International Date Line International Date Line The International Date Line (IDL) is an imaginary line of demarcation on the surface of Earth that runs from the North Pole to the South Pole and demarcates the change of one calendar day to the next. It passes through the middle of the Pacific Ocean, roughly following the 180° line of longitude but deviating to pass around some territories and island groups. The IDL is roughly based on the meridian of 180° longitude, roughly down the middle of the Pacific Ocean, and halfway around the world from the Greenwich meridian. In many places, the IDL follows
Waldseemüller map far as Cattigara 180 degrees, which is 12 hours". This shows that Christopher Columbus overestimated the distance eastward between Portugal and Cattigara as being 225 degrees instead of Ptolemy's estimate of 180 degrees, permitting him to believe the distance westward was only 135 degrees and therefore that the land he found was the East Indies. As noted by Nunn, in accordance with this calculation, the Colon/Zorzi maps employ the longitude estimate of Claudius Ptolemy from Cape St. Vincent eastward to Cattigara, but the longitude calculation of Marinus and Columbus is employed for the space between Cape St. Vincent westward to