“By 1821 a curious anomaly had arisen – a recently discovered planet was not behaving as it should”
Uranus overtakes Neptune once per orbit. The effect this has on Uranus, shown below, led to the belief that Neptune existed before its discovery found that the planet frequently strayed from its predicted path. Perplexed, he realised that these discrepancies implied the presence of a large unseen body, tugging Uranus off course. Both John Couch Adams and Urbain Le Verrier predicted where this new planet might be. But it was Johann Galle and Heinrich d’Arrest who claimed victory, and the discovery of Neptune on the night of 23 September 1846 went to them.
So near, and yet...
Mathematicians set to work finalising the details of Neptune’s orbit, but they found the discrepancies in Uranus’s path around the Sun didn’t completely vanish as expected. Something was still causing Uranus to wander. The problem was of keen interest to Percival Lowell, who had established a large observatory in the frontier town of Flagstaff, Arizona, in 1894. Lowell concluded that Uranus and Neptune were being drawn off course by another, more distant planet, which he named ‘Planet X’. In 1906 he began searching for it, but after a decade Planet X had eluded him and he died in 1916. The
Tombaugh with the blink comparator, the device he used to find Pluto in 1930
NEPTUNEURANUS 1. Neptune exerts a gravitational pull on Uranus, accelerating it 2. As Uranus overtakes, it is decelerated again by Neptune’s gravitySUN