1 A half of a sphere.
1.1 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.
1.2 A half of the celestial sphere.
1.3 Each of the two parts of the cerebrum (left and right) in the brain of a vertebrate.
Late Middle English (in the sense ‘half the celestial sphere, the sky’): from Old French emisphere, via Latin from Greek hēmisphairion, from hēmi- ‘half’ + sphaira ‘sphere’.
We must skitter across half the globe to explore the southern hemisphere.
(Well, actually we probably didn’t actually start at the north pole.)