We just learned about the Arctic Ocean
We learned that the middle of the earth halfway between North and South
is called the equator.
If we want to draw a line like that from the top of the map to the bottom,
to split it into East and West we use the Prime Meridian.
(from: wikipedia - prime meridian)
The main location on the map where the prime meridian is marked is a place called
the Royal Observatory Greenwich, in London England.