III.

Motion of Neptune

Neptune orbits about 4,490 million km (about 2,790 million mi), or 30 astronomical units (AU) from the Sun, beyond Uranus. An AU is equal to the average distance between the Earth and the Sun, or about 150 million km (93 million mi). Neptune takes 164.79 years to complete a single revolution around the Sun, so a year on Neptune is 164.79 times longer than a year on Earth. The orbit of Neptune traces out a flat region of space called the planet’s orbital plane. The orbital plane of Neptune lies close to Earth’s orbital plane. As a result, Neptune always crosses the same region of Earth’s sky.

The planet spins around its axis once every 16 hours in a counterclockwise direction, just as Earth spins once every 24 hours. The axis of rotation on Neptune tilts 29.6° into its orbital plane (the plane created by Neptune’s orbit around the Sun). This tilt gives Neptune almost Earthlike seasons. (Seasons on Earth result from our planet’s 23.5° tilt into its orbital plane.)