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How long is a day on Neptune?

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Clouds in Neptune's atmosphere — Neptune A photograph of clouds in Neptune's atmosphere, taken by Voyager 2 in August 1989. The view is from below the planet's equator, and north is up. The Great Dark Spot (center left) is 13,000 km (8,100 miles)—about the diameter of Earth—in its longer dimension. Accompanying it are bright, wispy clouds thought to comprise methane ice crystals. At higher southern latitudes lies a smaller, eye-shaped dark spot with a light core (bottom left). Just above that spot is a bright cloud dubbed Scooter. Each of these cloud features was seen to travel eastward but at a different rate, the Great Dark Spot moving the slowest.

The rotation period of Neptune, which defines the length of its day, is 16 hours and 7 minutes. This value was determined by the Voyager 2 spacecraft, which detected radio bursts associated with the planet's magnetic field. Astronomers infer this period to represent the rotation of the planet's interior, where the magnetic field is rooted.12