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.
Most often that color is black, hence the name. But the tongue can also turn brown, yellow, green, or a variety of other colors. Certain lifestyle habits and conditions can make people more likely to develop black hairy tongue. They include: poor oral hygiene. smoking tobacco. drinking a lot of coffee or tea.