A great circle divides the Earth into the Eastern and Western Hemispheres.That circle is referenced by two Meridians.
The Prime Meridian is the line of longitude at which longitude is defined to be 0°.
The Prime Meridian intersects eight (8) countries, (shown to the left) and those countries are therefore in both the Eastern and Western Hemispheres.
The 180th Meridian, on the opposite side of the planet, is the line of longitude at which longitude is defined to be 180°.
The 180th Meridian directly intersects four (4) countries, and those countries are therefore in both the Eastern and Western Hemispheres.
NOTES: The far western end of Alaska's Aleutian Islands are disected by the 180th Meridian, thus putting the USA into both hemispheres. Kiribati is the only country in all four hemispheres.
COUNTRIES IN BOTH THE NORTHERN and SOUTHERN HEMISPHERES
There are twelve (12) countries located directly on the Equator (0° latitude) and all are therefore in both the Northern and Southern Hemispheres. For a printable map and list of those countries go here!