An Etymological Dictionary of Astronomy and Astrophysics
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The eighth planet from the Sun and the fourth largest by size in the → solar system. The equatorial radius of Neptune is 24,764 km (3.883 Earths), its → semi-major axis is 30.11 → astronomical units (4.50 × 10⁹ km), and its → orbital period is 164.8 yr. Neptune has at least 14 moons, the largest ones are → Triton, → Proteus, and → Nereid, whereas its smaller moons are: Naiad, Thalassa, Despina, Galatea, Larissa, Halimede, Sao, Laomedeia, Psamathe, and Neso. Neptune has an incredibly thick atmosphere comprised of 74% → hydrogen, 25% → helium, and approximately 1% → methane. Particles of icy methane in its upper atmosphere give Neptune its deep blue color. Large storms whirl through Neptune's upper atmosphere, and high-speed winds track around the planet at up 600 m/s, fastest recorded in the solar system. One of the largest storms ever seen was recorded in 1989. Called the → Great Dark Spot, it lasted about five years. Neptune has a very thin collection of → rings. They are likely made up of ice particles mixed with → dust grains and possibly coated with a carbon-based substance.

Named for the Roman god of the sea Neptune, Gk. Poseidon.