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.
Other symptoms of hairy tongue include: a burning sensation on the tongue; a gagging or tickling sensation caused by the lengthened cells on the tongue; bad breath caused by bacteria or yeast growing on the tongue; abnormal taste on the tongue, or a lack of taste caused by covered-up taste buds; Learn more about hairy tongue. Lichen planus