The IAU, the party officially recognized for naming bodies outside of earth's atmosphere, officially recognizes the moon's name as... well.. The Moon. Luna is the Latin term for Moon. Earth's name is Earth, not Terra. The name of the sun is The Sun, not sol. Luna is simply the latin term for Moon, and the Moon's official name is Moon. Why are moons of other planets called moons as well? Not sure. Perhaps that is just the laymen term for satellite.