triton, neptune's largest moon, has an atmosphere, but it is so thin that its sky is still black, possibly with some pale haze at the horizon. because triton orbits with synchronous rotation, neptune always appears in the same position in its sky. as neptune orbits the sun, triton's polar regions take turns facing the sun for 82 years at a stretch, resulting in radical seasonal changes as one pole, then the other, moves into the sunlight.