Happy First 'Birthday' Neptune! Or, to be more accurate, I should say: "Happy First Orbit Since Being Discovered On Sept. 23, 1846, Neptune!"
Yes, Neptune has only completed one whole orbit around the sun after being discovered 164.8 (Earth) years ago.
Despite Neptune's size, however, the "ice giant" wasn't easy to spot and it took some ingenuity to track down.
(Note: As Neptune and Uranus possess a higher proportion of water, ammonia and methane "ices" in their atmospheres than "gas giants" Jupiter and Saturn, they are sometimes known as "ice giants.")
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Tugging Uranus
The first clue of Neptune's existence didn't come from directly spotting a "wandering star" synonymous with the orbit of an unknown planet; it actually came from observing the "wobble" in the orbit of a neighboring planet.
In 1781, British astronomer Sir William Herschel was the first to notice something strange about Uranus's orbit. By 1821, French astronomer Alexis Bouvard surmised that Uranus was being perturbed by the gravity of another massive planet in the outer solar system. There had to be something out there tugging at the 7th planet from the sun.
ANALYSIS: Uranus Pathfinder: Mission to the Mysterious Ice Giant
Then in the 1840's, English and French astronomers John Couch Adams and Urbain Le Verrier independently went on to calculate where this mystery planet should be in the night sky by purely measuring these little 'wobbles' in Uranus' path.
55 years after Herschel noticed Uranus' perturbations, the distant planet was officially discovered by German astronomer Johann Galle in the location predicted by Couch Adams and Le Verrier. It was named Neptune.
As Neptune is located so far away from the sun (approximately 4.5 billion kilometers, 30 Astronomical Units (AU), or 30-times the sun-Earth distance), it takes over 164 Earth years to complete one full orbit around our star.
As the first direct observation of the blue-green ice giant was made on Sept. 23, 1846, today Neptune arrived back in exactly the same spot (relative to its orbit around the sun, or "heliocentric longitude") as where it was first spotted.
SEE ALSO: Diamond Oceans Possible on Uranus, Neptune
Deja Vu?
If you're feeling some deja vu after reading this article, it might be because Discovery News actually ran a similar story last year when other news outlets began reporting on "Neptune's first orbit" on Aug. 20, 2010.
However, corrections were made to the article after it transpired that the news was premature. The confusion arose due to the apparent location of Neptune in the night's sky made it appear it had returned to the location in its orbit as when it was first discovered. As investigated by Nancy Atkinson over at Universe Today, it was a very easy astronomical error to make.
It all boils down to perspective, but one thing is for sure, today (July 12, 2011) is the official day of Neptune's first orbit since its discovery on Sept. 23, 1846 -- certainly a red letter day for astronomical history.
Now we just need a mission to the ice giants, as they remain one of the biggest (literally) mysteries of our solar system.
Image: These four images of Neptune were taken by NASA's Hubble Space Telescope in June 2011 during the planet's 16-hour rotation. The snapshots were taken at roughly four-hour intervals, offering a full view of the blue-green planet. For more information, see HubbleSite. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
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