Okay, I have my Mom looking at this with me. From the information on the map it seems the answer is the Prime Meridian at 0 degrees longitude and its twin line of longitude opposite to the Prime Meridian at 80 degrees longitude. However, I told my Mom it is the Prime Meridian and the International Dateline. She does not know??
You're right about the Prime Meridian which goes through Greenwich, England and western Africa.
On the other side of the world, longitude 180 degrees (not 80 degrees) is the other dividing line.
The International Dateline follows most of the 180 degree meridian, but not all of it. People in that area decide for themselves exactly where the International Dateline is. A couple of Pacific Islands changed the Dateline just before 2000 so that they could be the first to welcome in the new millenium.
posted by Ms. Sue