Unlike the equator and the Tropic of Cancer and the Tropic of Capricorn, the Prime Meridian and all lines of longitude are completely imaginary lines and have no significance with regard to the Earth or to its relationship with the sun.
Black hairy tongue typically results when projections on the tongue called papillae grow longer and don't shed like normal. This makes the tongue look hairy. Debris, bacteria or other organisms can collect on the papillae and result in discoloration. The cause of black hairy tongue can't always be determined.
Q: What causes geographic tongue? A: The cause of geographic tongue is unknown. Several factors have been proposed as possible causes such as emotional stress, psychological factors, habits, allergies, diabetes and hormonal disturbances. However, none of these factors have been conclusively linked to geographic tongue. A relationship between geographic tongue and psoriasis (a skin disease) has been reported.