What separates the westerns and eastern hemisphere?

By most standards, the prime meridian (which runs through Greenwich, England) as the eastern edge of the Western Hemisphere (and the western edge of the eastern hemisphere) and its longitudinal opposite at 180 degrees longitude (west or east, which runs through the Pacific Ocean) being the western edge of the Western Hemisphere and the eastern edge of the eastern hemisphere.

The time zones known as Greenwich Mean Time and the International Date Line are sometimes alternatively used to demark the western from eastern hemispheres, but the would not be true hemispheres as those time zones are not straight longitudes.