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.
The name black hairy tongue may sound scary, but the condition is harmless. Black hairy tongue is caused by bacteria or fungi in the mouth, which make the tongue appear black and hairy. It's easily remedied by good old-fashioned oral hygiene.