Neptunus

Neptunus was the Roman god of freshwater — springs, rivers, and lakes — before the Romans identified him with the Greek Poseidon and expanded his domain to the sea. His name has no certain etymology, though some scholars connect it to an Indo-European root meaning 'moist' or to the Vedic water deity Apam Napat, 'grandson of the waters.' What is certain is that by the late Republic, Neptune was the god of the sea, depicted with a trident and attended by hippocampi — half-horse, half-fish creatures.

In 1846, the French mathematician Urbain Le Verrier predicted the existence of an eighth planet by analyzing irregularities in Uranus's orbit. Something massive was pulling Uranus off course. Le Verrier calculated where the unknown planet should be and sent his coordinates to Johann Galle at the Berlin Observatory. On September 23, 1846, Galle pointed his telescope at Le Verrier's predicted position and found Neptune within one degree of where the math said it would be. It was the first planet discovered by prediction rather than observation.

The name Neptune was Le Verrier's own suggestion, though he initially tried to name it after himself. The international astronomical community settled on Neptune because the planet's blue color, caused by methane in its atmosphere absorbing red light, evoked the sea. The British astronomer John Couch Adams had independently made similar calculations, and the question of priority between Le Verrier and Adams sparked a Franco-British rivalry that lingered for decades.

In 1940, the American physicists Edwin McMillan and Philip Abelson synthesized element 93, the first transuranium element. Following the pattern of the periodic table — uranium was named after Uranus — they named the new element neptunium, after Neptune. The sea god's name now designates a planet, an element, and the adjective Neptunian, used in geology to describe rocks formed by water. Neptune rules water in every sense.