Why is the prime meridian a half circle?

The prime meridian is considered a half circle because it represents the line of 0 degrees longitude, which divides the Earth into the Eastern and Western Hemispheres. This line runs from the North Pole to the South Pole, forming a semicircle on the globe. Since the Earth is a sphere, half of its circumference is 180 degrees, allowing the prime meridian to serve as a reference point for measuring longitude both east and west.