Illustration by Paul Hudson, National Geographic

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The planet Neptune, as seen from above its largest moon, Triton.

Illustration by Paul Hudson, National Geographic

Neptune Discovered a Year Ago Today*

*One Neptunian year, that is, which is about 165 Earth years long.

Today marks one full year since astronomers discovered the planet Neptune—one Neptunian year, that is.

At an average distance of 2.8 billion miles (4.5 billion kilometers) from the sun, Neptune takes about 165 Earth years to finish a single orbit. (Explore an interactive solar system.)

That means July 12 marks the planet's first complete orbit around the sun since astronomers first observed it in 1846. The planet officially reaches this milestone at 22:27 UT.

How Galileo Missed the Mark

Technically, Neptune's existence was mathematically predicted years earlier, based on the gravitational effect the ice giant planet was having on neighboring Uranus.

By looking for discrepancies in Uranus's orbit, Englishman John Couch Adams and Frenchman Urbain Le Verrier each independently calculated the unseen world's mass and position.

But Neptune's existence wasn't confirmed until September 23, 1846, when German astronomer Johann Galle used a telescope to search the predicted location in the sky and spotted a tiny blue-green disk. The planet was eventually named Neptune, after the Roman god of the sea.

Ironically, Galileo Galilei spotted Neptune more than 200 years earlier but wrongly assumed the planet was just a star.

"He observed it and he thought it moved relative to background stars, but it became cloudy and he was unable to observe it further," said Rocky Alvey, director of the Vanderbilt Dyer Observatory in Brentwood, Tennessee.

"If it hadn't been for clouds, Galileo may have been the discoverer of Neptune."

Neptune Mysteriously Stormy

The closest approach to Neptune by a spacecraft occurred in 1989, when NASA's Voyager 2 passed within 3,000 miles (4,950 kilometers) of the planet's north pole.

But even with more than a century and a half of observations, many unresolved questions remain about the farthest planet from the sun.

For example, it's still unclear how Neptune is generating the heat needed to power the changes astronomers have observed in its atmosphere.

"It only gets 0.1 percent as much sunlight as Earth, but Neptune has storms and its appearance changes over time," said Erich Karkoschka, an astronomer at the University of Arizona.

Neptune was much darker in the 1970s and '80s than it is now, and large dark spots corresponding to giant storms have previously been observed in the planet's atmosphere by Voyager 2 and the Hubble Space Telescope.

"If we can understand how a planet with so little sunlight can have a very active atmosphere," Karkoschka said, "then I think we'll understand Earth's atmosphere and the planetary atmospheres of the more than 500 exoplanets around other stars better."

Find Neptune in the July Sky

Although Neptune is not among the planets visible to the naked eye, early birds with telescopes in the United States can spot Neptune this month—if you know where to look.

Through August the faint planet will appear just above the horizon near the constellation Aquarius in predawn skies.

"Prime viewing time is early morning, from midnight to 4 a.m." central time, the Vanderbilt Dyer Observatory's Alvey said.

"It's just beyond the visual limit, so ... you'll be able to spot Neptune with a small telescope or even just a pair of binoculars."