Neptune’s existence was predicted by both English mathematician John Adams and French mathematician U.J.J. Leverrier, based on irregularities in Uranus’ orbit. The German astronomer J.G. Galle began searching for a new planet and found it very near the predicted position.

In August 1989, the robotic spacecraft Voyager 2 flew past Neptune, radioing back close-up pictures of the planet and its family of moons. Voyager also discovered a large storm system, similar to Jupiter’s Great Red Spot, which is called The Great Dark Spot.

Due to the elliptical shape of Pluto’s orbit, Neptune is the furthest planet from the Sun for about 20 years out of every 248 years. This was the case from about 1979 to 1999.

The basics

Order (outwards from the Sun): 8 or 9
Average distance from the Sun: 4 504 400 000 km (30.11 astronomical units)
Period of revolution (length of year): 163.73 years
Period of rotation (length of day): 16 hours 3 minutes at equator
Diameter: 49 530 km at the equator
Surface gravity: 1.19 times greater than Earth’s
Composition: Hydrogen and helium ice
Atmosphere: Hydrogen, helium, methane
Satellites: 11 moons and 5 incomplete ring arcs
Surface temperature: -205 degrees Celsius at the cloud tops

Appearance of surface

Perhaps a rocky core surrounded by massive layers of ice. May be covered by a deep ocean of liquid hydrogen under the visible cloud tops.

Appearance in Earth’s sky

Neptune is so far away it cannot be seen without the aid of a telescope. Neptune revolves so slowly that it spends years in basically the same area of the sky.

Telescopic appearance

Even in large telescopes, Neptune appears as a small pale blue speck, with no detail visible.


Canadian Heritage Information Network
Australian Museums & Galleries Online, Australia; Centre of the Universe; Gemini Observatory, Hawaii; Glenbow Museum; The Manitoba Museum; National Research Council Canada; Planétarium de Montréal

© Canadian Heritage Information Network, 2003

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