* A hemisphere is half of a sphere. The Earth is very close to a sphere, so a hemisphere is indeed half of the Earth's surface.
* However, the Earth is not perfectly split in half by hemispheres. We typically divide the Earth into the Northern and Southern Hemispheres along the Equator, and the Eastern and Western Hemispheres along the Prime Meridian. These lines are arbitrary, and there are other ways to divide the Earth into hemispheres.
So, a hemisphere is half of the Earth's surface, but it's not a perfect division of the Earth into two equal parts.
