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.
I posted a while back looking for help with Hairy Tongue. Several more months have passed and I still have hairy tongue. It started almost a year ago when I took a 1 week course of Metronodizole (Flagyl). The antibiotic did its job but caused this condition called hairy tongue as a side effect that seems permanent. My tongue looks like it has a white coating of thousands of little elongate papillae. Almost like my once healthy pink tonge now has a 1970's shag-carpet growing on it.