Space.com | Night Sky | 18 Aug 2010
Home Run! Neptune Completes First Orbit Since DiscoveryThe planet Neptune will be in opposition — when the sun, Earth, and a planet fall in a straight line on Aug. 20. The planet will be exactly opposite the sun in the sky, being highest in the sky at local midnight. Usually this is also the point where the planet is closest to the Earth.
This opposition is special because Neptune will be returning close to the spot where it was discovered in 1846, marking its first complete trip around the sun since its discovery.
Coincidentally opposition in 1846 also fell on Aug. 20, although the planet wasn't actually spotted until over a month later, on Sept. 23.
This Neptune sky map shoes where to find the planet as it completes its first orbit since astronomers first discovered it.
Source: Starry Night Education
Discovery News | 18 Aug 2010
As we approach the 164th anniversary of Neptune's discovery, there's another reason why we should celebrate this event in astronomical history: the gas giant is about to complete its first orbit since it was first spotted.
Although this is pretty exciting, the lead-up to Neptune's first sighting reads like a detective novel, a testament to the ingenuity of astronomers in the 18th and 19th Centuries.
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 Neptune 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 -- 164 years ago -- Neptune is about to arrive back in approximately the same spot as where it was first spotted by Galle.
Interestingly, Neptune was also observed just after it had reached opposition with the Earth -- a time when the sun, Earth and Neptune are roughly in alignment and when Neptune usually makes its closest approach to our planet. Opposition will occur on Aug. 20.
Source: Space.com
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