When you look out the aft dining room windows in the Tuscan Grill it seems like you are in the center of a large circle moving across a grey featureless sea. It is all too easy to believe that the world I’d flat; especially out here in the ocean with nothing else in view. Falling off the edge or meeting monsters seems all too real.
To balance that thought from a scientific point of view, you can draw imaginary lines on the globe to help define locations and relationships. Incidentally this helps out those with the need to navigate the ocean but I am not sure at all those concerns were in the forefront if the English minds that chose the Zero.
Night before last we crosses the Arctic Circle, the most northern of the five latitudinal divisions.
And today ~ 1100 we cross the Zero (Greenwich) Meridian that point on the globe where east and west are defined as extending 180* around, defining both positions and time zones.
There was no mark in the water, no line floating on the sea. Just the knowledge that the ship was in position and on the correct heading for our next land fall.