“By 1821 a cu­ri­ous anom­aly had arisen – a re­cently dis­cov­ered planet was not be­hav­ing as it should”

Sky at Night Magazine - - THE SKY GUIDE -

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

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