What is the prime meridian?

• A. Zero degrees longitude, also known as the Greenwich Meridian
• B. Zero degrees latitude
• C. The 180th meridian
• D. The equator

Answer: (A)

The prime meridian is the zero-degree reference line for measuring longitude. Longitudes are imagined lines running from the North Pole to the South Pole, and the prime meridian sets the starting point from which east and west distances are measured. It passes through Greenwich, England, at the Royal Observatory, which is why this line is commonly called the Greenwich Meridian.

This line is distinct from the equator, which is the zero-degree line for latitude and divides the Earth into northern and southern hemispheres. The longitude values increase from 0° at the prime meridian up to 180° to the east or west, with the opposite side of the globe at 180° often tied to the International Date Line. So the prime meridian is the line of zero degrees longitude, the Greenwich Meridian.