Earth is a sphere, as is any other planet. When Earth is presented as a map, reaching one edge wraps us to another; for example, when we move east and reach the furthest point in the Pacific Ocean, we are wrapped to the west side of the map and we continue moving towards America. Furthermore, to make the movement easier, we can define the map as a grid. That grid should have length and height expressed as an x-axis and y-axis. That grid should have maximum length (x) and height (y).