Nereid is one of the moons of the planet Neptune. Discovered in 1949, it is named after the sea nymphs in Greek mythology. Nereid is unique because of its highly eccentric orbit, which means it travels in a stretched-out path around Neptune. It is the third-largest moon of Neptune and has a diameter of about 340 kilometers (211 miles). Nereid's surface is thought to be composed of water ice and possibly other materials, giving it a bright appearance. It orbits Neptune at an average distance of about 5.5 million kilometers (3.4 million miles) and takes approximately 360 days to complete one orbit.