A meridian line is an imaginary line that runs from the North Pole to the South Pole, dividing the Earth into the Eastern and Western Hemispheres. It is used in geography and navigation to help determine longitude, which is the measurement of how far east or west a location is from the prime meridian, located at Greenwich, England. Meridian lines are essential for mapping and understanding time zones. Each meridian is spaced 15 degrees apart, which corresponds to one hour of time difference. This system helps in coordinating activities across different regions of the world, making it easier to communicate and travel.