If its the shortest distance over the earth it is a Great Circle. An Agonic Line is a line where there is zero magnetic variation. As this is a crossword (?) do you have number of letters and any that you have.
I seem to remember that it asked for an imaginery line between two points of equal (or was it zero) magnetism. The answer - Agonic line - fitted the spaces and the clue, in my opinion.
Daisy, it has to be zero variation for Agonic. Lines joining points of equal magnetic variation are Isogonals. I am only interested in all this as it is part of my day job!