Neptune was discovered in September 1845. It was discovered by means of mathematics before being seen through a telescope. It began in 1843 when John Adams, a young English astronomer and mathematician, began working to find thew location of the unknown planet. Adams predicted the planned would be about 1.6 billion kilometres farther from the sun then Uranus. That's when he completed his work in September 1845. Also Urbain Leverriar, unknown to Adams, began working on the same project. By mid-1846, Leverriar also had predicted Neptune's position. He also sent in similar to what Adams had discovered. To this day both of these bright-minded men take the credit for discovering Neptune. Astronomers had noticed that Uranus, which they thought was the most distant planet, was not always in the position they predicted that it would be in. The force of gravity at some unknown planet seemed to be influencing the motion and position of Uranus. So that is what mainly influenced Adams to see what was causing Uranus to keep moving. In August 1989, the Voyager 2 spacecraft provided the first close-up views of Neptune and its satellites and rings. The planet was named after Neptune, the Roman sea God.
Neptune's diameter is about 49,100 kilometres, or almost 4 times that of Earth. The planet is about 13 times as massive as Earth, but it is not so dense as earth. It has 8 satellites (moons). Astronomers have also detected several rings around Neptune. Neptune cannot be seen without a telescope. Pluto crossed Neptune's orbit on January 23, 1979, and will remain within it until March 15, 1999. So that is how Neptune was discovered.