In 1781, British astronomer Sir William Herschel was the first to notice something strange about Uranus’ 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.

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 gas giant was made on Sept. 23, 1846, Neptune will arrive back in approximately the same spot as where it was first spotted on July 12, 2011.

via When Will Neptune Complete Its First Orbit Since Discovery? : Discovery News.