General

Like Uranus its close relative, Neptune is a gas giant planet, a huge rapidly rotating largely fluid world with a density more than twice that of water, greater than that of Uranus, and with a considerable atmosphere. The equatorial diameter is 31 403 miles (50 538 km) and the polar diameter is 30 589 miles (49 229 km) so that the ratio of the polar to the equatorial diameter is 0.98. Neptune is about four times bigger than the Earth (fig. 12.1) and is slightly smaller than Uranus. The rotational axis is inclined 29.6° to the plane of its orbit so that it does not share the remarkable axial tilt of Uranus. The axial rotation period is 16 hours 3 minutes (16 hours 7 minutes for the magnetic field). Neptune's distance from the sun varies from an aphelion distance of 2819.2 million miles (4537.0 million km) to a perihelion distance of 2771.4 million miles (4460.2 million km).

The mean orbital velocity is 3.37 miles (5.43 km) per second and the orbital eccentricity is 0.009. The orbit is inclined 1° 46′ to the plane of the ecliptic. Neptune's orbital (sidereal) revolution period is 164.8 Earth years and its synodic period is 367.5 Earth days.

Neptune is encircled by eight satellites and a system of rings. Two of the satellites, Triton and Nereid, were discovered by Earth-based observation, the remainder by the Voyager 2 spacecraft. The orbits of Triton and Nereid around Neptune are shown in fig 12.2.