Meridians of Longitude are evenly spaced all the way around the earth at the Equator. They too are divided into degrees. This meridian is so named because it passes through Greenwich, England. Each meridian or degree of longitude on either side of the Greenwich Meridian is described as East or West, culminating at 180 Degrees in the Pacific Ocean. After proceeding in either direction all the way around to 180 Degrees, the direction changes from East to West, or vice versa.
Any meridians passing through the United States are degrees of West longitude.
Proceeding westerly into the Pacific Ocean one would eventually come to the 180th Degree of Longitude. If one were to cross that meridian, one would then be in degrees of East longitude. Conversely, if one were to leave Greenwich in an easterly direction, eventually crossing the 180th meridian of longitude, one would then be in degrees of West longitude. Take a look at the Longitude Scales which generally appear along the top and bottom of a chart. This scale must NOT be used to measure distance, because as previously mentioned, meridians converge at the poles.