What is the imaginary line at Greenwich, England, that is used to measure longitude called?

The imaginary line at Greenwich, England, used to measure longitude is known as the Prime Meridian. This line is defined as zero degrees longitude, serving as the reference point from which all other longitudes are measured. The Prime Meridian runs from the North Pole to the South Pole and divides the Eastern and Western Hemispheres of the Earth. This fundamental geographical reference is crucial for navigation and for the functioning of global time zones, as it establishes the standard for Coordinated Universal Time (UTC).

In contrast, the Equator is an imaginary line that divides the Earth into the Northern and Southern Hemispheres and is used to measure latitude, not longitude. The International Date Line, situated approximately opposite the Prime Meridian, primarily serves as the demarcation for calendar dates, while the Tropic of Cancer is a specific latitude that indicates the northernmost point where the sun can be directly overhead. Each of these terms refers to different geographical concepts that serve distinct purposes.