Tenth parallel: religious and political faultline

The tenth parallel is the horizontal band that rings the earth seven hundred miles north of the equator If Africa is shaped like a rumpled sock, with South Africa at the toe and Somalia at the heel, then the tenth parallel runs across the ankle. Along the tenth parallel, in Sudan, and in most of inland Africa, two world collide: the mostly Muslim, Arab-influenced north meets a black African south inhabited by Christians and those who follow indigenous religions—which include those who venerate ancestors and the spirits of animals, land, and sky. 

Eliza Griswold, The Tenth Parallel: Dispatches from the Fault Line Between Christianity and Islam (New York: Farrar, Straus and Giroux, 2010), p. 3

 To the east, five thousand miles off the Africn coast and over the Indian Ocean, natural forces also shaped the encounter of Christianity and Islam in the Southeast Asian nations of Indonesia, Malaysia, and the Philippines. The trade winds—high-pressure air currents that move steadily from either pole toward the equator—filled the sails of both Muslim and Christian merchants from the northern hemisphere beginning in the eighth century.

Id., p. 8.