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The average distance between Neptune and the Sun is 4.5 billion km (about 30.1 astronomical units (AU), the mean distance from the Earth to the Sun), and it completes an orbit on average every 164.79 years, subject to a variability of around ±0.1 years. The perihelion distance is 29.81 AU, and the aphelion distance is 30.33 AU.
Average distance from the Sun – Earth: 1.00 – Average distance of Earth's orbit from the Sun (sunlight travels for 8 minutes and 19 seconds before reaching Earth) – Mars: 1.52 – Average distance from the Sun – Jupiter: 5.2 – Average distance from the Sun – Light-hour: 7.2 – Distance light travels in one hour – Saturn: 9.5 –
The Kuiper belt and Neptune may be treated as a marker of the extent of the Solar System, alternatives being the heliopause and the distance at which the Sun's gravitational influence is matched by that of other stars (estimated to be between 50 000 AU and 125 000 AU). [22]
Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth and slightly larger than Neptune. Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 astronomical units (4.50 × 10 9 km).
Proteus is the second-largest moon of Neptune and is the largest of its regular prograde moons. It is about 420 km (260 mi) in diameter, larger than Nereid, Neptune's third-largest moon. It was not discovered by Earth-based telescopes because Proteus orbits so close to Neptune that it is lost in the glare of reflected sunlight. [15]
The distance of Alpha Centauri from the Earth is now reckoned at 4.396 light-years or 4.159 × 10 13 km.) α Centauri A (left) is of the same stellar type G2 as the Sun, while α Centauri B (right) is a K1-type star.
Mercury can come as near as 82,200,000 km (0.549 astronomical units; 51.1 million miles) to Earth, and that is slowly declining: The next approach to within 82,100,000 km (51 million mi) is in 2679, and to within 82,000,000 km (51 million mi) in 4487, but it will not be closer to Earth than 80,000,000 km (50 million mi) until 28,622. [119]
[21] [69] This planet would be perhaps six times as massive as Earth. [70] It would have a highly eccentric orbit, and its average distance from the Sun would be about 15 times that of Neptune (which orbits at an average distance of 30.1 astronomical units (4.50 × 10 9 km)). Accordingly, its orbital period would be approximately 7,000 to ...