.................... movement of the magnetic lines from 1590 to 1990.....................
Now I wonder how they came up with the magnetic lines since 1590??
The chronometer was not available until the 1700’s, so a mariner had no way to pinpoint where he was on the earth’s surface in an east/west direction.
I am not sure of the 1590 lines, but I do remember that the orientation of magnetic lines can be found in volcanic rocks, based on when it cooled. Beyond that, I really don’t know.