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.
Smooth Tongue. The most common cause of a smooth tongue is the use of dentures. Nutritional deficiencies include iron, folate and vitamin B12 deficiency. B12 deficiency will also make the tongue sore and beefy-red in color. Glossitis, by causing swelling of the tongue, may also cause the tongue to appear smooth.