♆ Neptune

Named for the Roman god Neptune (Greek: Poseidon)

The sea Fresh water Horses

Which sign is Neptune in now?

Neptune’s Horses by Walter Crane
Neptune’s Horses Walter Crane, 1892 Neue Pinakothek, Munich White horses gallop forward on a breaking wave, with a rider above them. Public domain, via Wikimedia Commons

God of fresh water and the sea, and, like the Greek Poseidon, of horses.

The god

Neptune is the Roman god of fresh water and the sea, the counterpart of the Greek Poseidon. He was probably linked with freshwater springs before the sea: his festival, the Neptunalia, came in the heat and drought of summer, probably on July 23, when people built huts of branches and leaves. Poseidon was the trident-carrying god of the sea, earthquakes and horses, and the Romans also worshipped Neptune as a god of horses, Neptunus equestris, and patron of horse racing. In Greek myth, Poseidon made the very first horse as a gift when he competed with Athena to rule Athens.

How the planet got its name

Neptune was the first planet found by mathematical prediction rather than by watching the sky. Uranus wasn’t moving quite as expected, so Urbain Le Verrier in Paris and John Couch Adams in Cambridge each worked out where an unknown planet might be. Le Verrier sent his prediction to Johann Galle at the Berlin Observatory, who found Neptune on his first night of searching, September 23–24, 1846, within a degree of the predicted spot, and Le Verrier quickly proposed the name Neptune. Galileo had already recorded it as a star in 1612 and 1613. Its largest moon, Triton, was found seventeen days after the planet.

Symbols

Trident Horses

The planet

An ice giant like Uranus: most of its mass is a hot, dense fluid of water, methane and ammonia over a small rocky core, under an atmosphere of hydrogen, helium and methane. Its winds whip clouds of frozen methane along at more than 2,000 km/h, against about 400 km/h for Earth’s strongest. In 1989 Voyager 2, the first spacecraft to fly past Neptune, saw a huge oval storm in its southern hemisphere, the Great Dark Spot.

Diameter
49,528 km 3.9 × Earth
Mass
102 × 10²⁴ kg 17 × Earth
Gravity
11.0 m/s² 1.1 × Earth's
Length of day
16.1 hours sunrise to sunrise
Length of year
163.7 Earth years one trip around the Sun
Distance from the Sun
4,515 million km 30.18 AU (Earth = 1)
Average temperature
-200 °C -328 °F
Moons
16
Rings
Yes

Figures from NASA’s Planetary Fact Sheet; the temperature is its “mean temperature.” Comparisons with Earth are worked out from the same sheet.

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