A common term in geography, the term "longitude" refers to the lines that run perpendicular on a map with latitude, meaning that longitude runs vertically. Lines of longitude are also sometimes known as meridians.
Every line of longitude is part of a great circle, and there is a distinct spot where longitude is measured at 0 degrees. In fact, the 0 degree mark is internationally recognized as being located in Greenwich, England. The Earth is divided into 360 degrees of longitude, with each part being equal. The prime meridian, or point where longitude is measured at 0 degrees, has both an east and a west. On each side of the prime meridian, there are 180 degrees of longitude.