Geographical Longitude

Terrestrial longitude measures one's distance east or west of an arbitrarily-chosen arc on the surface of the earth that runs from the north pole to the south pole. Longitude circles are not parallel to each other, but pass through both poles and intersect the equator perpendicularly. All longitude circles are great circles of equal diameter.

The prime meridian, which equals zero degrees longitude, is the longitude arc from the North pole to the South pole that runs through Greenwich Observatory in England. The longitude arc 180 degrees opposite (the other half of the same great circle) is the ideal international dateline (the actual dateline zigs and zags for political reasons). Longitudes are measured in degrees east or west of the Greenwich meridian. This produces the curious result that the northernmost, easternmost, and westernmost state of the USA is Alaska, since it lies on both sides of 180 degrees longitude, with an island or two in the eastern hemisphere. (Okay, trivia buffs: Hawaii is the southernmost state.)