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 causes of black, hairy tongue include poor oral hygiene and excessive use of medications containing bismuth, such as Pepto-Bismol. Other causes include antibiotic use, which can change the normal bacteria content in the mouth. Heavy tobacco use is also a cause.