National Geographic Daily News
An illustration of Neptune as seen from its moon Triton.

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

Illustration by Paul Hudson, National Geographic

Ker Than

for National Geographic News

Published July 12, 2011

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.

(Related: "Massive Comet Impact Detected on Neptune.")

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.

(Related: "Uranus, Neptune Swapped Spots, New Model Says.")

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."

(Read more about Galileo and the birth of telescopic astronomy.)

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."

(Also see "'Dead Zone' Asteroid Found Following Neptune.")

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."

0 comments

How to Feed Our Growing Planet

  • Feed the World

    Feed the World

    National Geographic explores how we can feed the growing population without overwhelming the planet in our food series.

See blogs, stories, photos, and news »

The Innovators Project

  • Teen Wonder: Taylor Wilson

    Teen Wonder: Taylor Wilson

    After achieving nuclear fusion at age 14, Taylor, now 19, is working with subatomic particles for solutions to nuclear terrorism and cancer.

See more innovators »

Phenomena

See more posts »

Latest News Video

  • How a T. Rex Packs for a Road Trip

    How a T. Rex Packs for a Road Trip

    The nation's most complete Tyrannosaurus rex specimen is taking a 2,000-mile road trip from Montana to its new home in Washington, D.C.

See more videos »

See Us on Google Glass

Shop Our Space Collection

  • Be the First to Own <i>Cosmos: A Spacetime Odyssey</i>

    Be the First to Own Cosmos: A Spacetime Odyssey

    The updated companion book to Carl Sagan's Cosmos, featuring a new forward by Neil deGrasse Tyson is now available. Proceeds support our mission programs, which protect species, habitats, and cultures.

Shop Now »