and France. In 1972 the term was changed to Universal Time, which is now used by astronomers. Noon can be up to 16 minutes off for when the sun is exactly over Greenwich, due to how the earth is shaped.