Longitude lines around the globe run north-south, and measure position east or west of a reference line. In 1884 an international conference agreed that the reference line would be the Greenwich Meridian, the longitude line that runs north-south through the observatory in Greenwich, England, outside of London. Maine's longitude ranges from 67° to 71° West longitude. While there are approximately 60 nautical miles per degree of longitude at the equator, that distance diminishes to nothing at the poles, where all of the longitude lines converge. Longitude was not easily measured until chronometers became available.