Longitude

Longitude, as a geographical coordinate, identifies east-west position on Earth, with lines of longitude, or meridians, running from pole to pole. Because of the Earth’s rotation, the difference in longitude between two locations is equivalent to the difference in their local times: one degree of longitude equals a four-minute time difference, and 15 degrees is equal to one hour (making 360 degrees, or 24 hours, in total). There is no naturally occurring zero point, or prime meridian, and so it is for historical and contingent reasons that we have come to measure longitude from a meridian that runs through Greenwich, near London in the United Kingdom. Longitude was far more difficult than latitude (north-south position) to measure by astronomical observation. Latitude is defined in relation to the equator and could be calculated by observing the maximum altitudes of the Sun or other stars using instruments such as astrolabes, cross-staffs and back-staffs. Longitude, in contrast, came to be defined in the early modern period as a difficult technical problem that was either insoluble or a challenge to be met with improved mathematics, astronomy and instrumentation.