Neptune is a gas giant planet, with a diameter of 49,500 km. At a distance of 4,497,000,000 km from the Sun, it is the most distant planet from the Sun, now that Pluto is no longer considered to be a planet. Neptune is composed mostly of hydrogen and helium, with small amounts of other gasses. Neptune takes 164.8 years to complete one orbit of the Sun, but rotates on its axis once every 19.2 hours. Neptune's orbit is almost perfectly circular; only Venus has a more circular orbit. Even at its closest approach to Earth, light traveling to us from Neptune requires 4.04 hours to make the journey.
In the 1800's, astronomers predicted that another planet lay beyond the orbit of Uranus, because they could detect irregularities in the orbit of Uranus that appeared to be caused by another body pulling and tugging on the planet . Based upon the motions of Uranus, the probable position of Neptune was calculated mathematically. In 1846, Neptune was discovered near the predicted position. Neptunes' largest moon Triton (with a diameter of 2,700 km) was found at the time the planet was discovered, and in 1949, another satellite named Nereid was discovered.
Few details were known about the planet until the Voyger 2 space probe flew past it in 1989. Voyager discovered six additional moons of Neptune, bringing the total number of known moons at that time to eight. Voyager 2 also discovered faint rings around the planet that are not visible from Earth. Much to everyone's surprise, Voyager 2 showed that Neptune's clouds contained active weather systems and raging storms. This is not the cold dead world that we had previously envisioned.
This image was taken from my back yard in Scottsdale, Arizona, using a Mewlon 210 telescope at prime focus with an SBIG ST-8E and a CFW-8. On the date that this image was taken, Neptune was about magnitude 7.9, and the size of its image was a tiny 2.2 arc-seconds. Even at opposition, Neptune is only marginally better, reaching a size of 2.3 arc-seconds at magnitude 7.8. Neptune cannot be seen with the naked eye, but can be discerned with binoculars under a dark sky. Magnifications of over 200x are usually necessary to see the planet as a disk rather than as a point of light. At best, the planet is only a tiny bluish dot in amateur size telescopes. I also imaged Neptune with the Steward Observatory 61-inch Kuiper Telescope, and that image includes Neptune's moon Triton.
RA: 21h 02m 26.6s Dec: -16d 51' 12"
May 4, 2003
Image by Sid Leach
Complete list of images.
Description of equipment used to acquire images.
Feedback and comments should go to Sid