Editing International Date Line (section)

===Circumnavigating the globe===

People traveling westward [[circumnavigation|around the world]] must set their clocks: 
*Back by one hour for every 15° of longitude crossed, and 
*Forward by 24 hours upon crossing the International Date Line.

People traveling eastward must set their clocks: 
*Forward by one hour for every 15° of longitude crossed, and 
*Back by 24 hours upon crossing the International Date Line.

Moving forward or back 24 hours generally also implies a one day date change.

The 14th century Arab geographer [[Abulfeda]] predicted that circumnavigators would accumulate a one-day offset to the local date.<ref>{{cite book |last1=Gunn |first1=Geoffrey C. |title=Overcoming Ptolemy: The Revelation of an Asian World Region |date=15 October 2018 |publisher=Lexington Books |location=Lanham, Maryland |isbn=9781498590143 |pages=47–48}}</ref> This phenomenon was confirmed in 1522 at the end of the [[Magellan's circumnavigation|Magellan–Elcano expedition]], the first successful circumnavigation. After sailing westward around the world from [[Spain]], the expedition called at [[Cape Verde]] for provisions on Wednesday, 9 July 1522 (ship's time). However, the locals told them that it was actually Thursday, 10 July 1522. The crew was surprised, as they had recorded each day of the three-year journey without omission.<ref>{{cite book|last=Neal|first=Larry|title=The Rise of Financial Capitalism: International Capital Markets in the Age of Reason|url=https://books.google.com/books?id=ey8h1WC5w7kC|year=1993|publisher=Cambridge University Press|isbn=978-0-521-45738-5|page=[https://books.google.com/books?id=ey8h1WC5w7kC&dq=%229+july%22+%2210+july%22&pg=PA8-IA1 1]}}</ref> Cardinal [[Gasparo Contarini]], the Venetian ambassador to Spain, was the first European to give a correct explanation of the discrepancy.<ref>{{cite book |last1=Winfree |first1=Arthur T. |title=The Geometry of Biological Time |edition=2nd |date=2001 |publisher=Springer Science & Business Media |location=New York |isbn=978-1-4757-3484-3 |page=10 |language=en}}</ref>