Neptune

If you're here already, you probably already know a lot about Neptune, but in case you don't, here's a small list of factoids about the blue pearl of the solar system for your consideration!

• Neptune is the eighth and final planet in the Solar System (no, Pluto is not a planet, it's a planetoid; scientific research does not just stop making improvements and reclassifications because you have a weird sense of entitled nostalgia, folks).
• It's the fourth-largest planet in diameter, the third-most massive, and comes out on top as the most dense in the Solar System.
• Like its almost-twin Uranus, Neptune is an ice giant, a giant planet consisting mainly of elements that are heavier than the hydrogen and helium that make up its cousins the gas giants, Jupiter and Saturn.
• Neptune is not visible in the night sky (or the daytime sky, either, natch) to the naked human eye -- to see it you'll need a telescope or binoculars. Human eyes can only see objects with a brightness of more than magnitude 6.5, while Neptune hovers around 7.7 on average, depending on the time -- before 1980, it hit its lowest at 8.0! Currently (September 2019) it's sitting at 7.82.
• It was the only planet that wasn't discovered by empirical observation, but rather by mathematical guesswork. French astronomer Alexis Bouvard looked at the unexpected changes in Uranus' orbit, and reasoned that there must be another planet influencing it. After his death, fellow French astronomer and mathematician Urbain Le Verrier used Bouvard's observations to calculate Neptune's likely position. Incidentally, so did British astronomer John Couch Adams, completely independently of Le Verrier! However, Le Verrier's calculations ended up being just that bit more accurate than Adams', so after much reviewing of the historical papers involved, modern astronomers have come to the conclusion that Le Verrier's discovery is considered the legitimate one.
• The planet was observed by someone who knew exactly what they were looking at via telescope for the first time on the 23rd of September, 1846 by German astronomer Johann Gottfried Galle.
• Neptune is named after the Roman god of the sea (or, technically, the Roman name for the Greek sea god, Poseidon), matching the theme of the majority of the planets and planetoids in the Sol system. It was originally proposed by Urbain Le Verrier himself...and claimed that the French Bureau des Longitudes had approved of the name (spoiler: they hadn't). Other names suggested before Neptune was officially adopted as its title (sometime after German-Russian astronomer Friedrich Georg Wilhelm von Struve put forward his favour of the name to the Saint Petersburg Academy of Sciences on the 29th of December, 1846) internationally included Janus, suggested by Galle, and Oceanus, put forwards by English astronomer and physicist James Challis (who is best remebered for "NOT finding Neptune"!). Le Verrier did a u-turn on the name in the October of 1846, wishing to call it Le Verrier, which was not a popular choice outside of the French astronomical community, to say the least. Still, for a time, French almanacs referred to Neptune Leverrier and Uranus as Herschel (after its own discoverer, Sir William Herschel).
• Neptune's names in Japanese, Chinese, Vietnamese, and Korean all translate to the same thing -- sea king star. They are, respectively, 海王星 (Kaiousei), 海王星 (Hǎiwángxīng -- Cantonese Hoi²wong⁴sing¹), Sao Hải Vương, and 해왕성 (Haewangseong).
• Neptune has fourteen known natural satellites, all named for water-related deities in Greek mythology. They are, from largest to smallest: Triton, Proteus, Nereid, Larissa, Galatea, Despina, Thalassa, Naiad, Halimede, Neso, Sao, Laomedeia, Psamathe, and Hippocamp. The first discovered was Triton in 1846, and the most recent is Hippocamp, only discovered in 2013!
• There are two types of Neptunian moons: regular and irregular. Regular moons follow a circular orbit around the planet, while irregular moons follow eccentric ones, and are mostly retrograde. The regular moons are the seven "inner" moons of Naiad, Thalassa, Despina, Galatea, Larissa, Hippocamp, and Proteus, and the remainder outer moons are irregular.
• Though very faint, Neptune has rings, just like all the gas giants in the solar system. They are in fact so faint that they were only discovered on the 22nd of July, 1984 -- 138 years after the planet's initial observation. There are five main rings named after astronomers involved in its general discovery process: Galle (the innermost ring), Le Verrier, Lassell (after English astronomer William Lassell, discoverer of Triton), Arago (after French physicist François Arago, who was instrumental in Le Verrier's discovery of the planet and supporter of the aforementioned's attempt at naming the planet), and Adams (the outermost ring). Some publications, however, don't consider Arago a true ring. There is also an even fainter, as-yet unnamed ring that is coincident with its satellite Galatea.
• The Adams ring is not even -- it is thick in some areas, and thin in others. These areas of thickness are known as arcs, and were the first parts of Neptune's rings to be discovered. The names of these five arcs are, in order from thickest (thus brightest) to thinnest are Fraternité, Égalité 1, Égalité 2, Liberté, and Courage, French for 'fraternity', 'equality', 'freedom', and...surprise surprise...'courage'.
• The inner moons are closely entwined with Neptune's rings. The existence of the arcs in the Adams ring may be due to the aforementioned Galatea, which functions as Neptune's only discovered shepherd moon -- so far, although Despina has also shown some evidence of being a shepherd for the Le Verrier ring. The two innermost moons, Naiad and Thalassa, orbit between the Galle and Le Verrier rings.