- Neptune Nears End of First Orbit Since Its Discovery in 1846

This story has been corrected to reflect that Neptune will actually complete its first orbit around the sun, since being discovered, in 2011.

The planet Neptune will be in opposition — when the sun, Earth, and a planet fall in a straight line on Aug. 20. The planet will be exactly opposite the sun in the sky, being highest in the sky at local midnight. Usually this is also the point where the planet is closest to the Earth.

This opposition is special because Neptune will be returning close to the spot where it was discovered in 1846, marking its first complete trip around the sun since its discovery. Neptune is close, but still not quite at the finish line of its first orbit since being discovered yet. That will occur in 2011, according to NASA.

Coincidentally, opposition in 1846 also fell on Aug. 20, although the planet wasn't actually spotted until over a month later, on Sept. 23.

This Neptune sky map shows where to find the planet as it completes it hits opposition this year.

Strange path to discovery

The discovery of Neptune has an interesting prehistory.

The planet Uranus was discovered more or less by accident in 1781 by Sir William Herschel, in the course of his search for deep sky objects. As time went by, Uranus' position wasn't quite what astronomer's predicted, and mathematical astronomers began to suspect that there was another planet out there whose gravity was influencing Uranus' motion.

In the mid-1840s an Englishman named John Couch Adams and a Frenchman named Urbain Le Verrier independently calculated where this new planet would have to be located to have the observed effect on Uranus, but both had trouble getting observational astronomers interested in looking for it.

Finally the German astronomer Johann Galle actually looked at the predicted location and discovered the tiny blue-green disk of the planet that eventually came to be known as Neptune. The date was Sept. 23, 1846. This led to a drawn out battle between French and English astronomers as to who pointed to Neptune first; in the end, a three-way tie was declared and Adams, Le Verrier, and Galle share the honor of discovering Neptune.

Ironically, Galle was not the first person to observe Neptune. That honor goes to none other than Galileo Galilei, who twice observed Neptune but mistook it for a star, on Dec. 28, 1612, and Jan. 27, 1613. Galileo had two strikes against him: first, the small size and poor quality of his telescopes, and secondly he happened to observe Neptune when it was stationary, as happens to all planets from time to time because of the relative motions of the planet and Earth.

For nearly a century Neptune was the planet farthest from the sun, only losing that honor when tiny Pluto was discovered by Clyde Tombaugh in 1930. Now that the International Astronomical Union has downgraded Pluto's status, Neptune is once again the farthest known planet from the sun — at least in our solar system.

Because of its great distance from the sun, 30 astronomical units out (1 AU is the distance from the sun to Earth), and its relatively small diameter (30,800 miles/49,500 km), Neptune is a dim and tiny object in amateur telescopes. While Uranus can just be glimpsed with the naked eye under perfect dark sky conditions, Neptune requires binoculars or a small telescope to be seen.

Finding Neptune now

For somewhat seasoned backyard astronomers, this Neptune map can help to locate the planet.

Around 1 a.m. this week look for the large but faint triangle of Capricornus, to the left of Sagittarius and the Milky Way. The two stars at the left end of the triangle point the way to Neptune, just a little bit short of and above the star Iota in the neighboring constellation Aquarius.

In a small telescope, Neptune will look just like a star; what gives it away is its distinctive blue-green color.

Although tiny in a telescope and dwarfed by giants Jupiter and Saturn, Neptune is still four times the diameter of the Earth. Like all the gas giant planets, it shows only an atmosphere, in this case fairly featureless. When the Voyager 2 passed by in 1989 it photographed a huge "Blue Spot" in Neptune's upper atmosphere, perhaps similar to the Great Red Spot on Jupiter. Like all the gas giants, Neptune has a system of rings, but these are far fainter than Saturn's famous rings.

Although Neptune's face appears serene, its atmosphere boasts winds which travel almost at supersonic speeds. Its 13 moons range in size from what are little more than boulders up to Triton, 1,680 miles (2,700 km.) in diameter.

• Gallery - Planets of the Solar System
• Telescopes for Beginners
• More Night Sky Features from Starry Night Education

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