On this day in 1846, Neptune was officially discovered. Neptune–the eighth and, since the reclassification of Pluto, farthest planet from the Sun in our Solar System–is a blue planet that takes its name from the Roman god of the sea. Although the German astronomer Johann Gottfried Galle identified it back on 23 September 1846, Neptune only celebrated the first “birthday” of its discovery in July 2011, as its vast orbits take well over a century and a half.
On 12 July 2011, it arrived at the very same location in space where it was first discovered by Galle, and the 165 Earth years that passed are equivalent to only one Neptune year. In any case, the discovery of this freezing cold ice giant is an unusual tale, as it was only found because of observations of a fellow ice giant, its inner neighbour Uranus.
British astronomers William and Caroline Herschel first found Uranus in 1781, and very soon afterwards they noted that its unusual orbit did not equate to its expected orbit, according to the Newtonian laws of gravity, and yet they were unaware of why this was. It would take a while for this conundrum to be resolved, almost four decades in fact.
In 1821 the French astronomer Alexis Bouvard speculated, correctly as it turns out, that another planet was tugging on Uranus and so altering its orbit through space–but he had no way of proving this without finding that mystery planet, and he had no idea where it was.
Then, over 20 years later, two mathematician-astronomers–the Frenchman Urbain Le Verrier and the Englishman John Couch Adams–working independently of one another, were both able to predict where Neptune must be by using complex mathematics to measure how a hypothetical planet’s gravitational pull might affect Uranus’ path. Immediately afterwards, in September 1846, Le Verrier wrote a letter to the astronomer Johann Gottfried Galle of the Berlin Observatory, explaining exactly where he believed the planet to be, and upon receiving it Galle instantly started searching the skies with his telescope.
Le Verrier’s calculations were so accurate that his predicted position was less than a degree away from Neptune’s actual position, and so it took only two nights for Galle to find it and confirm it as a planet.
Johann Gottfried Galle was only able to find and identify Neptune because of the invaluable assistance of his fellow 19th-century astronomers and mathematicians, from William and Caroline Herschel to Urbain Le Verrier. However, the real significance of Neptune’s discovery is not only that it was the first planet found by mathematical prediction rather than by empirical observation–which is now common practice–but that the methods used to find it confirmed theories about the application of Newton’s theory of gravity to space, and indeed our entire understanding of the universe.
Credit: NASA/courtesy of nasaimages.org.
Caption: A photograph of Neptune taken by Voyager 2 in 1989.