Triton is the largest moon of Neptune. It is about 1,680 miles (2,700 kilometers) in diameter, almost exactly7/9 as wide as Earth's moon. Triton is unusual in two ways: (1) it orbits Neptune in the direction opposite that of the planet's rotation, and (2) it is one of only two satellites in the solar system known to have an atmosphere that is dense enough to have weather. Saturn's moon Titan is the other one. Triton was discovered in 1846, only three weeks after the discovery of Neptune.