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.

I can help clarify the confusion! The Prime Meridian and the International Date Line are separate lines of longitude that play different roles.

The Prime Meridian is a reference line of longitude that passes through the Royal Observatory in Greenwich, London, dividing the Earth into the Eastern Hemisphere and the Western Hemisphere. It is assigned the value of 0 degrees longitude. This line is used as a global standard for measuring longitude.

On the other hand, the International Date Line (IDL) is a separate line of longitude, approximately 180 degrees from the Prime Meridian. The IDL serves as the dividing line between one calendar day and the next. When you cross the IDL from west to east, you move forward by one day, and when you cross it from east to west, you move back one day.

Now, coming back to your initial question, the lines at 0 degrees longitude and 80 degrees longitude are not the Prime Meridian and its twin line of longitude. The Prime Meridian is only at 0 degrees.

To find the twin line of longitude that is located 180 degrees opposite the Prime Meridian, you would need to look for a line running through 180 degrees longitude. This particular line does not have a specific name like the Prime Meridian but can be referred to as the antimeridian or the 180th meridian.

So, to recap, the Prime Meridian is located at 0 degrees longitude, while the International Date Line is roughly positioned at 180 degrees longitude. They serve different purposes and are not twin lines.