Galileo first discovered, during his observation between 1612 and 1613, the eighth planet from the sun, Neptune. However he recorded it as a fixed star.  After the discovery of Uranus it was observed that its orbit was not in accordance with Newton’s laws. Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky.


NASA Voyager 2

Neptune’s orbit is 2.8 million miles or 30.06 A.Us from Sun. It will take 165 Earth years for this ice giant to orbit around Sun.

Whatever we know about Neptune is know from it’s only visitor spacecraft, Voyager 2 in 1989. But fortunately, recent ground-based and Hubble Space Telescope observations have added a great deal, too.

Due to the Pluto’s eccentric orbit, which crosses Neptune’s, which makes it’s the distant planet form Sun for few years.

Storms On Neptune

Despite of its distance from the sun, Neptune’s winds are three times stronger than Jupiter’s and nine times stronger than Earth’s.

At the time of the Voyager encounter, Neptune’s most prominent feature was the Great Dark Spot (left) in the southern hemisphere. It was about half the size as Jupiter’s grate red spot (about the same diameter as Earth). Neptune’s winds blew the Great Dark Spot westward at 300 meters/second (700 mph). Voyager 2 also saw a smaller dark spot in the southern hemisphere and a small irregular white cloud that zips around Neptune every 16 hours or so now known as “The Scooter” (right). It may be a plume rising from lower in the atmosphere but its true nature remains a mystery.

However, Hubble Space Telescope(HST) observations of Neptune (left) in 1994 show that the Great Dark Spot has disappeared! It may have been either simply dissipated or is may be being masked by other aspects of the atmosphere. A few months later HST discovered a new dark spot in Neptune’s northern hemisphere. This indicates that Neptune’s atmosphere changes rapidly, perhaps due to slight changes in the temperature differences between the tops and bottoms of the clouds.



Neptune also has rings. Earth based observations show faint arcs. But the Voyager 2 images showed them as complete rings. The planet has six rings of varying thicknesses, confirmed by Voyager 2’s observations in 1989. This ice giant’s rings are believed to be relatively young and relatively short-lived.Like Uranus and Jupiter rings Neptune’s rings are also very dark. Their composition is unknown.

The interior of Neptune, like that of Uranus, is primarily composed of ices and rock.

 History of Observations

  • 1612: Galileo observed Neptune and noted it as a static star.
  • 1846: Using mathematical calculations, astronomers discover Neptune, increasing the number of known planets to eight. Neptune’s largest moon, Triton, is found the same year.
  • 1984: Astronomers find evidence for the existence of a ring system around this ice giant.
  • 1989: Voyager 2 becomes the first and only spacecraft to visit Neptune, passing about 4,800 km (2,983 miles) above the planet’s north pole.
  • 1998: Scientists using telescopes on Earth and in space image Neptune’s rings and ring arcs for the first time.
  • 2003: Using improved observing techniques, astronomers discover five new moons orbiting Neptune.
  • 2005: Scientists using the Keck Observatory image the outer rings and find that some of the ring arcs have deteriorated.
  • 2011: Neptune completes its first 165-year orbit of the sun since its discovery in 1846.
  • 2013: A scientist studying Neptune’s ring arcs in archival Hubble Space Telescope images finds a previously unknown 14th moon of Neptune, provisionally designated S/2004 N 1.