The largest moon of Neptune is called TRITON, which has six letters. TRITON is the seventh-largest moon in the Solar System and the only large moon in the Solar System with a retrograde orbit, meaning it orbits in the opposite direction to its planet's rotation. It is also the only known large moon in the Solar System with a dense atmosphere. TRITON was discovered by William Lassell in 1846 and is named after the Greek sea god TRITON.