How Neptune was discovered
Billions of stars may twinkle above our heads at night, but there still remain only eight planets in our solar system. Jupiter is massive; Venus is our closest neighbour; Saturn looks awesome; Mars has a chocolate named after it; and Pluto plays the role of the hipster's favourite. But our favourite is the much under-appreciated, Neptune.
On September 23, 1846, German astronomer Johann Gottfried Galle first glimpsed Neptune, 2.7 billion km from Earth, through his telescope, the achromatic refractor, invented by Joseph von Fraunhofer in 1829.
It helped that he knew where to look. Previous astronomers had detected irregularities in Uranus' orbit that could only be explained by the existence of a father planet's gravity. This spurred Urbanin Le Verrier and John Couch Adams, who worked in Paris and Cambridge respectively, to begin their own separate calculations to determine the nature and position of such a planet. Galle would find it less than one degree from the location predicted by Verrier.
Though initially reffered to as Le Verrier's planet, Galle suggested Janus, the Roman 'God of beginnings', but it was eventually named Neptune by Le Verrier himself - maintaining the trend of naming planets after deities in Greco-Roman mythology, in this case the Roman 'God of the sea'.
For 84 years Neptune remained the furthers point in our solar system, until the discovery of Pluto. However, with Pluto's status as a planet having long been disputed the decision was finally taken in 2006 for the International Astronomical Union to fully defined the word 'planet' for the first time. This definition meant that Pluto would be reclassified as a dwarf planet, therefore officially reinstating Neptune once again as the outermost known planet in our solar system.