From: Ian O'Neill, Discovery News
Published July 12, 2011 02:50 PM

Neptune Completes First Orbit Since Discovery

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.")

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.

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.

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