The imaginary line that runs north and south and serves as the starting point for east is the Prime Meridian.

Here's why:

* Longitude: The Prime Meridian is the zero-degree line of longitude. Longitude lines are imaginary circles that run from the North Pole to the South Pole, measuring distances east or west of the Prime Meridian.

* East and West: The Prime Meridian divides the Earth into the Eastern Hemisphere (to the east of the Prime Meridian) and the Western Hemisphere (to the west of the Prime Meridian).

* Starting Point: Therefore, the Prime Meridian serves as the starting point for measuring distances east or west.

Let me know if you have any other questions!