September 24, 2010
Neptune turns 164 on September 23rd! Actually the fourth largest planet in our solar system is billions of years old but it was discovered in 1846. Neptune is about 4 times Earth’s diameter and 30 times further from the Sun than Earth. Uranus and Neptune are the two planets in our solar system which are not visible to the naked eye. British astronomer and mathematician John C. Adams began calculating Neptune’s position in 1843. He submitted his extremely accurate prediction of the planet’s position to the Astronomer Royal of England who didn’t act on Adams’ prediction apparently because he lacked confidence it Adams’ calculations.
Image: Planet Neptune’s atmosphere is primarily methane. In this image Neptune’s moon Triton appears in the foreground. Image courtesy NASA/JPL Click to enlarge
At about the same time a French mathematician named Urbain J. J. Leverrier began working on the project. In mid-1846 he sent his calculations to the Urania Observatory in Berlin, Germany. The director of the observatory, Johann G. Galle, had just charted the area of the sky where both Adam’s and Leverrier’s calculations predicted the planet would be. On Sept. 23, 1846, Galle and his assistant, Heinrich L. d’Arrest, found Neptune near the position predicted by Leverrier. Both Adams and Leverrier are credited with the discovery.
Neptune orbits the Sun once every 165 years so it’s roughly in same part of the sky now as it was at the time of it’s discovery 164 years ago in the constellation Aquarius. You’ll need a telescope and fairly dark skies to spot it. The chart shows the position of the Moon, Jupiter, Uranus and Neptune on September 23rd. The view is to the southeast around 11 pm. The Moon’s position from night to night will move about 15 degrees to the east. Because of their much greater distances the planets positions will change very little from night to night. Click chart to enlarge