What line of longitude is designated as 0°?

The line of longitude that is designated as 0° is the Prime Meridian. This line serves as the starting point for measuring longitude, which is the angular distance east or west of the Prime Meridian, measured in degrees. The Prime Meridian runs through Greenwich, England, and is significant not only for navigation and mapping but also for establishing time zones around the world. Understanding the role of the Prime Meridian is crucial in geography as it helps to provide a frame of reference for global positioning and timekeeping.

The other options, such as the Tropic of Cancer and the Equator, are lines of latitude, which measure distances north or south of the Equator, while the International Date Line is situated at approximately 180° longitude and serves as a reference for calendar dates across the globe. These distinctions demonstrate the unique position of the Prime Meridian in the geographic coordinate system, making it pivotal in the study of geography and social studies.