Question

What is a hemisphere?

Answer 1

The hemisphere is the imaginary line which divides the Earth into two. Both divided parts have names, the northern hemisphere, and the southern hemisphere.

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Answer 2

Depends on which hemisphere you need.  it could be half a sphere or a half of the earth, usually as divided into northern and southern halves by the equator, or into western and eastern halves by an imaginary line passing through the poles.