The International Date Line is an imaginary line on the earth's surface, at approximately the 180th meridian (LONGITUDE). Traveling east across the line, takes the traveler back one day; traveling west, takes the traveler forward one day. Without the International Date Line travelers going westward would discover that when they returned home, one day more than they thought had passed, even though they had kept careful tally of the days. This first happened to Magellan's crew after the first circumnavigation of the globe!