The Far Planets - Neptune

Neptune is usually the eighth planet from the Sun. (Occasionally the erratic orbit of Pluto brings that planet in closer than Neptune.) Neptune was named after the mythical Roman god of the sea. The planet is very far from Earth and extremely difficult to observe. The planet has a distinctive bluish hue when viewed through a telescope. It orbits around the Sun once in 165 Earth years.

Neptune has at least thirteen moons. They are named after characters associated with Neptune or Poseidon (his Greek counterpart) or other sea-related individuals in ancient mythology.

Neptune's Controversial Discovery

Neptune's discovery is a twisted tale of mathematics, bureaucrats, and international competition. Following the discovery of Uranus in 1781, astronomers watched that planet for several decades. They were puzzled because its orbit did not follow the path expected. Some astronomers began to suspect that there might be another planet beyond Uranus. The effect of its gravity would explain the irregularities that astronomers saw in the orbit of Uranus.

During the 1840s two scientists used mathematics to figure out where this mystery planet could be located. Their names were Urbain Le Verrier (1811–77) of France and John Couch Adams (1819–92) of Britain. Adams presented his theory to the director of the Cambridge Observatory in England. For some reason the director did not pursue the matter and look for the unknown planet. Le Verrier submitted his theory to the Berlin Observatory. It was directed by Johann Galle (1812–1910). On the night of September 23, 1846, Galle used Le Verrier's notes to locate the planet in the sky.

When Le Verrier and Galle publicized the discovery, the director of the Cambridge Observatory complained that Adams had described the location of the planet months before Le Verrier. It turned into a heated argument between France and England. Astronomers decided to split the credit for the planet's discovery between Adams and Le Verrier. Galle is considered the first to observe the planet. However, a review of Galileo's notes from the 1600s revealed that Galileo actually spotted the planet centuries before, but thought it was a fixed star.

Lassell Discovers a Moon around Neptune

Only weeks after Neptune's discovery its first moon was discovered. In early October 1846 an amateur British astronomer named William Lassell (1799–1880) spotted the moon. It was named Triton after the sea-god son of Poseidon. The name was suggested by the French astronomer Camille Flammarion (1842–1925).

More Moons of Neptune

During the 1940s another moon around Neptune was discovered from a ground-based telescope. In 1989 images from the spacecraft Voyager 2 revealed six previously unknown moons. The latest discoveries occurred in 2002 and 2003 when five new moons were added to the list, bringing the total to thirteen.