Final answer:
The purple arrow on the map points to the International Date Line, which is a crucial demarcation for the change of the calendar day and is situated near longitude 180° but does not strictly follow it.
Step-by-step explanation:
The purple arrow on the map is pointing to the International Date Line, an important line on the Earth's surface near longitude 180°. The International Date Line is not to be confused with the Prime Meridian or the equator.
While the Prime Meridian, set at 0 degrees longitude, divides the Earth into the Eastern and Western Hemispheres, and the equator runs around the Earth's middle, the International Date Line serves as an arbitrary boundary where the date changes by one day.
When crossing the line from east to west, you add a day, and when crossing from west to east, you subtract a day. This line does not follow the 180° meridian exactly, as it has been modified in places to accommodate international agreements and geographical considerations, including avoiding dividing groups of islands or having a country split by the line.