On the Earth's surface, which point serves as the starting reference for measuring longitude?

The prime meridian serves as the starting reference for measuring longitude because it is defined as 0 degrees longitude. This meridian runs from the North Pole to the South Pole and passes through the Royal Observatory in Greenwich, London. Longitude measures the east-west position of a point on the Earth's surface, and the prime meridian is the baseline from which all other longitudes are calculated.

This reference point is essential for navigation and global positioning, as it provides a standardized point to define the geographical coordinates of locations around the world. Other options are relevant to geography but do not serve as the starting reference for measuring longitude. For instance, the equator is the reference line for measuring latitude, while the International Date Line, located at 180 degrees longitude, serves as a line for demarcating calendar days rather than a starting point for measuring longitude.