ABSTRACT

Have you ever seen the Equator, or the Arctic Circle? Of course you have not; both are invisible lines. You might have been told that you were precisely at the spot on earth where an equator or where an arctic circle was supposed to be. There even might have been a monument or marker to identify the location of the Equator or the Arctic Circle. But you have not actually seen the Equator or Arctic Circle as a line or as a physical phenomenon.