What are the definitions of the western hemisphere?

Answer 1

In discussing the geography of the planet Earth, it has become a matter of convention to refer to the Eastern Hemisphere and the Western Hemisphere. The division is specified by an imaginary line drawn between the North pole and the South pole that passes through Greenwich, England. Everything to the east of that line is the Eastern hemisphere and everything to the west is the Western Hemisphere, and the two hemispheres meet again at a line on exactly the opposite side of the Earth.

This derives from the need to specify locations on Earth by latitude and longitude. The lines of longitude start at 0 degrees and that line is the above mentioned line drawn between the North pole and the South pole that passes through Greenwich, England. It is accordingly also called the Prime Meridian. The precise location is through the through the Royal Observatory, Greenwich in southeast London, United Kingdom. The line on the opposite side of the Earth is called the 180th meridian or antemeridian.